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1.5: Heat Transfer, Specific Heat, and Calorimetry

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Learning Objectives

By the end of this section, you will be able to:

  • Explain phenomena involving heat as a form of energy transfer
  • Solve problems involving heat transfer

We have seen in previous chapters that energy is one of the fundamental concepts of physics. Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. As we learned earlier in this chapter, heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature. Heat transfer is fundamental to such everyday activities as home heating and cooking, as well as many industrial processes. It also forms a basis for the topics in the remainder of this chapter.

We also introduce the concept of internal energy, which can be increased or decreased by heat transfer. We discuss another way to change the internal energy of a system, namely doing work on it. Thus, we are beginning the study of the relationship of heat and work, which is the basis of engines and refrigerators and the central topic (and origin of the name) of thermodynamics.

Internal Energy and Heat

A thermal system has internal energy (also called thermal energy ) , which is the sum of the mechanical energies of its molecules. A system’s internal energy is proportional to its temperature. As we saw earlier in this chapter, if two objects at different temperatures are brought into contact with each other, energy is transferred from the hotter to the colder object until the bodies reach thermal equilibrium (that is, they are at the same temperature). No work is done by either object because no force acts through a distance (as we discussed in Work and Kinetic Energy ). These observations reveal that heat is energy transferred spontaneously due to a temperature difference. Figure \(\PageIndex{1}\) shows an example of heat transfer.

Figure a shows a soda can at temperature T1 and an ice cube, some distance away at temperature T2. T1 is greater than T2. Figure b shows the can and cube in contact with each other. Both are at temperature T prime.

The meaning of “heat” in physics is different from its ordinary meaning. For example, in conversation, we may say “the heat was unbearable,” but in physics, we would say that the temperature was high. Heat is a form of energy flow, whereas temperature is not. Incidentally, humans are sensitive to heat flow rather than to temperature.

Since heat is a form of energy, its SI unit is the joule (J). Another common unit of energy often used for heat is the calorie (cal), defined as the energy needed to change the temperature of 1.00 g of water by \(1.00^oC\)—specifically, between \(14.5^oC\) and \(15.5^oC\) since there is a slight temperature dependence. Also commonly used is the kilocalorie (kcal), which is the energy needed to change the temperature of 1.00 kg of water by \(1.00^oC\). Since mass is most often specified in kilograms, the kilocalorie is convenient. Confusingly, food calories (sometimes called “big calories,” abbreviated Cal) are actually kilocalories, a fact not easily determined from package labeling.

Mechanical Equivalent of Heat

It is also possible to change the temperature of a substance by doing work, which transfers energy into or out of a system. This realization helped establish that heat is a form of energy. James Prescott Joule (1818–1889) performed many experiments to establish the mechanical equivalent of heat — the work needed to produce the same effects as heat transfer . In the units used for these two quantities, the value for this equivalence is

\[1.000 \, kcal = 4186 \, J.\] We consider this equation to represent the conversion between two units of energy. (Other numbers that you may see refer to calories defined for temperature ranges other than \(14.5^oC\) to \(15.5^oC\).)

Figure \(\PageIndex{2}\) shows one of Joule’s most famous experimental setups for demonstrating that work and heat can produce the same effects and measuring the mechanical equivalent of heat. It helped establish the principle of conservation of energy. Gravitational potential energy ( U ) was converted into kinetic energy ( K ), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. Joule’s contributions to thermodynamics were so significant that the SI unit of energy was named after him.

An insulated cylindrical container is filled with known volume water. A vertical rod is immersed in it. This has paddles which would stir the water if the rod were rotated. The top portion of the rod is outside the water. A string is tied around it, both ends of which go over pulleys and support weights on either side. A lever at the top is used to rotate the rod. A thermometer is kept in the water. The distance from the cente of the weight and the pully to the base of the container is labeled measured height of descent.

Increasing internal energy by heat transfer gives the same result as increasing it by doing work. Therefore, although a system has a well-defined internal energy, we cannot say that it has a certain “heat content” or “work content.” A well-defined quantity that depends only on the current state of the system, rather than on the history of that system, is known as a state variable . Temperature and internal energy are state variables. To sum up this paragraph, heat and work are not state variables .

Incidentally, increasing the internal energy of a system does not necessarily increase its temperature. As we’ll see in the next section, the temperature does not change when a substance changes from one phase to another. An example is the melting of ice, which can be accomplished by adding heat or by doing frictional work, as when an ice cube is rubbed against a rough surface.

Temperature Change and Heat Capacity

We have noted that heat transfer often causes temperature change. Experiments show that with no phase change and no work done on or by the system, the transferred heat is typically directly proportional to the change in temperature and to the mass of the system, to a good approximation. (Below we show how to handle situations where the approximation is not valid.) The constant of proportionality depends on the substance and its phase, which may be gas, liquid, or solid. We omit discussion of the fourth phase, plasma, because although it is the most common phase in the universe, it is rare and short-lived on Earth.

We can understand the experimental facts by noting that the transferred heat is the change in the internal energy, which is the total energy of the molecules. Under typical conditions, the total kinetic energy of the molecules \(K_{total}\) is a constant fraction of the internal energy (for reasons and with exceptions that we’ll see in the next chapter). The average kinetic energy of a molecule \(K_{ave}\) is proportional to the absolute temperature. Therefore, the change in internal energy of a system is typically proportional to the change in temperature and to the number of molecules, N . Mathematically, \(\Delta U \propto \Delta K_{total} = NK_{ave} \propto N\Delta T\). The dependence on the substance results in large part from the different masses of atoms and molecules. We are considering its heat capacity in terms of its mass, but as we will see in the next chapter, in some cases, heat capacities per molecule are similar for different substances. The dependence on substance and phase also results from differences in the potential energy associated with interactions between atoms and molecules.

Heat Transfer and Temperature Change

A practical approximation for the relationship between heat transfer and temperature change is:

\[Q = mc\Delta T,\]

where \(Q\) is the symbol for heat transfer (“quantity of heat”), m is the mass of the substance, and \(\Delta T\) is the change in temperature. The symbol c stands for the specific heat (also called “ specific heat capacity ”) and depends on the material and phase. The specific heat is numerically equal to the amount of heat necessary to change the temperature of \(1.00 \, kg\) of mass by \(1.00^oC\). The SI unit for specific heat is \(J/(kg \times K)\) or \(J/(kg \times ^oC)\). (Recall that the temperature change \(\Delta T\) is the same in units of kelvin and degrees Celsius.)

Values of specific heat must generally be measured, because there is no simple way to calculate them precisely. Table \(\PageIndex{1}\) lists representative values of specific heat for various substances. We see from this table that the specific heat of water is five times that of glass and 10 times that of iron, which means that it takes five times as much heat to raise the temperature of water a given amount as for glass, and 10 times as much as for iron. In fact, water has one of the largest specific heats of any material, which is important for sustaining life on Earth.

The specific heats of gases depend on what is maintained constant during the heating—typically either the volume or the pressure. In the table, the first specific heat value for each gas is measured at constant volume, and the second (in parentheses) is measured at constant pressure. We will return to this topic in the chapter on the kinetic theory of gases.

In general, specific heat also depends on temperature. Thus, a precise definition of c for a substance must be given in terms of an infinitesimal change in temperature. To do this, we note that \(c = \frac{1}{m} \frac{\Delta Q}{\Delta T}\) and replace \(\Delta\) with d:

\[c = \dfrac{1}{m} \dfrac{dQ}{dT}.\]

Except for gases, the temperature and volume dependence of the specific heat of most substances is weak at normal temperatures. Therefore, we will generally take specific heats to be constant at the values given in the table.

Example \(\PageIndex{1}\): Calculating the Required Heat

A 0.500-kg aluminum pan on a stove and 0.250 L of water in it are heated from \(20.0^oC\) to \(80.0^oC\). (a) How much heat is required? What percentage of the heat is used to raise the temperature of (b) the pan and (c) the water?

We can assume that the pan and the water are always at the same temperature. When you put the pan on the stove, the temperature of the water and that of the pan are increased by the same amount. We use the equation for the heat transfer for the given temperature change and mass of water and aluminum. The specific heat values for water and aluminum are given in Table \(\PageIndex{1}\).

  • Calculate the temperature difference: \[\Delta t = T_f - T_i = 60.0^oC.\]
  • Calculate the mass of water. Because the density of water is \(1000 \, kg/m^3\), 1 L of water has a mass of 1 kg, and the mass of 0.250 L of water is \(m_w = 0.250 \, kg.\)
  • Calculate the heat transferred to the water. Use the specific heat of water in Table \(\PageIndex{1}\): \[Q_w = m_wc_w\Delta T = (0.250 \, kg)(4186 \, J/kg ^oC)(60.0 ^oC) = 62.8 \, kJ.\]
  • Calculate the heat transferred to the aluminum. Use the specific heat for aluminum in Table \(\PageIndex{1}\): \[Q_{A1} = m_{A1}c_{A1}\Delta T = (0.500 \, kg)(900 \, J/kg^oC)(60.0^oC) = 27.0 \, kJ.\]
  • Find the total transferred heat: \[Q_{Total} = Q_W + Q_{A1} = 89.8 \, kJ.\]

Significance

In this example, the heat transferred to the container is a significant fraction of the total transferred heat. Although the mass of the pan is twice that of the water, the specific heat of water is over four times that of aluminum. Therefore, it takes a bit more than twice as much heat to achieve the given temperature change for the water as for the aluminum pan.

Example \(\PageIndex{2}\) illustrates a temperature rise caused by doing work. (The result is the same as if the same amount of energy had been added with a blowtorch instead of mechanically.)

Calculating the Temperature Increase from the Work Done on a Substance.

Truck brakes used to control speed on a downhill run do work, converting gravitational potential energy into increased internal energy (higher temperature) of the brake material (Figure \(\PageIndex{3}\)). This conversion prevents the gravitational potential energy from being converted into kinetic energy of the truck. Since the mass of the truck is much greater than that of the brake material absorbing the energy, the temperature increase may occur too fast for sufficient heat to transfer from the brakes to the environment; in other words, the brakes may overheat.

Figure shows a truck on a road. There is smoke near its tires.

Calculate the temperature increase of 10 kg of brake material with an average specific heat of \(800 \, J/kg \cdot ^C\) if the material retains 10% of the energy from a 10,000-kg truck descending 75.0 m (in vertical displacement) at a constant speed.

We calculate the gravitational potential energy ( Mgh ) that the entire truck loses in its descent, equate it to the increase in the brakes’ internal energy, and then find the temperature increase produced in the brake material alone.

First we calculate the change in gravitational potential energy as the truck goes downhill:

\[Mgh = (10,000 \, kg)(9.80 \, m/s^2)(75.0 \, m) = 7.35 \times 10^6 \, J. \nonumber\]

Because the kinetic energy of the truck does not change, conservation of energy tells us the lost potential energy is dissipated, and we assume that 10% of it is transferred to internal energy of the brakes, so take \(Q = Mgh/10\). Then we calculate the temperature change from the heat transferred, using

\[\Delta T = \dfrac{7.35 \times 10^5 \, J}{(10 \, kg)(800 \, J/kg^oC)} = 92^oC. \nonumber\]

If the truck had been traveling for some time, then just before the descent, the brake temperature would probably be higher than the ambient temperature. The temperature increase in the descent would likely raise the temperature of the brake material very high, so this technique is not practical. Instead, the truck would use the technique of engine braking. A different idea underlies the recent technology of hybrid and electric cars, where mechanical energy (kinetic and gravitational potential energy) is converted by the brakes into electrical energy in the battery, a process called regenerative braking.

In a common kind of problem, objects at different temperatures are placed in contact with each other but isolated from everything else, and they are allowed to come into equilibrium. A container that prevents heat transfer in or out is called a calorimeter , and the use of a calorimeter to make measurements (typically of heat or specific heat capacity) is called calorimetry .

We will use the term “calorimetry problem” to refer to any problem in which the objects concerned are thermally isolated from their surroundings. An important idea in solving calorimetry problems is that during a heat transfer between objects isolated from their surroundings, the heat gained by the colder object must equal the heat lost by the hotter object, due to conservation of energy:

\[Q_{cold} + Q_{hot} = 0.\]

We express this idea by writing that the sum of the heats equals zero because the heat gained is usually considered positive; the heat lost, negative.

Calculating the Final Temperature in Calorimetry

Suppose you pour 0.250 kg of \(20.0^oC\) water (about a cup) into a 0.500-kg aluminum pan off the stove with a temperature of \(150^oC\). Assume no heat transfer takes place to anything else: The pan is placed on an insulated pad, and heat transfer to the air is neglected in the short time needed to reach equilibrium. Thus, this is a calorimetry problem, even though no isolating container is specified. Also assume that a negligible amount of water boils off. What is the temperature when the water and pan reach thermal equilibrium?

Originally, the pan and water are not in thermal equilibrium: The pan is at a higher temperature than the water. Heat transfer restores thermal equilibrium once the water and pan are in contact; it stops once thermal equilibrium between the pan and the water is achieved. The heat lost by the pan is equal to the heat gained by the water—that is the basic principle of calorimetry.

  • Use the equation for heat transfer \(Q = mc\Delta T\) to express the heat lost by the aluminum pan in terms of the mass of the pan, the specific heat of aluminum, the initial temperature of the pan, and the final temperature: \[Q_{hot} = m_{A1}c_{A1}(T_f - 150^oC). \nonumber\]
  • Express the heat gained by the water in terms of the mass of the water, the specific heat of water, the initial temperature of the water, and the final temperature: \[Q_{cold} = m_wc_w(T_f - 20.0^oC). \nonumber\]
  • Note that \(Q_{hot} <0\) and \(Q_{cold} > 0 \) and that as stated above, they must sum to zero: \[Q_{cold} + Q_{hot} = 0\]\[Q_{cold} = -Q_{hot}\]\[m_wc_w(T_f - 20.0 ^C) = -m_{A1}c_{A1} (T_f - 150^oC). \nonumber\]
  • This a linear equation for the unknown final temperature, \(T_f\). Solving for \(T_f\), \[T_f = \dfrac{m_{A1}c_{A1}(150^oC) + m_wc_w(20.0^oC)}{m_{A1}c_{A1} + m_wc_w}, \nonumber\] and insert the numerical values: \[T_f = \dfrac{(0.500 \, kg)(900 \, J/kg^oC)(150^oC) + (0.250 \, kg)(4186 \, J/kg^oC)(20.0^oC)}{(0.500 \, kg)(900 \, J/kg^oC) + (0.250 \, kg)(4186 \, J/kg^oC)} = 59.1 \, ^oC. \nonumber\]

Significance Why is the final temperature so much closer to \(20.0^oC\) than to \(150^oC\)? The reason is that water has a greater specific heat than most common substances and thus undergoes a smaller temperature change for a given heat transfer. A large body of water, such as a lake, requires a large amount of heat to increase its temperature appreciably. This explains why the temperature of a lake stays relatively constant during the day even when the temperature change of the air is large. However, the water temperature does change over longer times (e.g., summer to winter).

Exercise \(\PageIndex{3}\)

If 25 kJ is necessary to raise the temperature of a rock from \(25^oC\) to \(30^oC\), how much heat is necessary to heat the rock from \(45^oC\) to \(50^oC\)?

To a good approximation, the heat transfer depends only on the temperature difference. Since the temperature differences are the same in both cases, the same 25 kJ is necessary in the second case. (As we will see in the next section, the answer would have been different if the object had been made of some substance that changes phase anywhere between \(30^oC\) and \(50^oC\).)

Temperature-Dependent Heat Capacity

At low temperatures, the specific heats of solids are typically proportional to \(T^3\). The first understanding of this behavior was due to the Dutch physicist Peter Debye , who in 1912, treated atomic oscillations with the quantum theory that Max Planck had recently used for radiation. For instance, a good approximation for the specific heat of salt, NaCl, is \(c = 3.33 \times 10^4 \frac{J}{kg \cdot k}\left(\frac{T}{321 \, K}\right)^3\). The constant 321 K is called the Debye temperature of NaCl, \(\Theta_D\) and the formula works well when \(T < 0.04 \Theta_D\). Using this formula, how much heat is required to raise the temperature of 24.0 g of NaCl from 5 K to 15 K?

Because the heat capacity depends on the temperature, we need to use the equation \[c = \dfrac{1}{m} \dfrac{dQ}{dT}.\]

We solve this equation for Q by integrating both sides: \(Q = m \int_{T_1}^{T_2} cdT\).

Then we substitute the given values in and evaluate the integral:

\[Q = (0.024 \, kg) \int_{T1}^{T2} 333 \times 10^4 \dfrac{J}{kg \cdot K}\left(\dfrac{T}{321 \, K}\right)^3 dT = \left( 6.04 \times 10^{-4} \dfrac{J}{K^4}\right) T^4 |_{5 \, K}^{15 \, K} = 30.2 \, J.\]

Significance If we had used the equation \(Q = mc\Delta T\) and the room-temperature specific heat of salt, \(880 \, J/kg \cdot K\), we would have gotten a very different value.

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transfer of heat physics

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If you have been following along since the beginning of this lesson, then you have been developing a progressively sophisticated understanding of temperature and heat. You should be developing a model of matter as consisting of particles which vibrate (wiggle about a fixed position), translate (move from one location to another) and even rotate (revolve about an imaginary axis). These motions give the particles kinetic energy. Temperature is a measure of the average amount of kinetic energy possessed by the particles in a sample of matter. The more the particles vibrate, translate and rotate, the greater the temperature of the object. You have hopefully adopted an understanding of heat as a flow of energy from a higher temperature object to a lower temperature object. It is the temperature difference between the two neighboring objects that causes this heat transfer. The heat transfer continues until the two objects have reached thermal equilibrium and are at the same temperature. The discussion of heat transfer has been structured around some everyday examples such as the cooling of a hot mug of coffee and the warming of a cold can of pop. Finally, we have explored a thought experiment in which a metal can containing hot water is placed within a Styrofoam cup containing cold water. Heat is transferred from the hot water to the cold water until both samples have the same temperature.

  • What is happening at the particle level when energy is being transferred between two objects?
  • Why is thermal equilibrium always established when two objects transfer heat?
  • How does heat transfer work within the bulk of an object?
  • Is there more than one method of heat transfer? If so, then how are they similar and different than one another?

Conduction - A Particle View

Let's begin our discussion by returning to our thought experiment in which a metal can containing hot water was placed within a Styrofoam cup containing cold water. Heat is transferred from the hot water to the cold water until both samples have the same temperature. In this instance, the transfer of heat from the hot water through the metal can to the cold water is sometimes referred to as conduction . Conductive heat flow involves the transfer of heat from one location to another in the absence of any material flow. There is nothing physical or material moving from the hot water to the cold water. Only energy is transferred from the hot water to the cold water. Other than the loss of energy, there is nothing else escaping from the hot water. And other than the gain of energy, there is nothing else entering the cold water. How does this happen? What is the mechanism that makes conductive heat flow possible?

The container walls represent the perimeters of a sample of matter. Just as the perimeter of your property (as in real estate property) is the furthest extension of the property, so the perimeter of an object is the furthest extension of the particles within a sample of matter. At the perimeter, the little bangers are colliding with particles of another substance - the particles of the container or even the surrounding air. Even the wigglers that are fixed in a position along the perimeter are doing some banging. Being at the perimeter, their wiggling results in collisions with the particles that are next to them; these are the particles of the container or of the surrounding air.

The mechanism in which heat is transferred from one object to another object through particle collisions is known as conduction. In conduction, there is no net transfer of physical stuff between the objects. Nothing material moves across the boundary. The changes in temperature are wholly explained as the result of the gains and losses of kinetic energy during collisions.

Conduction Through The Bulk of an Object

We have discussed how heat transfers from one object to another through conduction. But how does it transfer through the bulk of an object? For instance, suppose we pull a ceramic coffee mug out of the cupboard and place it on the countertop. The mug is at room temperature - maybe at 26°C. Then suppose we fill the ceramic coffee mug with hot coffee at a temperature of 80°C. The mug quickly warms up. Energy first flows into the particles at the boundary between the hot coffee and the ceramic mug. But then it flows through the bulk of the ceramic to all parts of the ceramic mug. How does heat conduction occur in the ceramic itself?

The mechanism of heat transfer through the bulk of the ceramic mug is described in a similar manner as it before. The ceramic mug consists of a collection of orderly arranged wigglers. These are particles that wiggle about a fixed position. As the ceramic particles at the boundary between the hot coffee and the mug warm up, they attain a kinetic energy that is much higher than their neighbors. As they wiggle more vigorously, they bang into their neighbors and increase their vibrational kinetic energy. These particles in turn begin to wiggle more vigorously and their collisions with their neighbors increase their vibrational kinetic energy. The process of energy transfer by means of the little bangers continues from the particles at the inside of the mug (in contact with the coffee particles) to the outside of the mug (in contact with the surrounding air). Soon the entire coffee mug is warm and your hand feels it.

This mechanism of conduction by particle-to-particle interaction is very common in ceramic materials such as a coffee mug. Does it work the same in metal objects? For instance, you likely have noticed the high temperatures attained by the metal handle of a skillet when placed upon a stovetop. The burners on the stove transfer heat to the metal skillet. If the handle of the skillet is metallic, it too attains a high temperature, certainly high enough to cause a bad burn. The transfer of heat from the skillet to the skillet handle occurs by conduction. But in metals, the conduction mechanism is slightly more complicated. In a manner similar to electrical conductivity, thermal conductivity in metals occurs by the movement of free electrons . Outer shell electrons of metal atoms are shared among atoms and are free to move throughout the bulk of the metal. These electrons carry the energy from the skillet to the skillet handle. The details of this mechanism of thermal conduction in metals are considerably more complex than the discussion given here. The main point to grasp is that heat transfer through metals occurs without any movement of atoms from the skillet to the skillet handle. This qualifies the heat transfer as being categorized as thermal conduction.

Heat Transfer by Convection

Convection is the main method of heat transfer in fluids such as water and air. It is often said that heat rises in these situations. The more appropriate explanation is to say that heated fluid rises . For instance, as the heated air rises from the heater on a floor, it carries more energetic particles with it. As the more energetic particles of the heated air mix with the cooler air near the ceiling, the average kinetic energy of the air near the top of the room increases. This increase in the average kinetic energy corresponds to an increase in temperature. The net result of the rising hot fluid is the transfer of heat from one location to another location. The convection method of heat transfer always involves the transfer of heat by the movement of matter. This is not to be confused with the caloric theory discussed earlier in this lesson. In caloric theory, heat was the fluid and the fluid that moved was the heat. Our model of convection considers heat to be energy transfer that is simply the result of the movement of more energetic particles.

The two examples of convection discussed here - heating water in a pot and heating air in a room - are examples of natural convection . The driving force of the circulation of fluid is natural - differences in density between two locations as the result of fluid being heated at some source. (Some sources introduce the concept of buoyant forces to explain why the heated fluids rise. We will not pursue such explanations here.) Natural convection is common in nature. The earth's oceans and atmosphere are heated by natural convection. In contrast to natural convection, forced convection involves fluid being forced from one location to another by fans, pumps and other devices. Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations. This is an example of forced convection. The movement of the fluid from the hot location (near the furnace) to the cool location (the rooms throughout the house) is driven or forced by a fan. Some ovens are forced convection ovens; they have fans that blow heated air from a heat source into the oven. Some fireplaces enhance the heating ability of the fire by blowing heated air from the fireplace unit into the adjacent room. This is another example of forced convection.

Heat Transfer by Radiation

A final method of heat transfer involves radiation. Radiation is the transfer of heat by means of electromagnetic waves . To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin. The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. Thermal radiation can occur through matter or through a region of space that is void of matter (i.e., a vacuum). In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun.

All objects radiate energy in the form of electromagnetic waves. The rate at which this energy is released is proportional to the Kelvin temperature ( T ) raised to the fourth power.

Radiation rate = k•T 4

Our discussion on this page has pertained to the various methods of heat transfer. Conduction, convection and radiation have been described and illustrated. The macroscopic has been explained in terms of the particulate - an ongoing goal of this chapter of The Physics Classroom Tutorial. The last topic to be discussed in Lesson 1 is more quantitative in nature. On the next page , we will investigate the mathematics associated with the rate of heat transfer.

Check Your Understanding

1. Consider Object A which has a temperature of 65°C and Object B which has a temperature of 15°C. The two objects are placed next to each other and the little bangers begin colliding. Will any of the collisions result in the transfer of energy from Object B to Object A? Explain.

Answer: Most certainly yes.

The average kinetic energy of the particles in Object A is greater than the average kinetic energy of the particles in Object B. But there is a range of speeds and thus of kinetic energy in both objects. As such, there will be some highly energetic particles in Object B and some very non-energetic particles in Object A. When this combination of particles encounter a collision, there will a transfer of energy across the boundary from Object B (the colder object) to Object A (the hotter object). This is just one collision. Since majority of collisions result from the more energetic particles of Object A with less energetic particles of collision B, there will be a net kinetic energy transfer from Object A to Object B.

2. Suppose that Object A and Object B (from the previous problem) have reached a thermal equilibrium. Do the particles of the two objects still collide with each other? If so, do any of the collisions result in the transfer of energy between the two objects? Explain.

The collisions will still take place because the particles are still moving. Just because the temperatures are the same doesn't mean the collisions will stop. The fact that the temperature is identical means that the average kinetic energy of all the particles is the same for both objects. As such, there will be just as much energy transferred from Object B to Object A as there is energy transferred in the opposite direction. When the effect of these collisions is averaged , there is no net energy transfer. This explains why the temperature of the two objects remains the same. Thermal equilibrium persists.
  • What Does Heat Do?

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Physics library

Course: physics library   >   unit 10.

  • Specific heat and latent heat of fusion and vaporization
  • Thermal conduction, convection, and radiation
  • Thermal conduction
  • Thermal conductivity of metal and wood
  • Intuition behind formula for thermal conductivity

What is thermal conductivity?

What is thermal conduction, what's the equation for the rate of thermal conduction, what does each term represent in the thermal conduction equation, why do metals feel both colder in the winter, and hotter in the summer, what do solved examples involving thermal conduction look like, example 1: window makeover.

  • (Choice A)   double the area, double the thickness, quadruple the k constant A double the area, double the thickness, quadruple the k constant
  • (Choice B)   quadruple the area, double the thickness, cut the k constant in half B quadruple the area, double the thickness, cut the k constant in half
  • (Choice C)   cut the area in half, cut the thickness in half, and double the k constant C cut the area in half, cut the thickness in half, and double the k constant
  • (Choice D)   double the area, cut the thickness in half, cut the k constant in half D double the area, cut the thickness in half, cut the k constant in half

Example 2: Window heat loss

  • "Conduction" from Openstax College Physics. Download the original article free at http://cnx.org/contents/[email protected]:105/Conduction

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16. Heat transfer

Conduction is heat transfer that occurs due to physical contact between objects. Conduction also explains how heat is able to transfer within an object.

As discussed in the previous chapter, temperature relates to the average energy of molecules. If molecules are able to easily bump into each other, causing collisions that transfer energy from one place to the next, then those types of substances will be good conductors of heat. Good heat conductors tend to be solids , where molecules are in close physical proximity. In liquids (where the molecules are not rigidly bonded to each other) and gases (where molecules are spread farther apart), it’s harder for heat to conduct due to these atomic collisions.

In fact, the solid objects that make the best heat conductors tend to be those that also make the best electrical conductors : metals. In metals, electrons are capable of moving around readily, which provides for a very efficient transfer of thermal energy from one part of a metal to another.

In the video below, there are two pretty much identical ice cubes that have been placed onto different black plates. One of the plates is metal and the other one is foam. Metal is a good heat conductor. Foam is not a good heat conductor. Therefore, the metal is able to conduct heat from the surrounding air into the metal plate, and thus into the ice cube, very efficiently. That ice cube melts very rapidly.

On the other hand, the foam plate is a poor heat conductor (alternatively: it is a good thermal insulator). That means that it keeps the heat in the atmosphere away from the ice cube, and that ice cube barely melts at all. Note that both ice cubes are surrounded on five sides by air. Air, being a gas, is a rather poor heat conductor. When the ice cube on the metal plate melts, it melts predominantly from the bottom, the side that is in contact with the good heat conductor.

This demonstration also explains why metal objects tend to feel cold to the touch even when they are at the same temperature as their surroundings. When you touch something metal, that metal is able to conduct the heat from your hand away very readily. Similarly, objects made of bad heat conductors, things like plastic, wood, paper, cork, and other insulators, are not going to feel cold to the touch, unless they themselves are cold.

Even among metals, thermal conductivity can vary. In the video below, there is an apparatus with four different metals that connect to a central plate. Dr. Pasquale places candles into each of the metals then lights a Bunsen burner under the central plate. As that central plate heats up, it conducts heat to each of the four metals. The metal that heats up the fastest has the highest heat conduction, and will melt the candles fastest.

First, you can see that the heat is going to travel outward. That is to say, the candles closest to the central plate melt first. Second, you can see that the best heat conductor melts the candles the fastest.

That best heat conductor is copper, which contains all of the green candles. The next best heat conductor is aluminum, which contains all of the yellow candles. Then comes brass, with the blue candles. Finally, the worst of the heat conductors of these four metals is iron, which has the red candles.

Remember, heat conduction requires physical contact between materials, or within a material. When you put food onto a frying pan to cook it, that food will cook due to that physical contact: heat conduction. This is why frying pans are made out of metals: they are good heat conductors! The part of the frying pan we have to touch is frequently covered with plastic or is very long and far away from the center of the frying pan, keeping our hands safe from the heat of the stove. The frying pan in Figure 16.1 has a metal base and a plastic handle.

A photograph of a non-stick frying pan.

One last thing about heat conduction: don’t confuse heat conduction with specific heat capacity ! While they seem similar, they are actually two completely different things. Heat conduction quantifies the ability of a material to allow thermal energy to move through it from one place to another. Specific heat capacity quantifies how much heat is required to change the temperature of a certain amount of material.

Convection is heat transfer that comes about due to the motion of molecules themselves. The types of materials that are good at convection are the types of materials where molecules can move around very freely: liquids and gases.

If you’ve ever held your hands above a toaster while heating something up, you’ve experienced convection. Air in close proximity to the toaster coils heats up. As that air heats up, it becomes less dense than the surrounding air, causing it to rise. When you hold your hand above the toaster, you are experiencing that warm air rising up.

In the video below, there is a large beaker filled with water that’s a little bit colder than room temperature. Dr. Pasquale places inside of that beaker a smaller flask filled with hot dyed water. The dye is just there to help visualize the motion of the molecules in that flask. What happens is that the hot water, being less dense than the surrounding cool water, is going to experience a buoyant force that causes it to rise up. You can see this from the dyed water moving upward in the beaker.

Eventually, as the hot water moves upward, it exchanges heat energy with the cool water, causing the cool water to increase its temperature and the hot water to decrease its temperature. This process repeats until the entire mixture reaches thermal equilibrium.

Convection occurs in our atmosphere all the time. On hot summer days, the ground is heated up by the Sun ( in a process that will be discussed in the next section of this textbook ). The air directly above the ground heats up due to conduction, and then the convection process causes that heat to rise up into the atmosphere. This causes a lot of vertical motion in the air.

This is why, in the summer, we tend to see a lot of cumulus clouds (Figure 16.2), which have a lot of vertical height to them. They occur when there is a lot of convection in the atmosphere. Atmospheric convection also causes turbulence in airplanes.

A photograph of cumulus clouds as seen out an airplane's window. The clouds separate a blue sky from farm fields in north central Illinois. The right wing and wing strut of the airplane can be seen out the window.

Convection doesn’t happen as much in the winter, because the ground doesn’t heat up as much when it’s covered in snow. Winter days tend to cause stratus clouds (Figure 16.3), which are very flat, indicating that there isn’t a lot of convection, or vertical motion, in the atmosphere.

A photograph of stratus clouds as seen out an airplane's window. The clouds separate a gray sky from the city of DeKalb, Illinois. The right wing and wing strut of the airplane can be seen out the window.

Electromagnetic radiation

Electromagnetic radiation is heat that is transferred from one place to another through light waves. Heat from the Sun travels nearly 150 million km through the vacuum of space to our planet as light waves. When those light waves reach the surface of the Earth, objects on the Earth’s surface can absorb this energy and will heat up.

The amount that objects heat up has to do with how much of the light they absorb. Black objects absorb (almost) all of the visible wavelengths of light, which is why they appear to be black. This absorption makes black objects heat up much more readily than objects of a different color.

This may be something you experience if you have a dark-colored car. The Sun’s light is absorbed by the car, heating it up. This is very nice in cold weather, but can make it hard to cool the car down in hot weather. If you drive a white or silver car, it’s very likely you have the opposite experience. Your car remains cooler in hot weather, but does not heat up as much from the Sun’s rays in the cold.

This is demonstrated in the video below. There are temperature probes placed in otherwise identical cans. The only difference between the two cans is that one is silver, and the other has been painted black. Dr. Pasquale connects the temperature probes to Logger Pro and turns on a heat lamp. The heat lamp is placed so that it was equidistant from both cans. Right away, the black can starts recording higher temperatures, indicating that it heats up more readily from electromagnetic radiation due to its color. The silver can heats up, but not as quickly.

The data from this demonstration (while the cans were heating up) is shown in Figure 16.4. ( Download this data [XLSX, 47 kB] )

A graph with temperature (degrees C) on the y-axis and time (s) on the x-axis. Hundreds of data points are shown for two cases: a temperature probe placed in a black can and a temperature probe placed in a silver can. The temperature from the black can increases rapidly from 21 C to 36 C. The temperature from the silver can increases slowly from 21 C to 26 C. This data is collected over 5 minutes.

If a black object heats up faster, what can we say about how quickly it cools down? Dr. Pasquale removed the heat lamp and recorded the temperature of both cans as they cooled off. The black can not only heats up faster, but also cools down faster as well. The data from the experiment (while the cans were cooling down) is shown in Figure 16.5. ( Download this data [XLSX, 31 kB] )

A graph with temperature (degrees C) on the y-axis and time (s) on the x-axis. Hundreds of data points are shown for two cases: a temperature probe placed in a black can and a temperature probe placed in a silver can. The temperature from the black can decreases rapidly from 37 C to 28 C. The temperature from the silver can decreases slowly from 27 C to 26 C. This data is collected over about 4 minutes.

The Sun is not the only object that emits electromagnetic radiation. In fact, when the cans from the video above cool down, they are re-emitting that electromagnetic radiation into the air and surrounding objects. Humans also absorb and emit electromagnetic radiation. The reason we don’t see it is that the wavelengths of light we emit are in the infrared. Infrared light is not something that the human eye is able to see.

If we use an infrared camera, we can collect infrared light that is emitted from different objects. Figure 16.6 shows an infrared photo of one of the College of DuPage physics classes from a few years ago. Notice that people, who are warm compared to the air, emit a good amount of infrared light, which is then collected by the camera.

A photograph taken using infrared of students in a classroom. Students appear in shades of blue to red coloring, based on their temperature. Students with glasses appear to have black spots where their glasses are.

The students who wear glasses look like they have dark spots around their eyes. This is because glass does not allow infrared light to pass through it. It’s not that the students have cold eyes, it’s that the infrared radiation emitted by the student’s eyes is blocked from reaching the camera.

The hotter an object is, the higher the energy of the light that they will emit. Infrared has relatively low energy and is generally emitted from objects that are relatively cool, such as “room temperature” objects or humans. Things that are very hot, such as stars, are much hotter and emit higher energy light. Light with more energy than infrared is red light, then orange light, all the way through the visible spectrum to blue light and then ultraviolet. A blue star would therefore be much hotter than a red star. We’ll revisit these topics when we talk about electromagnetic waves and light emission later in this textbook.

In general, if an object absorbs more energy than it emits, its temperature will increase. If an object emits more energy than it absorbs, its temperature will go down.

The greenhouse effect

The greenhouse effect is a gradual warming process as heat is trapped by gases in the Earth’s atmosphere. In short, these gases are transparent to the higher energy light coming from the (hot) Sun. These same gases are opaque to the lower energy radiation emitted from the (cold) Earth. This is analogous to an actual greenhouse (Figure 16.7), which is a beneficial structure used to keep plants from dying off in cold weather. In the case of our planet acting as a greenhouse, warming up the atmosphere and contributing to global climate change, the greenhouse effect is decidedly less positive.

A photograph of a greenhouse with glass walls. There are many green plants inside the greenhouse on wooden shelves and in flower pots.

A greenhouse is generally a glass building filled with plants. Sunlight is able to enter into the greenhouse through the windows, and the heat from that light is absorbed by the plants. The plants will re-emit light. Because plants are relatively cool, they emit heat as infrared light. As seen in the infrared photo in Figure 16.6, infrared cannot transmit through glass. Instead, it gets reflected off of glass surfaces. This means that all of the heat from the plants is trapped inside the greenhouse, causing the greenhouse to stay warm even if the outside is cold.

In the video below, two otherwise identical transparent plastic bottles have been placed at equal distances from a heat lamp. One of the plastic bottles has been filled with carbon dioxide, a greenhouse gas. As the heat lamp shines on both of the bottles, the temperature recorded by the computer shows that the bottle filled with carbon dioxide warms up much faster.

The temperature data recorded during this experiment is graphed in Figure 16.8. ( Download this data [XLSX, 39 kB] )

A graph with temperature (degrees C) on the y-axis and time (min) on the x-axis. Hundreds of data points are shown for two cases: a temperature probe placed in a clear plastic soda bottle filled with air and a temperature probe placed in a clear plastic soda bottle filled with air and carbon dioxide. The temperature from the bottle without CO2 rises from 22 C to 44 C over 10 minutes. The temperature from the bottle with CO2 rises from 22 C to 47 C over 10 minutes.

On our planet, the presence of greenhouse gases causes the re-radiation of electromagnetic radiation that would otherwise be lost if there were no greenhouse gases. This is shown in Figure 16.9.

A graphic of the sun and Earth. There are arrows depicting rays of light from the sun. Some of the rays reflect back into space. The graphic states: "some sunlight that hits Earth is reflected back into space, while the rest becomes heat." Some of the rays are reflected by the Earth's atmosphere and directed back to the Earth's surface. The graphic states: "Greenhouse gases absorb and redirect heat radiated by Earth, insulating it from heat loss to space."

Globally, the greenhouse effect is an issue for our planet as our atmosphere contains a lot of carbon dioxide and other greenhouse gases that have accumulated in large quantities due to human activities. As our planet continues to warm up, this becomes a bigger and bigger problem.

To a certain extent, the fact that our planet is capable of trapping heat makes it possible for our planet to sustain life. However, when the atmosphere heats up to dangerous levels, we see catastrophic weather events. This is an area where technology and policy need to come together to create lasting change that will protect our planet and its inhabitants.

Engineering heat transfer

An insulated beverage container is a great example of how we can use the knowledge of heat transfer to engineer a container that can keep hot things hot and cold things cold. Insulated beverage containers are made from a double-walled material. Inside the two walls is a vacuum. The inside of the beverage container is silver in color. To keep things hot, or cold, we put a lid on top. (This is depicted in Figure 16.10.) How do all of those features keep our hot things hot and our cold things cold?

A graphic of a vacuum flask containing two walls insulated by a vacuum, and a stopper as a lid.

First, let’s answer the question of “how does an insulated beverage container know if something is hot, and should keep it hot, or if something is cold, and should keep it cold?” The answer is that those two concepts are identical. To keep something hot, we want to prevent heat from leaving the object. To keep something cold, we want to prevent heat from entering the object. Either way, we’re eliminating or reducing all opportunities for heat transfer to occur.

To eliminate or reduce heat conduction, the vacuum in the beverage container’s walls prevents heat from entering or leaving through the walls of the container. Gas in general is a bad heat conductor, and vacuum is incapable of conducting heat, as there are no molecules to collide in order to move heat around. The container lid is usually made of a heat insulator such as plastic or cork.

To eliminate or reduce convection, the vacuum between the beverage container’s walls is also incapable of convecting. Because there are no molecules in a vacuum, they cannot distribute heat by moving around. By placing a lid on the container, we are eliminating convection in the vertical direction by trapping heat inside the container.

Finally, the silver color of the interior of the beverage container greatly reduces heat transfer by electromagnetic radiation. Any heat inside the beverage container is going to reflect off the walls and stay inside. Any heat outside of the container is going to reflect off the walls and stay outside.

In the video below, Dr. Pasquale has placed hot water (90 degrees Celsius) into two different containers: a double-walled calorimeter with a lid, and a metal can (the same kind used on the inside of the calorimeter) without a lid. The calorimeter acts the most like an insulated beverage container. There are two silver cups surrounded by air (not a good heat conductor), and the plastic and cork lid prevents convection.

The data from this experiment is graphed in Figure 16.11. ( Download this data [XLSX, 37 kB] )

A graph with temperature (degrees C) on the y-axis and time (min) on the x-axis. Hundreds of data points are shown for two cases: a temperature probe placed in hot water in a calorimeter and a temperature probe placed in a hot water in a non-insulated cup. The temperature from calorimeter decreases from 90 C to 80 C over 10 minutes. The temperature from the non-insulated cup decreases from 90 C to 68 C over 10 minutes.

After placing the hot water into both containers, Dr. Pasquale recorded the temperature changes as they cooled down. Over a span of ten minutes, the double-walled calorimeter held heat much better than the open metal can did. Much of the heat in the metal can was lost due to convection. Even placing a lid on a pot, pan, or your favorite coffee mug will help keep your food and beverages warmer for a longer time.

Solar power

The Earth is constantly receiving energy from the Sun. Solar power can be used to generate electricity for use in our homes using a photovoltaic panel, known as a solar panel (Figure 16.12).

A photograph of multiple solar panels on the rooftop of a building.

In the video below, a solar panel is connected to a light-emitting diode (LED). When no light is shining onto the solar panel, there is no electricity generated, and the LED remains off. Once Dr. Pasquale shines a flashlight onto the solar panel, electricity is generated and the LED turns on. This is a similar process to what happens when sunlight shines onto solar panels connected to a home or to the power grid.

The rate at which we obtain that energy is known as power: solar power. On average, the Sun transmits 1,400 J of energy every second to every square meter of the Earth that it hits. In other words, we receive 1,400 W/m 2 . This value is known as the solar constant .

If we consume a certain amount of power, we can use that information to determine how large of a solar panel we would need to generate that amount of electricity. To obtain enough power from solar panels, our power needs cannot exceed the power we receive from the Sun. In equation form, this can be stated as

$$A_{minimum} = \frac{P_{home}}{1,400~\textrm{W/m}^2},$$

The amount of power we need depends on how much we run our appliances. On average, a household in the United States consumes 1,250 W of power in one day. If there are solar panels capable of taking the entire 1,400 W/m 2 of power from the Sun to convert to usable electricity, then solar panels that are

\begin{align*} A &= \frac{1250~\textrm{W}}{1400~\textrm{W/m}^2}\\ &= 0.89~\textrm{m}^2 \end{align*}

would be sufficient to power that household. That size is not very big!

However, solar panels aren’t necessarily 100% efficient. A good solar panel is at best 15% efficient as of the time this textbook was written. How does this affect the size of the solar panels needed to power the average household? The solar constant of 1,400 W/m 2 is now only converted at 15%, meaning that the solar panels will only create 210 W/m 2 of power. In this case, the size of the solar panels required will be

\begin{align*} A &= \frac{1250~\textrm{W}}{210~\textrm{W/m}^2}\\ &= 5.95~\textrm{m}^2 \end{align*}

This is a big difference from perfectly efficient solar panels! Making solar panels more efficient is a big topic in engineering and physics research. More efficient solar panels will make renewable energy much more competitive with fossil fuels, and more convenient.

Another factor we must consider, if we do not live at the equator, is that the amount of energy we receive from the Sun is going to change throughout the seasons. This is because the Sun does not necessarily hit all parts of the surface of the Earth at a direct 90 degree angle. The equator may receive that 90 degree light, which means that area will receive more solar power than areas of the Earth that are tilted away from the Sun.

In the northern hemisphere in summer, the Earth’s axis is tilted toward the Sun, providing us with more direct light. We therefore obtain more energy from the Sun, and the northern hemisphere climate warms up, giving us hot summers. In the winter, the Earth’s axis is tilted away from the Sun, providing us with less direct light. We obtain less energy from the Sun per square meter, and the northern hemisphere climate cools down, giving us cold winters.

A graphic depicting the seasonal variations in the northern and southern hemispheres due to the Earth’s tilt relative to its orbit around the Sun is shown in Figure 16.13.

A graphic depicting the Earth in relation to the sun during each of four seasons. The axial tilt of the Earth at each point contributes to the season experinced on each of the north or south hemispheres.

Newton’s law of cooling

\Delta T

That means when there’s a big temperature difference between an object and its surroundings, we will expect it to cool down quickly. But as it cools down, the temperature difference between the object and the surroundings decreases, causing the rate of cooling to decrease as well. This continues to decrease until the temperature of the object asymptotically reaches the temperature of the surroundings.

This type of relationship is known as a differential equation and is described by a function known as an exponential function. The mathematics behind this function is beyond the scope of this textbook. However, we will use the basics of Newton’s law of cooling to understand and interpret a graph of the cooling of an object.

In an experimental demonstration, Dr. Pasquale took water at an initial temperature of 94 o C and recorded the temperature change over time as it cooled down. The temperature of the room was also recorded at a constant value of 22.5 o C. The temperature recordings were measured for ten minutes.

After the experiment was completed, Dr. Pasquale used Logger Pro software to calculate the exponential equation that represented the data. This was used to extrapolate the data to see what trend would occur if the experiment had been performed for an entire hour. The experimental data and exponential extrapolation are shown in a graph in Figure 16.14. ( Download this data [XLSX, 43 kB] )

A graph of temperature (degrees C) on the y-axis and time (minutes) on the x-axis. Hundreds of data points from a temperature probe in hot water decrease exponentially from 94 C to 70 C over 10 minutes. The temperature of the room is recorded at a constant 22 C over 10 minutes. An exponential fit of the hot water data is shown extrapolated out for 60 minutes.

At the very beginning of the experiment, the temperature of the water was 94 degrees and the temperature of the room was 22.5 o C. The difference in temperature was 71.5 o C. At that time, the rate of cooling was 2.9 o C/min. That is, the slope of the line at that instant in time says that the water will cool off at that rate. Because the function isn’t a straight line, the value of the slope is constantly changing, but the value of 2.9 °C/min is valid at this particular data point.

At 7.2 minutes into the experiment, the water had cooled down to 76.1 o C. At that point the temperature difference is 53.6 o C, three quarters of the initial value. What we should expect is that the slope of the line at this point will also be three quarters of 2.9 o C/min. According to the extrapolation, it is 2.1 o C/min, which is in very close agreement with that expectation.

We can look at a few more points and see that this trend is satisfied for all of the data calculated in this experiment. 17.3 minutes in, we would expect the temperature of the water to be 58.3 o C and the difference in temperature to be 35.8 o C, one half the original value. Now the slope of the line at this point would be about one half the original value, 1.4 o C/min.

Finally, 34.6 minutes into the experiment, the temperature of the water should be 40.4 o C and the temperature difference should be 17.9 o C, one quarter of the initial value. We would expect the slope to be one quarter the initial slope: 0.7 o C/min.

The slope continues to decrease over time. That is to say, as the water gets closer to room temperature, it cools down slower and slower, until eventually it asymptotically reaches the temperature of the room and is in thermal equilibrium.

Further reading

  • Solar power – This page from the International Energy Agency gives information about solar power and its importance in reducing the effects of global climate change.

Practice questions

Numerical analysis.

  • How large would perfectly efficient solar panels need to be to power a business using 8,500 W of electrical power?
  • If solar panels are only 15% efficient, calculate the size of the solar panels required for the business using 8,500 W of electrical power.
  • A household has 1 m 2 of solar panels, generating 150 W of electrical power. How efficient are the solar panels?
  • …22.5 o C?

Conduction is heat transfer that occurs due to collisions among atoms, electrons, or molecules.

Heat is energy that is transferred from one object to another in response to a difference in temperature. (symbol: Q, unit: cal)

Temperature defines the average kinetic energy of an object. It quantifies the “hotness” or “coldness” of something. (symbol: T, unit: °C or K)

Energy is defined as the capability of an object (or collection of objects) to do useful work. (symbol: E, unit: J)

A solid is a substance where the molecules or atoms are very tightly bound together. This gives a solid a very rigid volume and shape. Solid is one of the four most common phases of matter.

A liquid is a state of matter in which the constituent molecules will change their shape or arrangement but cannot be easily compressed to change their volume. Liquid is one of the four most common phases of matter.

Gas is a state of matter where molecules are very free to move about and generally do not interact with each other except during collisions. This means that the shape and volume of a gas is free to change. Gas is one of the four most common phases of matter.

Conduction is the motion of electrical charge within a material or between different materials.

An electron is a fundamental building block of matter that has a negative charge and is found surrounding the nucleus of an atom.

Specific heat capacity defines how difficult it is to change the temperature of a substance. It describes the amount of heat required to change the temperature of a certain mass of a substance by a certain temperature. (symbol: c, unit: cal/(g°C))

Convection is heat transfer that comes about due to the motion of molecules themselves.

Electromagnetic radiation is heat that is transferred from one place to another through light waves.

The wavelength of a wave describes the shortest distance between two identical repeating points on a wave. (symbol: λ, unit: m)

Absorption of light occurs when light waves completely transfer their energy to a medium as they attempt to pass through it. The light is therefore blocked from transmitting through the medium.

Physics is a branch of science that focuses on the fundamentals of the workings of our universe.

An electromagnetic wave is a transverse wave that contains electric and magnetic fields oscillating at right angles to each other.

An object is transparent if it allows visible light to pass through without getting scattered or absorbed.

An object is opaque if it does not allow any light to transmit through it.

Technology is the outcome of using scientific principles to solve problems. This can be a product, innovation, or other object whose creation is based on science such as physics, chemistry, or biology.

Power quantifies how quickly work is done. (symbol: P, unit: W)

A diode is a circuit element that acts like a one-way valve in that it only allows current to flow in one direction but not the other.

The solar constant is the rate at which we obtain energy from the sun. The solar constant is approximately 1,400 W/m^2.

The orbit of a satellite describes its path around a planet or star.

Conceptual Physics Copyright © 2024 by Alyssa J. Pasquale, Ph.D.; David R. Fazzini, Ph.D.; and Carley Bennett, Ph.D. is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Introduction to Heat Transfer: How Does Heat Transfer?

What Heat Transfer Is and How Heat Moves From One Body to Another

  • Thermodynamics
  • Physics Laws, Concepts, and Principles
  • Quantum Physics
  • Important Physicists
  • Cosmology & Astrophysics
  • Weather & Climate

transfer of heat physics

  • M.S., Mathematics Education, Indiana University
  • B.A., Physics, Wabash College

What is heat? How does heat transfer take place? What are the effects on matter when heat transfers from one body to another? Here's what you need to know:

Heat Transfer Definition

Heat transfer is a process by which internal energy from one substance transfers to another substance. Thermodynamics is the study of heat transfer and the changes that result from it. An understanding of heat transfer is crucial to analyzing a thermodynamic process , such as those that take place in heat engines and heat pumps.

Forms of Heat Transfer

Under the kinetic theory, the internal energy of a substance is generated from the motion of individual atoms or molecules. Heat energy is the form of energy which transfers this energy from one body or system to another. This heat transfer can take place in a number of ways:

  • Conduction is when heat flows through a heated solid through a heat current moving through the material. You can observe conduction when heating a stove burner element or a bar of metal, which goes from red hot to white hot.
  • Convection is when heated particles transfer heat to another substance, such as cooking something in boiling water.
  • Radiation is when heat is transferred through electromagnetic waves, such as from the sun. Radiation can transfer heat through empty space, while the other two methods require some form of matter-on-matter contact for the transfer.

In order for two substances to affect each other, they must be in thermal contact with each other. If you leave your oven open while turned on and stand several feet in front of it, you are in thermal contact with the oven and can feel the heat it transfers to you (by convection through the air).

Normally, of course, you do not feel the heat from the oven when you're several feet away and that is because the oven has thermal insulation to keep the heat inside of it, thus preventing thermal contact with the outside of the oven. This is of course not perfect, so if you stand nearby you do feel some heat from the oven.

Thermal equilibrium is when two items that are in thermal contact no longer transfer heat between them.

Effects of Heat Transfer

The basic effect of heat transfer is that the particles of one substance collide with the particles of another substance. The more energetic substance will typically lose internal energy (i.e. "cool down") while the less energetic substance will gain internal energy (i.e. "heat up").

The most blatant effect of this in our day-to-day life is a phase transition, where a substance changes from one state of matter to another, such as ice melting from a solid to a liquid as it absorbs heat. The water contains more internal energy (i.e. the water molecules are moving around faster) than in the ice.

In addition, many substances go through either thermal expansion or thermal contraction as they gain and lose internal energy. Water (and other liquids) often expands as it freezes, which anyone who has put a drink with a cap in the freezer for too long has discovered.

Heat Capacity

The heat capacity of an object helps define how that object's temperature responds to absorbing or transmitting heat. Heat capacity is defined as the change in heat divided by the change in temperature.

  • Laws of Thermodynamics

Heat transfer is guided by some basic principles which have become known as the laws of thermodynamics , which define how heat transfer relates to work done by a system and place some limitations on what it is possible for a system to achieve.

Edited by Anne Marie Helmenstine, Ph.D.

  • An Overview of Thermodynamics
  • A Scientific Way to Define Heat Energy
  • What Is a Thermodynamic Process?
  • What Is Conduction?
  • Definition and Examples of Latent Heat
  • What Is the Zeroth Law of Thermodynamics?
  • What Is Isobaric Process?
  • The Isochoric Process
  • Thermodynamics: Adiabatic Process
  • What Is an Isothermal Process in Physics?
  • Understanding Calorimetry to Measure Heat Transfer
  • Calculating Heat Current
  • Enthalpy Definition in Chemistry and Physics
  • Convection and Weather
  • What Is Entropy and How to Calculate It

11.2 Heat, Specific Heat, and Heat Transfer

Section learning objectives.

By the end of this section, you will be able to do the following:

  • Explain heat, heat capacity, and specific heat
  • Distinguish between conduction, convection, and radiation
  • Solve problems involving specific heat and heat transfer

Teacher Support

The learning objectives in this section will help your students master the following standards:

  • (F) contrast and give examples of different processes of thermal energy transfer, including conduction, convection, and radiation.

Section Key Terms

[BL] [OL] [AL] Review concepts of heat, temperature, and mass.

[AL] Check prior knowledge of conduction and convection.

Heat Transfer, Specific Heat, and Heat Capacity

We learned in the previous section that temperature is proportional to the average kinetic energy of atoms and molecules in a substance, and that the average internal kinetic energy of a substance is higher when the substance’s temperature is higher.

If two objects at different temperatures are brought in contact with each other, energy is transferred from the hotter object (that is, the object with the greater temperature) to the colder (lower temperature) object, until both objects are at the same temperature. There is no net heat transfer once the temperatures are equal because the amount of heat transferred from one object to the other is the same as the amount of heat returned. One of the major effects of heat transfer is temperature change: Heating increases the temperature while cooling decreases it. Experiments show that the heat transferred to or from a substance depends on three factors—the change in the substance’s temperature, the mass of the substance, and certain physical properties related to the phase of the substance.

The equation for heat transfer Q is

where m is the mass of the substance and Δ T is the change in its temperature, in units of Celsius or Kelvin. The symbol c stands for specific heat , and depends on the material and phase. The specific heat is the amount of heat necessary to change the temperature of 1.00 kg of mass by 1.00 ºC. The specific heat c is a property of the substance; its SI unit is J/(kg ⋅ ⋅ K) or J/(kg ⋅ ⋅ °C °C ). The temperature change ( Δ T Δ T ) is the same in units of kelvins and degrees Celsius (but not degrees Fahrenheit). Specific heat is closely related to the concept of heat capacity . Heat capacity is the amount of heat necessary to change the temperature of a substance by 1.00 °C °C . In equation form, heat capacity C is C = m c C = m c , where m is mass and c is specific heat. Note that heat capacity is the same as specific heat, but without any dependence on mass. Consequently, two objects made up of the same material but with different masses will have different heat capacities. This is because the heat capacity is a property of an object, but specific heat is a property of any object made of the same material.

Values of specific heat must be looked up in tables, because there is no simple way to calculate them. Table 11.2 gives the values of specific heat for a few substances as a handy reference. We see from this table that the specific heat of water is five times that of glass, which means that it takes five times as much heat to raise the temperature of 1 kg of water than to raise the temperature of 1 kg of glass by the same number of degrees.

[BL] [OL] [AL] Explain that this formula only works when there is no change in phase of the substance. The transfer of thermal energy, heat, and phase change will be covered later in the chapter.

Misconception Alert

The units of specific heat are J/(kg ⋅ °C ⋅ °C ) and J/(kg ⋅ ⋅ K). However, degrees Celsius and Kelvins are not always interchangeable. The formula for specific heat uses a difference in temperature and not absolute temperature. This is the reason that degrees Celsius may be used in place of Kelvins.

Temperature Change of Land and Water

What heats faster, land or water? You will answer this question by taking measurements to study differences in specific heat capacity.

  • Open flame—Tie back all loose hair and clothing before igniting an open flame. Follow all of your teacher's instructions on how to ignite the flame. Never leave an open flame unattended. Know the location of fire safety equipment in the laboratory.
  • Sand or soil
  • Oven or heat lamp
  • Two small jars
  • Two thermometers

Instructions

  • Place equal masses of dry sand (or soil) and water at the same temperature into two small jars. (The average density of soil or sand is about 1.6 times that of water, so you can get equal masses by using 50 percent more water by volume.)
  • Heat both substances (using an oven or a heat lamp) for the same amount of time.
  • Record the final temperatures of the two masses.
  • Now bring both jars to the same temperature by heating for a longer period of time.
  • Remove the jars from the heat source and measure their temperature every 5 minutes for about 30 minutes.
  • The pond will reach 0 °C first because of water’s greater specific heat.
  • The field will reach 0 °C first because of soil’s lower specific heat.
  • They will reach 0° C at the same time because they are exposed to the same weather.
  • The water will take longer to heat as well as to cool. This tells us that the specific heat of water is greater than that of land.

Conduction, Convection, and Radiation

Whenever there is a temperature difference, heat transfer occurs. Heat transfer may happen rapidly, such as through a cooking pan, or slowly, such as through the walls of an insulated cooler.

There are three different heat transfer methods: conduction , convection , and radiation . At times, all three may happen simultaneously. See Figure 11.3 .

Conduction is heat transfer through direct physical contact. Heat transferred between the electric burner of a stove and the bottom of a pan is transferred by conduction. Sometimes, we try to control the conduction of heat to make ourselves more comfortable. Since the rate of heat transfer is different for different materials, we choose fabrics, such as a thick wool sweater, that slow down the transfer of heat away from our bodies in winter.

As you walk barefoot across the living room carpet, your feet feel relatively comfortable…until you step onto the kitchen’s tile floor. Since the carpet and tile floor are both at the same temperature, why does one feel colder than the other? This is explained by different rates of heat transfer: The tile material removes heat from your skin at a greater rate than the carpeting, which makes it feel colder.

[BL] [OL] [AL] Ask students what the current temperature in the classroom is. Ask them if all the objects in the room are at the same temperature. Once this is established, ask them to place their hand on their desk or on a metal object. Does it feel colder? Why? If their desk is Formica laminate, then it will feel cool to their hand because the laminate is a good conductor of heat and draws heat from their hand creating a sensation of “cold” due to heat leaving the body.

Some materials simply conduct thermal energy faster than others. In general, metals (like copper, aluminum, gold, and silver) are good heat conductors, whereas materials like wood, plastic, and rubber are poor heat conductors.

Figure 11.4 shows particles (either atoms or molecules) in two bodies at different temperatures. The (average) kinetic energy of a particle in the hot body is higher than in the colder body. If two particles collide, energy transfers from the particle with greater kinetic energy to the particle with less kinetic energy. When two bodies are in contact, many particle collisions occur, resulting in a net flux of heat from the higher-temperature body to the lower-temperature body. The heat flux depends on the temperature difference Δ T = T hot − T cold Δ T = T hot − T cold . Therefore, you will get a more severe burn from boiling water than from hot tap water.

Convection is heat transfer by the movement of a fluid. This type of heat transfer happens, for example, in a pot boiling on the stove, or in thunderstorms, where hot air rises up to the base of the clouds.

Tips For Success

In everyday language, the term fluid is usually taken to mean liquid. For example, when you are sick and the doctor tells you to “push fluids,” that only means to drink more beverages—not to breath more air. However, in physics, fluid means a liquid or a gas . Fluids move differently than solid material, and even have their own branch of physics, known as fluid dynamics , that studies how they move.

As the temperature of fluids increase, they expand and become less dense. For example, Figure 11.4 could represent the wall of a balloon with different temperature gases inside the balloon than outside in the environment. The hotter and thus faster moving gas particles inside the balloon strike the surface with more force than the cooler air outside, causing the balloon to expand. This decrease in density relative to its environment creates buoyancy (the tendency to rise). Convection is driven by buoyancy—hot air rises because it is less dense than the surrounding air.

Sometimes, we control the temperature of our homes or ourselves by controlling air movement. Sealing leaks around doors with weather stripping keeps out the cold wind in winter. The house in Figure 11.5 and the pot of water on the stove in Figure 11.6 are both examples of convection and buoyancy by human design. Ocean currents and large-scale atmospheric circulation transfer energy from one part of the globe to another, and are examples of natural convection.

Radiation is a form of heat transfer that occurs when electromagnetic radiation is emitted or absorbed. Electromagnetic radiation includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays, all of which have different wavelengths and amounts of energy (shorter wavelengths have higher frequency and more energy).

[BL] [OL] Electromagnetic waves are also often referred to as EM waves. We perceive EM waves of different frequencies differently. Just as we are able to see certain frequencies as visible light, we perceive certain others as heat.

You can feel the heat transfer from a fire and from the sun. Similarly, you can sometimes tell that the oven is hot without touching its door or looking inside—it may just warm you as you walk by. Another example is thermal radiation from the human body; people are constantly emitting infrared radiation, which is not visible to the human eye, but is felt as heat.

Radiation is the only method of heat transfer where no medium is required, meaning that the heat doesn’t need to come into direct contact with or be transported by any matter. The space between Earth and the sun is largely empty, without any possibility of heat transfer by convection or conduction. Instead, heat is transferred by radiation, and Earth is warmed as it absorbs electromagnetic radiation emitted by the sun.

All objects absorb and emit electromagnetic radiation (see Figure 11.7 ). The rate of heat transfer by radiation depends mainly on the color of the object. Black is the most effective absorber and radiator, and white is the least effective. People living in hot climates generally avoid wearing black clothing, for instance. Similarly, black asphalt in a parking lot will be hotter than adjacent patches of grass on a summer day, because black absorbs better than green. The reverse is also true—black radiates better than green. On a clear summer night, the black asphalt will be colder than the green patch of grass, because black radiates energy faster than green. In contrast, white is a poor absorber and also a poor radiator. A white object reflects nearly all radiation, like a mirror.

Ask students to give examples of conduction, convection, and radiation.

Virtual Physics

Energy forms and changes.

In this animation, you will explore heat transfer with different materials. Experiment with heating and cooling the iron, brick, and water. This is done by dragging and dropping the object onto the pedestal and then holding the lever either to Heat or Cool. Drag a thermometer beside each object to measure its temperature—you can watch how quickly it heats or cools in real time.

Now let’s try transferring heat between objects. Heat the brick and then place it in the cool water. Now heat the brick again, but then place it on top of the iron. What do you notice?

Selecting the fast forward option lets you speed up the heat transfers, to save time.

  • Water will take the longest, and iron will take the shortest time to heat, as well as to cool. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific heat would prevent heat loss from the body.
  • Water will take the shortest, and iron will take the longest time to heat, as well as to cool. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific heat would prevent heat loss from the body.
  • Brick will take shortest and iron will take longest time to heat up as well as to cool down. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific heat would prevent heat loss from the body.
  • Water will take shortest and brick will take longest time to heat up as well as to cool down. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific heat would prevent heat loss from the body.

Have students consider the differences in the interactive exercise results if different materials were used. For example, ask them whether the temperature change would be greater or smaller if the brick were replaced with a block of iron with the same mass as the brick. Ask students to consider identical masses of the metals aluminum, gold, and copper. After they have stated whether the temperature change is greater or less for each metal, have them refer to Table 11.2 and check whether their predictions were correct.

Solving Heat Transfer Problems

Worked example, calculating the required heat: heating water in an aluminum pan.

A 0.500 kg aluminum pan on a stove is used to heat 0.250 L of water from 20.0 °C °C to 80.0 °C °C . (a) How much heat is required? What percentage of the heat is used to raise the temperature of (b) the pan and (c) the water?

The pan and the water are always at the same temperature. When you put the pan on the stove, the temperature of the water and the pan is increased by the same amount. We use the equation for heat transfer for the given temperature change and masses of water and aluminum. The specific heat values for water and aluminum are given in the previous table.

Because the water is in thermal contact with the aluminum, the pan and the water are at the same temperature.

  • Calculate the temperature difference. Δ T = T f − T i = 60.0 °C Δ T = T f − T i = 60.0 °C 11.8
  • Calculate the mass of water using the relationship between density, mass, and volume. Density is mass per unit volume, or ρ = m V ρ = m V . Rearranging this equation, solve for the mass of water. m w = ρ ⋅ V = 1000  kg/m 3 × ( 0 .250 L× 0 .001 m 3 1 L ) =0 .250 kg m w = ρ ⋅ V = 1000  kg/m 3 × ( 0 .250 L× 0 .001 m 3 1 L ) =0 .250 kg 11.9
  • Calculate the heat transferred to the water. Use the specific heat of water in the previous table. Q w = m w c w Δ T =   ( 0.250  kg ) ( 4186 J/kg °C ) ( 60 .0 °C )  = 62 .8 kJ Q w = m w c w Δ T =   ( 0.250  kg ) ( 4186 J/kg °C ) ( 60 .0 °C )  = 62 .8 kJ 11.10
  • Calculate the heat transferred to the aluminum. Use the specific heat for aluminum in the previous table. Q A l = m A l c A l Δ T =   ( 0.500  kg ) ( 900 J/kg °C ) ( 60 .0 °C )  = 27 .0 ×10 3 J = 27 .0 kJ Q A l = m A l c A l Δ T =   ( 0.500  kg ) ( 900 J/kg °C ) ( 60 .0 °C )  = 27 .0 ×10 3 J = 27 .0 kJ 11.11
  • Find the total transferred heat. Q T o t a l = Q w + Q A l = 62 .8 kJ + 27 .0 kJ = 89 .8 kJ Q T o t a l = Q w + Q A l = 62 .8 kJ + 27 .0 kJ = 89 .8 kJ 11.12

The percentage of heat going into heating the pan is

The percentage of heat going into heating the water is

In this example, most of the total heat transferred is used to heat the water, even though the pan has twice as much mass. This is because the specific heat of water is over four times greater than the specific heat of aluminum. Therefore, it takes a bit more than twice as much heat to achieve the given temperature change for the water than for the aluminum pan.

Water can absorb a tremendous amount of energy with very little resulting temperature change. This property of water allows for life on Earth because it stabilizes temperatures. Other planets are less habitable because wild temperature swings make for a harsh environment. You may have noticed that climates closer to large bodies of water, such as oceans, are milder than climates landlocked in the middle of a large continent. This is due to the climate-moderating effect of water’s large heat capacity—water stores large amounts of heat during hot weather and releases heat gradually when it’s cold outside.

Calculating Temperature Increase: Truck Brakes Overheat on Downhill Runs

When a truck headed downhill brakes, the brakes must do work to convert the gravitational potential energy of the truck to internal energy of the brakes. This conversion prevents the gravitational potential energy from being converted into kinetic energy of the truck, and keeps the truck from speeding up and losing control. The increased internal energy of the brakes raises their temperature. When the hill is especially steep, the temperature increase may happen too quickly and cause the brakes to overheat.

Calculate the temperature increase of 100 kg of brake material with an average specific heat of 800 J/kg ⋅ °C ⋅ °C from a 10,000 kg truck descending 75.0 m (in vertical displacement) at a constant speed.

We first calculate the gravitational potential energy ( Mgh ) of the truck, and then find the temperature increase produced in the brakes.

  • Calculate the change in gravitational potential energy as the truck goes downhill. M g h = ( 10 , 000  kg ) (9 .80 m/s 2 ) ( 75 .0 m ) = 7.35 × 10 6 J M g h = ( 10 , 000  kg ) (9 .80 m/s 2 ) ( 75 .0 m ) = 7.35 × 10 6 J 11.15

where m is the mass of the brake material (not the entire truck). Insert the values Q = 7.35×10 6 J (since the heat transfer is equal to the change in gravitational potential energy), m = = 100 kg, and c = = 800 J/kg ⋅ ⋅ °C °C to find

This temperature is close to the boiling point of water. If the truck had been traveling for some time, then just before the descent, the brake temperature would likely be higher than the ambient temperature. The temperature increase in the descent would likely raise the temperature of the brake material above the boiling point of water, which would be hard on the brakes. This is why truck drivers sometimes use a different technique for called “engine braking” to avoid burning their brakes during steep descents. Engine braking is using the slowing forces of an engine in low gear rather than brakes to slow down.

Practice Problems

How much heat does it take to raise the temperature of 10.0 kg of water by 1.0 °C ?

Calculate the change in temperature of 1.0 kg of water that is initially at room temperature if 3.0 kJ of heat is added.

Check Your Understanding

Use these questions to assess student achievement of the section’s learning objectives. If students are struggling with a specific objective, these questions will help identify which and direct students to the relevant content.

  • The mass difference between two objects causes heat transfer.
  • The density difference between two objects causes heat transfer.
  • The temperature difference between two systems causes heat transfer.
  • The pressure difference between two objects causes heat transfer.
  • The overall direction of heat transfer is from the higher-temperature object to the lower-temperature object.
  • The overall direction of heat transfer is from the lower-temperature object to the higher-temperature object.
  • The direction of heat transfer is first from the lower-temperature object to the higher-temperature object, then back again to the lower-temperature object, and so-forth, until the objects are in thermal equilibrium.
  • The direction of heat transfer is first from the higher-temperature object to the lower-temperature object, then back again to the higher-temperature object, and so-forth, until the objects are in thermal equilibrium.
  • conduction, radiation, and reflection
  • conduction, reflection, and convection
  • convection, radiation, and reflection
  • conduction, radiation, and convection

True or false—Conduction and convection cannot happen simultaneously

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  • Heat Introduction Classification
  • Heat Transfer Conduction Convection And Radiation

Heat Transfer - Radiation, Convection And Conduction

Any matter which is made up of atoms and molecules has the ability to transfer heat. The atoms are in different types of motion at any time. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy. The more the motion of molecules, more will be the heat energy. However, talking about heat transfer, it is nothing but the process of transfer of heat from a high-temperature body to a low temperature one.

What is Heat Transfer?

According to thermodynamic systems, heat transfer is defined as

“The movement of heat across the border of the system due to a difference in temperature between the system and its surroundings.”

Interestingly, the difference in temperature is said to be a ‘potential’ that causes the transfer of heat from one point to another.

transfer of heat physics

How is Heat Transferred?

Heat can travel from one place to another in several ways . The different modes of heat transfer include :

Meanwhile, if the temperature difference exists between the two systems, heat will find a way to transfer from the higher to the lower system.

Convention,Conduction, Radiation

What is Conduction?

Conduction is defined as

The process of transmission of energy from one particle of the medium to another with the particles being in direct contact with each other.

An area of higher kinetic energy transfers thermal energy towards the lower kinetic energy area. High-speed particles clash with particles moving at a slow speed, as a result, slow speed particles increase their kinetic energy . This is a typical form of heat transfer and takes place through physical contact. Conduction is also known as thermal conduction or heat conduction.

Conduction Equation

The rate of conduction can be calculated by the following equation:

  • Q is the transfer of heat per unit time
  • K is the thermal conductivity of the body
  • A is the area of heat transfer
  • T hot is the temperature of the hot region
  • T cold is the temperature of the cold region
  • d is the thickness of the body

The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction.

Conduction Examples

Following are the examples of conduction:

  • Ironing of clothes is an example of conduction where the heat is conducted from the iron to the clothes.
  • Heat is transferred from hands to ice cube resulting in the melting of an ice cube when held in hands.
  • Heat conduction through the sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.

What is Convection?

Convection is defined as

The movement of fluid molecules from higher temperature regions to lower temperature regions.

Convection Equation

As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows:

  • Q is the heat transferred per unit time
  • h c is the coefficient of convective heat transfer
  • T s is the surface temperature
  • T f is the fluid temperature

Convection Examples

Examples of convection include:

  • Boiling of water, that is molecules that are denser move at the bottom while the molecules which are less dense move upwards resulting in the circular motion of the molecules so that water gets heated.
  • Warm water around the equator moves towards the poles while cooler water at the poles moves towards the equator.
  • Blood circulation in warm-blooded animals takes place with the help of convection, thereby regulating the body temperature.

Learn more about Convection

transfer of heat physics

What is Radiation?

Radiant heat is present in some or other form in our daily lives. Thermal radiations are referred to as radiant heat. Thermal radiation is generated by the emission of electromagnetic waves . These waves carry away the energy from the emitting body. Radiation takes place through a vacuum or transparent medium which can be either solid or liquid. Thermal radiation is the result of the random motion of molecules in matter. The movement of charged electrons and protons is responsible for the emission of electromagnetic radiation. Let us know more about radiation heat transfer.

Radiation heat transfer is measured by a device known as thermocouple. A thermocouple is used for measuring the temperature. In this device sometimes, error takes place while measuring the temperature through radiation heat transfer.

Radiation Equation

As temperature rises, the wavelength in the spectra of the radiation emitted decreases and shorter wavelengths radiations are emitted. Thermal radiation can be calculated by Stefan-Boltzmann law:

  • P is the net power of radiation
  • A is the area of radiation
  • Tr is the radiator temperature
  • Tc is the surrounding temperature
  • e is emissivity and σ is Stefan’s constant (σ = 5.67 × 10 -8 Wm -2 K -4

Radiation Example

Following are the examples of radiation:

  • Microwave radiation emitted in the oven is an example of radiation.
  • UV rays coming from the sun is an example of radiation.
  • The release of alpha particles during the decaying of Uranium-238 into Thorium-234 is an example of radiation.

Unit of Heat Transfer

To know more about heat transfer in detail, click on the video below.

transfer of heat physics

Frequently Asked Questions – FAQs

What are the different modes of heat transfer.

The different modes of heat transfer are:

Give an example of radiation.

What is the si unit of heat.

SI unit of heat is Joules.

How is electromagnetic radiation emitted?

What is the movement of molecules in fluids from higher temperature regions to lower temperature regions known as.

It is known as convection.

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5 hacks to heat your home that actually work

I ’m afraid it’s happened: we’re now regularly seeing single-digit temperatures, and it’s only going to get colder as winter approaches.

Given that we’re in a cost-of-living crisis , a lot of people are thinking of ways to keep warm at home without constantly having their central heating on full blast .

One way to think about home heating is to go back to some basic physics that you’ve probably forgotten you learned at school.

Specifically, we need to consider the three kinds of heat transfer: convection, conduction and radiation.

Hack: Draught excluders 

Getting some draught excluders and filling any gaps, for example where pipes enter rooms, is an obvious, good idea. 

The science 

It’s all about convection, the form of heat transfer that’s most commonly used to heat our home.

We use central heating to warm the air with radiators, creating a cycle where the less dense warm air expands and rises, and cooler, denser air contracts and sinks.

If you happen to live in an older flat or house with high ceilings, it’s obvious why convection isn’t an efficient way to heat your home (unless you happen to be extremely tall, I suppose): the warm air rises uselessly upwards, away from where you’re sitting.

Not only that, but if warm air escapes through the roof of your house, perhaps because of poor insulation, the resulting convection loop can draw more cold air into the room. Aside from getting better roof and cavity-wall insulation, covering or filling up those cracks will stop this from happening.

Hack: Leave your curtains open 

One hack to heat your home without putting the heating on is to leave curtains open during the day to let the sunlight and warmth in. 

But there’s a reason to do so even after the sun is down. If you’ve ever sat next to a window and felt a sudden shiver, you might have put it down to a draught caused by a crack or an ill-fitting frame. That might be the case, but there’s another reason, too, and it’s called “window convection”.

Window convection comes about because the coldness of the window cools the air around it, causing it to sink and be replaced with warmer air, creating a miniature convection cycle around the window. That’s why you can sometimes feel the air moving near a window, even if there definitely aren’t any gaps to the outside.

Counterintuitively, it’s recommended that you leave curtains or blinds a little bit open to allow some warm air from inside to reach the window, thus raising its temperature and stopping a window convection current from getting going.

Getting windows double-glazed, and walls and roofs insulated, will also help with avoiding window convection.

Hack: Get a rug 

Walls and windows are one thing, but don’t forget about the floor.

If you have non-carpeted floors it might make sense to get a rug to add extra floor insulation (and stop you getting freezing feet when walking around).

The science

A rug will help avoid the second kind of heat transfer: conduction, where substances directly touching one another transfer heat from the warmer one to the cooler,. Specifically, you want to avoid the conduction of heat from your feet into the cold floor.

Hack: A hot water bottle

Conduction also explains the old-fashioned hot water bottle (don’t write it off!). More modern electrically-heated blankets – either for your bed or elsewhere – also rely on this method of heat transfer to keep you warm even if the rest of the room is chilly.

Back in the days before central heating , people used to rely on conduction heating to a surprising degree. Certain kinds of stoves, for example, were built with attached seats that conducted heat from the burning stove outwards to anyone sitting on them; the same kinds of technologies were used thousands of years ago in China and Korea to produce heated stone beds and floors.

Of course, sitting near a stove all day can’t have been healthy due to the air pollution from the burning, and it’s also not particularly efficient unless you’re sitting on it the whole time.

Hack: Rearrange your furniture 

Keeping furniture, like sofas and chairs, away from windows might ruin the view but will also keep you away from window convection currents.

It’s possible that you can rearrange the furniture in your room to make the most of the final kind of heat source, radiation.

A “radiant” heat source doesn’t rely on warming up the air in a room (like in convection) and doesn’t need to be touched (like in conduction). It can directly warm people by transferring heat to them via infrared radiation. That’s why you often see infrared heat lamps at outdoor bar and restaurant tables: they’re not trying to warm the air, since that would be daft to even attempt outdoors – they’re transferring heat directly to the people sitting at the table.

Again, this is an old-school method of heating your home: think of historical families sitting around a fire or stove: although these will have produced convection currents (and conduction for anyone who accidentally touched them), the majority of the benefit to the people sitting nearby was via radiation.

More modern versions of radiant heaters, like electric fires or infrared lamps, can provide heat without the convection current: that’s a big advantage for people with allergies since it doesn’t kick up dust and other particles.

Maximising the effect of radiant heaters was also something our forebears worked out in the past: high-backed chairs that trapped the heat from a radiant source, and folding screens placed behind seats with lower backs, ensured that less of the warmth escaped.

Another advantage of a radiant heat source is that it can be used in just one room, and might be less wasteful than heating the whole home. You might not think that’s an advantage when you step out of that specific room into the arctic temperatures of the hallway, but you might feel better when you see your heating bill.

So there we have it: that boring school physics lesson about convection, conduction, and radiation did have some practical implications after all. Knowing about all three of these processes can not only save you some money, but it can help you have a winter with as little teeth-chattering as possible.

One hack to heat your home without putting the heating on is to leave curtains open (Photo: Tara Moore/Getty)

COMMENTS

  1. 1.5: Heat Transfer, Specific Heat, and Calorimetry

    A practical approximation for the relationship between heat transfer and temperature change is: Q = mcΔT, where Q is the symbol for heat transfer ("quantity of heat"), m is the mass of the substance, and ΔT is the change in temperature. The symbol c stands for the specific heat (also called " specific heat capacity ") and depends on ...

  2. Heat Transfer

    The three types of heat transfer differ according to the nature of the medium that transmits heat: Conduction requires contact. Convection requires fluid flow. Radiation does not require any medium. Conduction is heat transfer directly between neighboring atoms or molecules. Usually, it is heat transfer through a solid.

  3. Heat transfer

    heat transfer, any or all of several kinds of phenomena, considered as mechanisms, that convey energy and entropy from one location to another. The specific mechanisms are usually referred to as convection, thermal radiation, and conduction ( see thermal conduction ). Conduction involves transfer of energy and entropy between adjacent molecules ...

  4. Thermal conduction, convection, and radiation

    About. Transcript. There are three forms of thermal energy transfer: conduction, convection, and radiation. Conduction involves molecules transferring kinetic energy to one another through collisions. Convection occurs when hot air rises, allowing cooler air to come in and be heated. Thermal radiation happens when accelerated charged particles ...

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    Heat transfer physics describes the kinetics of energy storage, transport, and energy transformation by principal energy carriers: phonons (lattice vibration waves), electrons, fluid particles, and photons. Heat is thermal energy stored in temperature-dependent motion of particles including electrons, atomic nuclei, individual atoms, and molecules. Heat is transferred to and from matter by the ...

  6. Heat transfer

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  7. 1.4 Heat Transfer, Specific Heat, and Calorimetry

    Heat Transfer and Temperature Change. A practical approximation for the relationship between heat transfer and temperature change is: Q = mcΔT, 1.5. where Q is the symbol for heat transfer ("quantity of heat"), m is the mass of the substance, and ΔT is the change in temperature. The symbol c stands for the specific heat (also called ...

  8. 1.6 Mechanisms of Heat Transfer

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  9. Methods of Heat Transfer

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  11. Heat Transfer

    Heat Convection Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law).Hot water is likewise less dense than cold water and rises, causing convection currents which ...

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  13. What is thermal conductivity? (article)

    d ‍ : A third factor in the mechanism of conduction is the thickness d ‍ of the material through which heat transfers. The figure above shows a slab of material with different temperatures on either side. Suppose that T 2 ‍ is greater than T 1 ‍ , so that heat is transferred from left to right.Heat transfer from the left side to the right side is accomplished by a series of molecular ...

  14. 16. Heat transfer

    16. Heat transfer. Summary. Heat transfer describes how heat moves from one place to another. It is useful to understand everything from how a stove works to cook food to how our climate works. Understanding how heat moves around also enables us to create technology that can keep things hot or cold.

  15. Heat Transfer

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  16. Introduction to Heat Transfer: How Does Heat Transfer?

    Heat Transfer Definition. Heat transfer is a process by which internal energy from one substance transfers to another substance. Thermodynamics is the study of heat transfer and the changes that result from it. An understanding of heat transfer is crucial to analyzing a thermodynamic process, such as those that take place in heat engines and ...

  17. 11.2 Heat, Specific Heat, and Heat Transfer

    where m is the mass of the substance and ΔT is the change in its temperature, in units of Celsius or Kelvin.The symbol c stands for specific heat, and depends on the material and phase.The specific heat is the amount of heat necessary to change the temperature of 1.00 kg of mass by 1.00 ºC. The specific heat c is a property of the substance; its SI unit is J/(kg ⋅ ⋅ K) or J/(kg ⋅ ⋅ ...

  18. What Is Heat Transfer? Conduction, Convection, Radiation and FAQs

    According to thermodynamic systems, heat transfer is defined as. "The movement of heat across the border of the system due to a difference in temperature between the system and its surroundings.". Interestingly, the difference in temperature is said to be a 'potential' that causes the transfer of heat from one point to another. 2,48,152.

  19. Heat transfer

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  20. Heat Transfer: Conduction, Convection And Radiation

    In this animated lecture, you will learn about: heat transfer, conduction, convection and radiation with examples. #Convection #Conduction #Radiation Subscri...

  21. 5 hacks to heat your home that actually work

    Specifically, we need to consider the three kinds of heat transfer: convection, conduction and radiation. Hack: Draught excluders . Getting some draught excluders and filling any gaps, for example ...

  22. Understanding Heat Transfer in Physics for Engineers:

    Notes in PHYSICS for Engineers / mads. 1. The convection heat transfer coefficient between a surface at 50oC and ambient air at 30o C is 20 W/ m2 o C. Calculate the heat flux (flow of energy per unit area ) leaving the surface by convection. (400 W/m2 ) 2. Air at 300o C flows over a flat plate of dimensions 0.5 m by0.25 m.

  23. Multiobjective optimization of Vortex Generators for heat transfer

    Convective heat transfer can be enhanced by streamwise vortices and coherent flow structures produced downstream Vortex Generators (VGs). The thermal performance depends on the VG shape which controls the intensity and topology of the vortices. Thus, VG shape optimization is a major challenge in designing sustainable and efficient heat exchangers. In the present study, a multi-objective ...