• 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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transfer of heat physics

  • Newton's Laws
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  • Methods of Heat Transfer

On previous pages of this lesson, we have learned that heat is a form of energy transfer from a high temperature location to a low temperature location. The three main methods of heat transfer - conduction, convection and radiation - were discussed in detail on the previous page . Now we will investigate the topic of the rate of heat transfer. This topic is of great importance because of the frequent need to either increase or decrease the rate at which heat flows between two locations. For instance, those of us who live in colder winter climates are in constant pursuit of methods of keeping our homes warm without spending too much money. Heat escapes from higher temperature homes to the lower temperature outdoors through walls, ceilings, windows and doors. We make efforts to reduce this heat loss by adding better insulation to walls and attics, caulking windows and doors, and buying high efficiency windows and doors. As another example, consider electricity generation. Household electricity is most frequently manufactured by using fossil fuels or nuclear fuels . The method involves generating heat in a reactor. The heat is transferred to water and the water carries the heat to a steam turbine (or other type of electrical generator) where the electricity is produced . The challenge is to efficiently transfer the heat to the water and to the steam turbine with as little loss as possible. Attention must be given to increasing heat transfer rates in the reactor and in the turbine and decreasing heat transfer rates in the pipes between the reactor and the turbine.

So what variables would affect the heat transfer rates? How can the rate of heat transfer be controlled? These are the questions to be discussed on this page of Lesson 1. Our discussion will be restricted to the variables affecting the rate of heat transfer by conduction . Once the variables affecting the rate of heat transfer are discussed, we will look at a mathematical equation that expresses the dependence of rate upon these variables.  

Temperature Difference

In the graphs above, the slope of the line represents the rate at which the temperature of each individual sample of water is changing. The temperature is changing because of the heat transfer from the hot to the cold water. The hot water is losing energy, so its slope is negative. The cold water is gaining energy, so its slope is positive. The rate at which temperature changes is proportional to the rate at which heat is transferred. The temperature of a sample changes more rapidly if heat is transferred at a high rate and less rapidly if heat is transferred at a low rate. When the two samples reach thermal equilibrium, there is no more heat transfer and the slope is zero. So we can think of the slopes as being a measure of the rate of heat transfer. Over the course of time, the rate of heat transfer is decreasing. Initially heat is being transferred at a high rate as reflected by the steeper slopes. And as time progresses, the slopes of the lines are becoming less steep and more gently sloped.

What variable contributes to this decrease in the heat transfer rate over the course of time? Answer: the difference in temperature between the two containers of water. Initially, when the rate of heat transfer is high, the hot water has a temperature of 70°C and the cold water has a temperature of 5°C. The two containers have a 65°C difference in temperature. As the hot water begins to cool and the cold water begins to warm, the difference in their temperatures decrease and the rate of heat transfer decreases. As thermal equilibrium is approached, their temperatures are approaching the same value. With the temperature difference approaching zero, the rate of heat transfer approaches zero. In conclusion, the rate of conductive heat transfer between two locations is affected by the temperature difference between the two locations.

The first variable that we have identified as affecting the rate of conductive heat transfer is the temperature difference between the two locations. The second variable of importance is the materials involved in the transfer. In the previous discussed scenario, a metal can containing high temperature water was placed within a Styrofoam cup containing low temperature water. The heat was transferred from water through the metal to water. The materials of importance were water, metal and water. What would happen if the heat were transferred from hot water through glass to cold water? What would happen if the heat were transferred from hot water through Styrofoam to cold water? Answer: the rate of heat transfer would be different. Replacing the inner metal can with a glass jar or a Styrofoam cup would change the rate of heat transfer. The rate of heat transfer depends on the material through which heat is transferred.

The effect of a material upon heat transfer rates is often expressed in terms of a number known as the thermal conductivity . Thermal conductivity values are numerical values that are determined by experiment. The higher that the value is for a particular material, the more rapidly that heat will be transferred through that material. Materials with relatively high thermal conductivities are referred to as thermal conductors . Materials with relatively low thermal conductivity values are referred to as thermal insulators . The table below lists thermal conductivity values (k) for a variety of materials, in units of W/m/°C.

Source: http://www.roymech.co.uk/Related/Thermos/Thermos_HeatTransfer.html

As is apparent from the table, heat is generally transferred by conduction at considerably higher rates through solids (s) in comparison to liquids (l) and gases (g). Heat transfer occurs at the highest rates for metals (first eight items in left-hand column) because the mechanism of conduction includes mobile electrons (as discussed on a previous page ). Several of the solids in the right-hand column have very low thermal conductivity values and are considered insulators. The structure of these solids is characterized by pockets of trapped air interspersed between fibers of the solid. Since air is a great insulator, the pockets of air interspersed between these solid fibers gives these solids low thermal conductivity values. One of these solid insulators is expanded polystyrene, the material used in Styrofoam products. Such Styrofoam products are made by blowing an inert gas at high pressure into the polystyrene before being injected into the mold. The gas causes the polystyrene to expand, leaving air filled pockets that contribute to the insulating ability of the finished product. Styrofoam is used in coolers, pop can insulators, thermos jugs, and even foam boards for household insulation. Another solid insulator is cellulose. Cellulose insulation is used to insulate attics and walls in homes. It insulates homes from heat loss as well as sound penetration. It is often blown into attics as loose fill cellulose insulation . It is also applied as fiberglass batts (long sheets of paper backed insulation) to fill the spacing between 2x4 studs of the exterior (and sometimes interior) walls of homes.  

Thickness or Distance

A final variable that affects the rate of conductive heat transfer is the distance that the heat must be conducted. Heat escaping through a Styrofoam cup will escape more rapidly through a thin-walled cup than through a thick-walled cup. The rate of heat transfer is inversely proportional to the thickness of the cup. A similar statement can be made for heat being conducted through a layer of cellulose insulation in the wall of a home. The thicker that the insulation is, the lower the rate of heat transfer. Those of us who live in colder winter climates know this principle quite well. We are told to dress in layers before going outside. This increases the thickness of the materials through which heat is transferred, as well as trapping pockets of air (with high insulation ability) between the individual layers.  

A Mathematical Equation

So far we have learned of four variables that affect the rate of heat transfer between two locations. The variables are the temperature difference between the two locations, the material present between the two locations, the area through which the heat will be transferred, and the distance it must be transferred. As is often the case in physics, the mathematical relationship between these variables and the rate of heat transfer can be expressed in the form of an equation. Let's consider the transfer of heat through a glass window from the inside of a home with a temperature of T 1 to the outside of a home with a temperature of T 2 . The window has a surface area A and a thickness d . The thermal conductivity value of the window glass is k . The equation relating the heat transfer rate to these variables is

Rate = k•A•(T 1 - T 2 )/d

The units on the rate of heat transfer are Joule/second, also known as a Watt. This equation is applicable to any situation in which heat is transferred in the same direction across a flat rectangular wall . It applies to conduction through windows, flat walls, slopes roofs (without any curvature), etc. A slightly different equation applies to conduction through curved walls such as the walls of cans, cups, glasses and pipes. We will not discuss that equation here.  

Example Problem

To solve this problem, we will need to know the surface area of the window. Being a rectangle, we can calculate the area as width • height.

Area = (1.2 m)•(1.8 m) = 2.16 m 2 .

We will also need to give attention to the unit on thickness (d). It is given in units of cm; we will need to convert to units of meters in order for the units to be consistent with that of k and A .

d = 6.2 mm = 0.0062 m

Now we are ready to calculate the rate of heat transfer by substitution of known values into the above equation.

Rate = (0.27 W/m/°C)•(2.16 m 2 )•(21°C - -4°C)/(0.0062 m) Rate = 2400 W (rounded from 2352 W)

It is useful to note that the thermal conductivity value of a house window is much lower than the thermal conductivity value of glass itself. The thermal conductivity of glass is about 0.96 W/m/°C. Glass windows are constructed as double and triple pane windows with a low pressure inert gas layer between the panes. Furthermore, coatings are placed on the windows to improve efficiency. The result is that there are a series of substances through which heat must consecutively pass in order to be transferred out of (or into) the house. Like electrical resistors placed in series , a series of thermal insulators has an additive effect on the overall resistance offered to the flow of heat. The accumulative effect of the various layers of materials in a window leads to an overall conductivity that is much less than a single pane of uncoated glass.

Lesson 1 of this Thermal Physics chapter has focused on the meaning of temperature and heat. Emphasis has been given to the development of a particle model of materials that is capable of explaining the macroscopic observations. Efforts have been made to develop solid conceptual understandings of the topic in the absence of mathematical formulas. This solid conceptual understanding will serve you well as you approach Lesson 2 . The chapter will turn slightly more mathematical as we investigate the question: how can the amount of heat released from or gained by a system be measured? Lesson 2 will pertain to the science of calorimetry.

Check Your Understanding

1. Predict the effect of the following variations upon the rate at which heat is transferred through a rectangular object by filling in the blanks.

a. If the area through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ (increased, decreased) by a factor of _________ (number). b. If the thickness of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________. c. If the thickness of the material through which heat is transferred is decreased by a factor of 3, then the rate of heat transfer is ________________ by a factor of _________. d. If the thermal conductivity of the material through which heat is transferred is increased by a factor of 5, then the rate of heat transfer is ________________ by a factor of _________. e. If the thermal conductivity of the material through which heat is transferred is decreased by a factor of 10, then the rate of heat transfer is ________________ by a factor of _________. f. If the temperature difference on opposite sides of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________.
a. If the area through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is increased by a factor of 2 . b. If the thickness of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is decreased by a factor of 2 . c. If the thickness of the material through which heat is transferred is decreased by a factor of 3, then the rate of heat transfer is increased by a factor of 3 . d. If the thermal conductivity of the material through which heat is transferred is increased by a factor of 5, then the rate of heat transfer is increased by a factor of 5 . e. If the thermal conductivity of the material through which heat is transferred is decreased by a factor of 10, then the rate of heat transfer is decreased by a factor of 10 . f. If the temperature difference on opposite sides of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is increased by a factor of 2 .

2. Use the information on this page to explain why the 2-4 inch thick layer of blubber on a polar bear helps to keep polar bears warm during frigid artic weather.

The blubber has insulating qualities, preventing the escape of heat from the interiors of the polar bear. The thicker the blubber, the lower the rate of heat transfer.

3. Consider the example problem above. Suppose that the area where the window is located is replaced by a wall with thick insulation. The thermal conductivity of the same area will be decreased to 0.0039 W/m/°C and the thickness will be increased to 16 cm. Determine the rate of heat transfer through this area of 2.16 m 2 .

Answer: 1.3 W

Solution: Rate = (0.0039 W/m/°C)•(2.16 m 2 )•(21°C - -4°C)/(0.16 m) Rate = 1.3 W (rounded from 1.2352 W)
  • What Does Heat Do?

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Heat (Physics): Definition, Formula & Examples

Everyone is familiar with the concept of being too hot or too cold or feeling heat from the sun on a warm day, but what specifically does the word "heat" mean? Is it a property of something "hot?" Is it the same thing as temperature? It turns out that heat is a measurable quantity that physicists have precisely defined.

What Is Heat?

Heat is what scientists call the form of energy that is transferred between two materials of different temperature. This transfer of energy occurs because of differences in the average translational kinetic energy per molecule in the two materials. Heat flows from the material with higher temperature to the material with lower temperature until thermal equilibrium is reached.

The SI unit of heat is the joule, where 1 joule = 1 newton × meter.

To understand better what is happening when this energy transfer occurs, imagine the following scenario: Two different containers are filled with tiny rubber balls bouncing all around. In one of the containers, the average speed of the balls (and hence their average kinetic energy) is much larger than the average speed of the balls in the second container (though the speed of any individual ball could be anything at any point in time as so many collisions cause a continual transfer of energy between the balls.)

If you place these containers so that their sides touch, then remove the walls separating their contents, what would you expect to happen?

The balls from the first container will begin interacting with the balls from the second container. As more and more collisions between the balls occur, gradually the average speeds of the balls from both containers become the same. Some of the energy from the balls from the first container becomes transferred to the balls in the second container until this new equilibrium is reached.

This is essentially what is happening at a microscopic level when two objects of different temperature come in contact with each other. Energy from the object at a higher initial temperature is transferred in the form of heat to the object with a lower initial temperature until a final temperature at equilibrium is reached.

What Is Temperature?

Temperature is a measure of average translational kinetic energy per molecule in a substance. In the balls-in-container analogy, it is a measure of the average kinetic energy per ball in a given container. On the molecular level, atoms and molecules all vibrate and jiggle around. You can’t see this motion because it happens on such a small scale.

Common temperature scales are Fahrenheit, Celsius and Kelvin, with Kelvin being the scientific standard. The Fahrenheit scale is most common in the United States. On this scale, water freezes at 32 degrees and boils at 212 degrees. On the Celsius scale, which is common in most other places in the world, water freezes at 0 degrees and boils at 100 degrees.

The scientific standard, however, is the Kelvin scale. While the size of an increment on the Kelvin scale is the same as the size of a degree on the Celsius scale, its 0 value is set at a different place. Zero degrees Kelvin is equal to -273.15 degrees Celsius.

Why such an odd choice for 0? It turns out this is much less of an odd choice than the Celsius scale’s zero value. 0 Kelvin is the temperature at which all molecular motion stops. It is the absolute coldest temperature theoretically possible.

In this scientific light, the Kelvin scale makes much more sense than the Celsius scale. Think about how distance is measured, for example. It would be strange to create a distance scale where the 0 value was equivalent to the 1 m mark. On such a scale, what would it mean for something to be twice the length of something else?

Temperature vs. Internal Energy

The total internal energy of a substance is the total of the kinetic energies of all of its molecules. It depends on the temperature of the substance (the average kinetic energy per molecule) and the total amount of the substance (the number of molecules).

Temperature and internal energy are very similar to density and mass. While both measure a related attribute of the object, the total temperature of the system is independent of the number of molecules, while internal energy can be described as the temperature times the number of molecules.

Through this relation, as an object’s temperature increases, internal energy will increase proportionally.

It’s possible for two objects to have the same total internal energy while having entirely different temperatures. For example, a cooler object will have a lower average kinetic energy per molecule, but if the number of molecules is large, then it can still end up with the same total internal energy of an object with high temperature and fewer molecules.

A surprising result of this relationship between total internal energy and temperature is the fact that a large block of ice can end up with more energy than a lit match head, even though the match head is so hot it’s on fire!

How Heat Transfers

There are three main methods by which heat energy transfers from one object to another. They are conduction, convection and radiation.

‌ Conduction ‌ occurs when energy transfers directly between two materials in thermal contact with each other. This is the type of transfer that occurs in the rubber ball analogy described earlier in this article. When two objects are in direct contact, energy is transferred via collisions between their molecules. This energy slowly makes its way from the point of contact to the rest of the initially cooler object until thermal equilibrium is achieved.

Not all objects or substances conduct energy in this way equally well, however. Some materials, called good thermal conductors, can transfer heat energy more readily than other materials, called good thermal insulators.

You’ve likely had experience with such conductors and insulators in your daily life. On a cold winter morning, how does stepping barefoot on a tile floor compare to stepping barefoot on carpet? It probably seems like the carpet is somehow warmer, however this is not the case. Both floors are likely the same temperature, but the tile is a much better thermal conductor. Because of this, it causes the heat energy to leave your body much more quickly.

‌ Convection ‌; is a form of heat transfer that occurs in gases or fluids. Gases, and to a lesser extent, fluids, experience changes in their density with temperature. Usually the warmer they are, the less dense they are. Because of this, and because the molecules in gasses and fluids are free to move, if the bottom portion becomes warm, it will expand and hence rise to the top due to its lower density.

If you place a pan of water on the stove, for example, the water on the bottom of the pan warms up, expands and rises to the top as the cooler water sinks. The cooler water then warms, expands, and rises and so on, creating convection currents that cause the heat energy to disperse through the system via mixing of the molecules within the system (as opposed to the molecules all staying in roughly the same place as they jiggle back and forth, bouncing into each other.)

Convection is why heaters work best to warm a house if they are placed near the floor. A heater placed near the ceiling would warm the air near the ceiling, but that air would stay put.

The third form of heat transfer is ‌ radiation ‌. Radiation is the transfer of energy via electromagnetic waves. Objects that are warm can give off energy in the form of electromagnetic radiation. This is how heat energy from the sun reaches the Earth, for example. Once that radiation comes in contact with another object, the atoms in that object can gain energy by absorbing it.

Specific Heat Capacity

Two different materials of the same mass will undergo different temperature changes despite having the same total energy added due to differences in a quantity called ‌ specific heat capacity ‌. Specific heat capacity is dependent on the material in question. You will typically look up a material's specific heat capacity in a table.

More formally, specific heat capacity of a substance is defined as the amount of heat energy that must be added per unit mass of the substance in order to raise the temperature by 1 degree Celsius. The SI units for specific heat capacity, usually denoted by ‌ c ‌, are

The specific heat of water is 1. Meaning it takes 1 joule of energy to increase the temperature of 1 kilogram of water by 1 Kelvin.

Think about it like this: Suppose you have two different substances that weigh exactly the same and are at exactly the same temperature. The first substance has a high specific heat capacity, and the second substance has a low specific heat capacity. Now suppose you add exactly the same amount of heat energy to both of them. The first substance – the one with the higher heat capacity – will not go up as much in temperature as the second substance.

Factors That Affect Temperature Change

There are many factors that affect how the temperature of a substance will change when a given amount of heat energy is transferred to it. These factors include the mass of the material (a smaller mass will undergo a greater temperature change for a given amount of heat added), the specific heat capacity ‌ c, ‌ and the thermal conductivity.

If there is a heat source supplying power ‌ P ‌, then the total heat added depends on ‌ P ‌ and time ‌ t ‌. The following equation describes this total heat energy ‌ Q: ‌

The rate of temperature change is another interesting factor to consider. Do objects change their temperatures at a constant rate? It turns out that the rate of change depends on the temperature difference between the object and its surroundings in combination with their thermal conductivity. Newton’s law of cooling describes this change. The closer an object is to the surrounding temperature, the slower it approaches equilibrium.

Temperature Changes and Phase Changes

The formula that relates the change in temperature to an object’s mass, specific heat capacity and heat energy added or removed is as follows:

This formula only applies, however, if the substance is not undergoing a phase change. When a substance is changing from solid to liquid or changing from liquid to gas, the heat added to it is put to use causing this phase change and will not result in a temperature change until the phase change is complete.

A quantity called the latent heat of fusion, denoted ‌ L f ‌, describes how much heat energy per unit mass is required to change a substance from a solid to a liquid. Just as with specific heat capacity, its value depends on the physical properties of the material in question and is often looked up in tables. The equation which relates heat energy ‌ Q ‌ to the mass of a material ‌ m ‌ and the latent heat of fusion is:

The same thing occurs when changing from liquid to gas. In such a situation, a quantity called the latent heat of vaporization, denoted ‌ L v ‌, describes how much energy per unit mass must be added to cause the phase change. The resulting equation is identical except for subscript:

Heat, Work, and Internal Energy

Internal energy ‌ E ‌ is the total internal kinetic energy, or thermal energy, in a material. Assuming an ideal gas where any potential energy between molecules is negligible, it is given by the formula:

where ‌ n ‌ is the number of moles, ‌ T ‌ is temperature in Kelvin and the universal gas constant ‌ R ‌ = 8.3145 J/molK. The internal energy becomes 0 J at absolute 0 K.

In thermodynamics, the relationship between changes in internal energy, heat transferred and work done on or by a system are related via:

This relationship is known as the first law of thermodynamics. In essence it is a statement of conservation of energy. Thermodynamics often describes the world through another quantity – entropy – which describes the disorder and chaos that follows energy and heat.

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About the Author

Gayle Towell is a freelance writer and editor living in Oregon. She earned masters degrees in both mathematics and physics from the University of Oregon after completing a double major at Smith College, and has spent over a decade teaching these subjects to college students. Also a prolific writer of fiction, and founder of Microfiction Monday Magazine, you can learn more about Gayle at gtowell.com.

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Heat Transfer – Conduction, Convection, Radiation

Types of Heat Transfer

Heat transfer occurs when thermal energy moves from one place to another. Atoms and molecules inherently have kinetic and thermal energy, so all matter participates in heat transfer. There are three main types of heat transfer, plus other processes that move energy from high temperature to low temperature.

What Is Heat Transfer?

Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics . The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal). The unit for the rate of heat transfer is the kilowatt (KW).

The Three Types of Heat Transfer With Examples

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. For example, the metal handle of a pan on a stove becomes hot due to convection. Touching the hot pan conducts heat to your hand.
  • Convection is heat transfer via the movement of a fluid, such as air or water. Heating water on a stove is a good example. The water at the top of the pot becomes hot because water near the heat source rises. Another example is the movement of air around a campfire. Hot air rises, transferring heat upward. Meanwhile, the partial vacuum left by this movement draws in cool outside air that feeds the fire with fresh oxygen.
  • Radiation is the emission of electromagnetic radiation. While it occurs through a medium, it does not require one. For example, it’s warm outside on a sunny day because solar radiation crosses space and heats the atmosphere. The burner element of a stove also emits radiation. However, some heat from a burner comes from conduction between the hot element and a metal pan. Most real-life processes involve multiple forms of heat transfer.

Conduction requires that molecules touch each other, making it a slower process than convection or radiation. Atoms and molecules with a lot of energy have more kinetic energy and engage in more collisions with other matter. They are “hot.” When hot matter interacts with cold matter, some energy gets transferred during the collision. This drives conduction. Forms of matter that readily conduct heat are called thermal conductors .

Examples of Conduction

Conduction is a common process in everyday life. For example:

  • Holding an ice cube immediately makes your hands feel cold. Meanwhile, the heat transferred from your skin to the ice melts it into liquid water.
  • Walking barefoot on a hot road or sunny beach burns your feet because the solid material transmits heat into your foot.
  • Iron clothes transfers heat from the iron to the fabric.
  • The handle of a coffee cup filled with hot coffee becomes warm or even hot via conduction through the mug material.

Conduction Equation

One equation for conduction calculates heat transfer per unit of time from thermal conductivity, area, thickness of the material, and the temperature difference between two regions:

Q = [K ∙ A ∙ (T hot – T cold )] / d

  • Q is heat transfer per unit time
  • K is the coefficient of thermal conductivity of the substance
  • 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

Convection is the movement of fluid molecules from higher temperature to lower temperature regions. Changing the temperature of a fluid affects its density, producing convection currents. If the volume of a fluid increases, than its density decreases and it becomes buoyant.

Examples of Convection

Convection is a familiar process on Earth, primarily involving air or water. However, it applies to other fluids, such as refrigeration gases and magma. Examples of convection include:

  • Boiling water undergoes convection as less dense hot molecules rise through higher density cooler molecules.
  • Hot air rises and cooler air sinks and replaces it.
  • Convection drives global circulation in the oceans between the equators and poles.
  • A convection oven circulates hot air and cooks more evenly than one that only uses heating elements or a gas flame.

Convection Equation

The equation for the rate of convection relates area and the difference between the fluid temperature and surface temperature:

Q = h c  ∙ A ∙ (T s  – T f )

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

Radiation is the release of electromagnetic energy. Another name for thermal radiation is radiant heat. Unlike conduction or convection, radiation requires no medium for heat transfer. So, radiation occurs both within a medium (solid, liquid, gas) or through a vacuum.

Examples of Radiation

There are many examples of radiation:

  • A microwave oven emits microwave radiation, which increases the thermal energy in food
  • The Sun emits light (including ultraviolet radiation) and heat
  • Uranium-238 emits alpha radiation as it decays into thorium-234

Radiation Equation

The Stephan-Boltzmann law describes relationship between the power and temperature of thermal radiation:

P = e ∙ σ ∙ A· (Tr – Tc) 4

  • 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
  • σ is Stefan’s constant (σ = 5.67 × 10 -8 Wm -2 K -4 )

More Heat Transfer – Chemical Bonds and Phase Transitions

While conduction, convection, and radiation are the three modes of heat transfer, other processes absorb and release heat. For example, atoms release energy when chemical bonds break and absorb energy in order to form bonds. Releasing energy is an exergonic process, while absorbing energy is an endergonic process. Sometimes the energy is light or sound, but most of the time it’s heat, making these processes exothermic and endothermic .

Phase transitions between the states of matter also involve the absorption or release of energy. A great example of this is evaporative cooling, where the phase transition from a liquid into a vapor absorbs thermal energy from the environment.

  • Faghri, Amir; Zhang, Yuwen; Howell, John (2010). Advanced Heat and Mass Transfer . Columbia, MO: Global Digital Press. ISBN 978-0-9842760-0-4.
  • Geankoplis, Christie John (2003). Transport Processes and Separation Principles (4th ed.). Prentice Hall. ISBN 0-13-101367-X.
  • Peng, Z.; Doroodchi, E.; Moghtaderi, B. (2020). “Heat transfer modelling in Discrete Element Method (DEM)-based simulations of thermal processes: Theory and model development”. Progress in Energy and Combustion Science . 79: 100847. doi: 10.1016/j.pecs.2020.100847
  • Welty, James R.; Wicks, Charles E.; Wilson, Robert Elliott (1976). Fundamentals of Momentum, Heat, and Mass Transfer (2nd ed.). New York: Wiley. ISBN 978-0-471-93354-0.

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  • 2.6 Problem-Solving Basics for One-Dimensional Kinematics
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  • 2.8 Graphical Analysis of One-Dimensional Motion
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  • 3.2 Vector Addition and Subtraction: Graphical Methods
  • 3.3 Vector Addition and Subtraction: Analytical Methods
  • 3.4 Projectile Motion
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  • 4.1 Development of Force Concept
  • 4.2 Newton’s First Law of Motion: Inertia
  • 4.3 Newton’s Second Law of Motion: Concept of a System
  • 4.4 Newton’s Third Law of Motion: Symmetry in Forces
  • 4.5 Normal, Tension, and Other Examples of Forces
  • 4.6 Problem-Solving Strategies
  • 4.7 Further Applications of Newton’s Laws of Motion
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  • 6.1 Rotation Angle and Angular Velocity
  • 6.2 Centripetal Acceleration
  • 6.3 Centripetal Force
  • 6.4 Fictitious Forces and Non-inertial Frames: The Coriolis Force
  • 6.5 Newton’s Universal Law of Gravitation
  • 6.6 Satellites and Kepler’s Laws: An Argument for Simplicity
  • Introduction to Work, Energy, and Energy Resources
  • 7.1 Work: The Scientific Definition
  • 7.2 Kinetic Energy and the Work-Energy Theorem
  • 7.3 Gravitational Potential Energy
  • 7.4 Conservative Forces and Potential Energy
  • 7.5 Nonconservative Forces
  • 7.6 Conservation of Energy
  • 7.8 Work, Energy, and Power in Humans
  • 7.9 World Energy Use
  • Introduction to Linear Momentum and Collisions
  • 8.1 Linear Momentum and Force
  • 8.2 Impulse
  • 8.3 Conservation of Momentum
  • 8.4 Elastic Collisions in One Dimension
  • 8.5 Inelastic Collisions in One Dimension
  • 8.6 Collisions of Point Masses in Two Dimensions
  • 8.7 Introduction to Rocket Propulsion
  • Introduction to Statics and Torque
  • 9.1 The First Condition for Equilibrium
  • 9.2 The Second Condition for Equilibrium
  • 9.3 Stability
  • 9.4 Applications of Statics, Including Problem-Solving Strategies
  • 9.5 Simple Machines
  • 9.6 Forces and Torques in Muscles and Joints
  • Introduction to Rotational Motion and Angular Momentum
  • 10.1 Angular Acceleration
  • 10.2 Kinematics of Rotational Motion
  • 10.3 Dynamics of Rotational Motion: Rotational Inertia
  • 10.4 Rotational Kinetic Energy: Work and Energy Revisited
  • 10.5 Angular Momentum and Its Conservation
  • 10.6 Collisions of Extended Bodies in Two Dimensions
  • 10.7 Gyroscopic Effects: Vector Aspects of Angular Momentum
  • Introduction to Fluid Statics
  • 11.1 What Is a Fluid?
  • 11.2 Density
  • 11.3 Pressure
  • 11.4 Variation of Pressure with Depth in a Fluid
  • 11.5 Pascal’s Principle
  • 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement
  • 11.7 Archimedes’ Principle
  • 11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
  • 11.9 Pressures in the Body
  • Introduction to Fluid Dynamics and Its Biological and Medical Applications
  • 12.1 Flow Rate and Its Relation to Velocity
  • 12.2 Bernoulli’s Equation
  • 12.3 The Most General Applications of Bernoulli’s Equation
  • 12.4 Viscosity and Laminar Flow; Poiseuille’s Law
  • 12.5 The Onset of Turbulence
  • 12.6 Motion of an Object in a Viscous Fluid
  • 12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes
  • Introduction to Temperature, Kinetic Theory, and the Gas Laws
  • 13.1 Temperature
  • 13.2 Thermal Expansion of Solids and Liquids
  • 13.3 The Ideal Gas Law
  • 13.4 Kinetic Theory: Atomic and Molecular Explanation of Pressure and Temperature
  • 13.5 Phase Changes
  • 13.6 Humidity, Evaporation, and Boiling
  • 14.2 Temperature Change and Heat Capacity
  • 14.3 Phase Change and Latent Heat
  • 14.4 Heat Transfer Methods
  • 14.5 Conduction
  • 14.6 Convection
  • 14.7 Radiation
  • Introduction to Thermodynamics
  • 15.1 The First Law of Thermodynamics
  • 15.2 The First Law of Thermodynamics and Some Simple Processes
  • 15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency
  • 15.4 Carnot’s Perfect Heat Engine: The Second Law of Thermodynamics Restated
  • 15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators
  • 15.6 Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy
  • 15.7 Statistical Interpretation of Entropy and the Second Law of Thermodynamics: The Underlying Explanation
  • Introduction to Oscillatory Motion and Waves
  • 16.1 Hooke’s Law: Stress and Strain Revisited
  • 16.2 Period and Frequency in Oscillations
  • 16.3 Simple Harmonic Motion: A Special Periodic Motion
  • 16.4 The Simple Pendulum
  • 16.5 Energy and the Simple Harmonic Oscillator
  • 16.6 Uniform Circular Motion and Simple Harmonic Motion
  • 16.7 Damped Harmonic Motion
  • 16.8 Forced Oscillations and Resonance
  • 16.10 Superposition and Interference
  • 16.11 Energy in Waves: Intensity
  • Introduction to the Physics of Hearing
  • 17.2 Speed of Sound, Frequency, and Wavelength
  • 17.3 Sound Intensity and Sound Level
  • 17.4 Doppler Effect and Sonic Booms
  • 17.5 Sound Interference and Resonance: Standing Waves in Air Columns
  • 17.6 Hearing
  • 17.7 Ultrasound
  • Introduction to Electric Charge and Electric Field
  • 18.1 Static Electricity and Charge: Conservation of Charge
  • 18.2 Conductors and Insulators
  • 18.3 Coulomb’s Law
  • 18.4 Electric Field: Concept of a Field Revisited
  • 18.5 Electric Field Lines: Multiple Charges
  • 18.6 Electric Forces in Biology
  • 18.7 Conductors and Electric Fields in Static Equilibrium
  • 18.8 Applications of Electrostatics
  • Introduction to Electric Potential and Electric Energy
  • 19.1 Electric Potential Energy: Potential Difference
  • 19.2 Electric Potential in a Uniform Electric Field
  • 19.3 Electrical Potential Due to a Point Charge
  • 19.4 Equipotential Lines
  • 19.5 Capacitors and Dielectrics
  • 19.6 Capacitors in Series and Parallel
  • 19.7 Energy Stored in Capacitors
  • Introduction to Electric Current, Resistance, and Ohm's Law
  • 20.1 Current
  • 20.2 Ohm’s Law: Resistance and Simple Circuits
  • 20.3 Resistance and Resistivity
  • 20.4 Electric Power and Energy
  • 20.5 Alternating Current versus Direct Current
  • 20.6 Electric Hazards and the Human Body
  • 20.7 Nerve Conduction–Electrocardiograms
  • Introduction to Circuits and DC Instruments
  • 21.1 Resistors in Series and Parallel
  • 21.2 Electromotive Force: Terminal Voltage
  • 21.3 Kirchhoff’s Rules
  • 21.4 DC Voltmeters and Ammeters
  • 21.5 Null Measurements
  • 21.6 DC Circuits Containing Resistors and Capacitors
  • Introduction to Magnetism
  • 22.1 Magnets
  • 22.2 Ferromagnets and Electromagnets
  • 22.3 Magnetic Fields and Magnetic Field Lines
  • 22.4 Magnetic Field Strength: Force on a Moving Charge in a Magnetic Field
  • 22.5 Force on a Moving Charge in a Magnetic Field: Examples and Applications
  • 22.6 The Hall Effect
  • 22.7 Magnetic Force on a Current-Carrying Conductor
  • 22.8 Torque on a Current Loop: Motors and Meters
  • 22.9 Magnetic Fields Produced by Currents: Ampere’s Law
  • 22.10 Magnetic Force between Two Parallel Conductors
  • 22.11 More Applications of Magnetism
  • Introduction to Electromagnetic Induction, AC Circuits and Electrical Technologies
  • 23.1 Induced Emf and Magnetic Flux
  • 23.2 Faraday’s Law of Induction: Lenz’s Law
  • 23.3 Motional Emf
  • 23.4 Eddy Currents and Magnetic Damping
  • 23.5 Electric Generators
  • 23.6 Back Emf
  • 23.7 Transformers
  • 23.8 Electrical Safety: Systems and Devices
  • 23.9 Inductance
  • 23.10 RL Circuits
  • 23.11 Reactance, Inductive and Capacitive
  • 23.12 RLC Series AC Circuits
  • Introduction to Electromagnetic Waves
  • 24.1 Maxwell’s Equations: Electromagnetic Waves Predicted and Observed
  • 24.2 Production of Electromagnetic Waves
  • 24.3 The Electromagnetic Spectrum
  • 24.4 Energy in Electromagnetic Waves
  • Introduction to Geometric Optics
  • 25.1 The Ray Aspect of Light
  • 25.2 The Law of Reflection
  • 25.3 The Law of Refraction
  • 25.4 Total Internal Reflection
  • 25.5 Dispersion: The Rainbow and Prisms
  • 25.6 Image Formation by Lenses
  • 25.7 Image Formation by Mirrors
  • Introduction to Vision and Optical Instruments
  • 26.1 Physics of the Eye
  • 26.2 Vision Correction
  • 26.3 Color and Color Vision
  • 26.4 Microscopes
  • 26.5 Telescopes
  • 26.6 Aberrations
  • Introduction to Wave Optics
  • 27.1 The Wave Aspect of Light: Interference
  • 27.2 Huygens's Principle: Diffraction
  • 27.3 Young’s Double Slit Experiment
  • 27.4 Multiple Slit Diffraction
  • 27.5 Single Slit Diffraction
  • 27.6 Limits of Resolution: The Rayleigh Criterion
  • 27.7 Thin Film Interference
  • 27.8 Polarization
  • 27.9 *Extended Topic* Microscopy Enhanced by the Wave Characteristics of Light
  • Introduction to Special Relativity
  • 28.1 Einstein’s Postulates
  • 28.2 Simultaneity And Time Dilation
  • 28.3 Length Contraction
  • 28.4 Relativistic Addition of Velocities
  • 28.5 Relativistic Momentum
  • 28.6 Relativistic Energy
  • Introduction to Quantum Physics
  • 29.1 Quantization of Energy
  • 29.2 The Photoelectric Effect
  • 29.3 Photon Energies and the Electromagnetic Spectrum
  • 29.4 Photon Momentum
  • 29.5 The Particle-Wave Duality
  • 29.6 The Wave Nature of Matter
  • 29.7 Probability: The Heisenberg Uncertainty Principle
  • 29.8 The Particle-Wave Duality Reviewed
  • Introduction to Atomic Physics
  • 30.1 Discovery of the Atom
  • 30.2 Discovery of the Parts of the Atom: Electrons and Nuclei
  • 30.3 Bohr’s Theory of the Hydrogen Atom
  • 30.4 X Rays: Atomic Origins and Applications
  • 30.5 Applications of Atomic Excitations and De-Excitations
  • 30.6 The Wave Nature of Matter Causes Quantization
  • 30.7 Patterns in Spectra Reveal More Quantization
  • 30.8 Quantum Numbers and Rules
  • 30.9 The Pauli Exclusion Principle
  • Introduction to Radioactivity and Nuclear Physics
  • 31.1 Nuclear Radioactivity
  • 31.2 Radiation Detection and Detectors
  • 31.3 Substructure of the Nucleus
  • 31.4 Nuclear Decay and Conservation Laws
  • 31.5 Half-Life and Activity
  • 31.6 Binding Energy
  • 31.7 Tunneling
  • Introduction to Applications of Nuclear Physics
  • 32.1 Medical Imaging and Diagnostics
  • 32.2 Biological Effects of Ionizing Radiation
  • 32.3 Therapeutic Uses of Ionizing Radiation
  • 32.4 Food Irradiation
  • 32.5 Fusion
  • 32.6 Fission
  • 32.7 Nuclear Weapons
  • Introduction to Particle Physics
  • 33.1 The Yukawa Particle and the Heisenberg Uncertainty Principle Revisited
  • 33.2 The Four Basic Forces
  • 33.3 Accelerators Create Matter from Energy
  • 33.4 Particles, Patterns, and Conservation Laws
  • 33.5 Quarks: Is That All There Is?
  • 33.6 GUTs: The Unification of Forces
  • Introduction to Frontiers of Physics
  • 34.1 Cosmology and Particle Physics
  • 34.2 General Relativity and Quantum Gravity
  • 34.3 Superstrings
  • 34.4 Dark Matter and Closure
  • 34.5 Complexity and Chaos
  • 34.6 High-temperature Superconductors
  • 34.7 Some Questions We Know to Ask
  • A | Atomic Masses
  • B | Selected Radioactive Isotopes
  • C | Useful Information
  • D | Glossary of Key Symbols and Notation

Chapter Outline

  • Define heat as transfer of energy.
  • Observe heat transfer and change in temperature and mass.
  • Calculate final temperature after heat transfer between two objects.
  • Examine heat transfer.
  • Calculate final temperature from heat transfer.
  • Discuss the different methods of heat transfer.
  • Calculate thermal conductivity.
  • Observe conduction of heat in collisions.
  • Study thermal conductivities of common substances.
  • Discuss the method of heat transfer by convection.
  • Discuss heat transfer by radiation.
  • Explain the power of different materials.

Energy can exist in many forms and heat is one of the most intriguing. Heat is often hidden, as it only exists when in transit, and is transferred by a number of distinctly different methods. Heat transfer touches every aspect of our lives and helps us understand how the universe functions. It explains the chill we feel on a clear breezy night, or why Earth’s core has yet to cool. This chapter defines and explores heat transfer, its effects, and the methods by which heat is transferred. These topics are fundamental, as well as practical, and will often be referred to in the chapters ahead.

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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?
  • What Is Isobaric Process?
  • The Isochoric Process
  • What Is an Isothermal Process in Physics?
  • Thermodynamics: Adiabatic Process
  • Understanding Calorimetry to Measure Heat Transfer
  • Why Matter Changes State
  • What Is Entropy and How to Calculate It
  • Specific Heat Capacity in Chemistry
  • Temperature Definition in Science
  • Laws of Thermodynamics as Related to Biology
  • Calculate the Change in Entropy From Heat of Reaction

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MIT physicists capture the first sounds of heat “sloshing” in a superfluid

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In most materials, heat prefers to scatter. If left alone, a hotspot will gradually fade as it warms its surroundings. But in rare states of matter, heat can behave as a wave, moving back and forth somewhat like a sound wave that bounces from one end of a room to the other. In fact, this wave-like heat is what physicists call “second sound.”

Signs of second sound have been observed in only a handful of materials. Now MIT physicists have captured direct images of second sound for the first time.

The new images reveal how heat can move like a wave, and “slosh” back and forth, even as a material’s physical matter may move in an entirely different way. The images capture the pure movement of heat, independent of a material’s particles.

“It’s as if you had a tank of water and made one half nearly boiling,” Assistant Professor Richard Fletcher offers as analogy. “If you then watched, the water itself might look totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat goes back and forth, while the water looks totally still.”

Led by Martin Zwierlein, the Thomas A Frank Professor of Physics, the team visualized second sound in a superfluid — a special state of matter that is created when a cloud of atoms is cooled to extremely low temperatures, at which point the atoms begin to flow like a completely friction-free fluid. In this superfluid state, theorists have predicted that heat should also flow like a wave, though scientists had not been able to directly observe the phenomenon until now.

The new results, reported today in the journal Science , will help physicists get a more complete picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars.

“There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars,” Zwierlein says. “Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach.”

Zwierlein and Fletcher’s co-authors on the study are first author and former physics graduate student Zhenjie Yan and former physics graduate students Parth Patel and Biswaroop Mukherjee, along with Chris Vale at Swinburne University of Technology in Melbourne, Australia. The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).

Super sound

When clouds of atoms are brought down to temperatures close to absolute zero, they can transition into rare states of matter. Zwierlein’s group at MIT is exploring the exotic phenomena that emerge among ultracold atoms, and specifically fermions — particles, such as electrons, that normally avoid each other.

Under certain conditions, however, fermions can be made to strongly interact and pair up. In this coupled state, fermions can flow in unconventional ways. For their latest experiments, the team employs fermionic lithium-6 atoms, which are trapped and cooled to nanokelvin temperatures.

In 1938, the physicist László Tisza proposed a two-fluid model for superfluidity — that a superfluid is actually a mixture of some normal, viscous fluid and a friction-free superfluid. This mixture of two fluids should allow for two types of sound, ordinary density waves and peculiar temperature waves, which physicist Lev Landau later named “second sound.”  

Since a fluid transitions into a superfluid at a certain critical, ultracold temperature, the MIT team reasoned that the two types of fluid should also transport heat differently: In normal fluids, heat should dissipate as usual, whereas in a superfluid, it could move as a wave, similarly to sound.

“Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it,” Zwierlein says. “The character of the heat wave could not be proven before.”

Zwierlein and his team sought to isolate and observe second sound, the wave-like movement of heat, independent of the physical motion of fermions in their superfluid. They did so by developing a new method of thermography — a heat-mapping technique. In  conventional materials one would use infrared sensors to image heat sources.

But at ultracold temperatures, gases do not give off infrared radiation. Instead, the team developed a method to use radio frequency to “see” how heat moves through the superfluid. They found that the lithium-6 fermions resonate at different radio frequencies depending on their temperature: When the cloud is at warmer temperatures, and carries more normal liquid, it resonates at a higher frequency. Regions in the cloud that are colder resonate at a lower frequency.

The researchers applied the higher resonant radio frequency, which prompted any normal, “hot” fermions in the liquid to ring in response. The researchers then were able to zero in on the resonating fermions and track them over time to create “movies” that revealed heat’s pure motion — a sloshing back and forth, similar to waves of sound.

“For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth,” Zwierlein says.

The experiments mark the first time that scientists have been able to directly image second sound, and the pure motion of heat in a superfluid quantum gas. The researchers plan to extend their work to more precisely map heat’s behavior in other ultracold gases. Then, they say their findings can be scaled up to predict how heat flows in other strongly interacting materials, such as in high-temperature superconductors, and in neutron stars.

“Now we will be able to measure precisely the thermal conductivity in these systems, and hope to understand and design better systems,” Zwierlein concludes.

This work was supported by the National Science Foundation (NSF), the Air Force Office of Scientific Research, and the Vannevar Bush Faculty Fellowship. The MIT team is part of the MIT-Harvard Center for Ultracold Atoms (an NSF Physics Frontier Center) and affiliated with the MIT Department of Physics and the Research Laboratory of Electronics (RLE).

Share this news article on:

Press mentions, popular mechanics.

For the first time, MIT physicists have successfully imaged how heat travels in a superfluid, known as a “second sound,” reports Darren Orf for Popular Mechanics . “While exotic superfluids may not fill up our lives (yet),” writes Orf, “understanding the properties of second wave movement could help questions regarding high-temperature superconductors (again, still at very low temperatures) or the messy physics that lie at the heart of neutron stars.”

Gizmodo reporter Isaac Schultz writes that MIT scientists have captured images of heat moving through a superfluid, a phenomenon that “may explain how heat moves through certain rare materials on Earth and deep in space.”  Schultz notes that the researchers believe their examination of heat flow in a superfluid “can be used to determine heat flow in high-temperature superconductors, or even in neutron stars, the roiling, ultra-dense relics of ordinary stars.”

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  • Department of Physics

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13.1: Introduction

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learning objectives

  • Distinguish three modes of heat transfer

Introduction to Heat and Heat Transfer

Energy can exist in many forms and heat is one of the most intriguing. Heat is often hidden, as it only exists when in transit, and is transferred by a number of distinctly different methods. Heat transfer touches every aspect of our lives and helps us understand how the universe functions. It explains the chill we feel on a clear breezy night, or why Earth’s core has yet to cool. This module defines and explores heat transfer, its effects, and the methods by which heat is transferred. These topics are fundamental, as well as practical, and will often be referred to in the modules ahead.

image

Examples of Heat Transfer : (a) The chilling effect of a clear breezy night is produced by the wind and by radiative heat transfer to cold outer space. (b) There was once great controversy about the Earth’s age, but it is now generally accepted to be about 4.5 billion years old. Much of the debate is centered on the Earth’s molten interior. According to our understanding of heat transfer, if the Earth is really that old, its center should have cooled off long ago. The discovery of radioactivity in rocks revealed the source of energy that keeps the Earth’s interior molten, despite heat transfer to the surface, and from there to cold outer space.

Definitions

Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat , was one of many who began to build on the already established idea that heat was something to do with matter in motion. This was the same idea put forwards by Sir Benjamin Thompson in 1798, who said he was only following on from the work of many others. One of Maxwell’s recommended books was Heat as a Mode of Motion by John Tyndall. Maxwell outlined four stipulations for the definition of heat:

  • It is something which may be transferred from one body to another.
  • It is a measurable quantity, and thus treated mathematically.
  • It cannot be treated as a substance, because it may be transformed into something that is not a substance, such as mechanical work.
  • Heat is one of the forms of energy.

In the following sections, we will define heat more rigorously, paying particular attention to how it can be measured and quantified.

Estimation of Quantity of Heat

The quantity of heat transferred by some process can either be directly measured, or determined indirectly through calculations based on other quantities. Direct measurement is by calorimetry and is the primary empirical basis of the idea of quantity of heat transferred in a process. The transferred heat is measured by changes in a body of known properties, for example, temperature rise, change in volume or length, or phase change, such as melting of ice. Indirect estimations of quantity of heat transferred rely on the law of conservation of energy, and, in particular cases, on the first law of thermodynamics (explored in the following sections). Indirect estimation is the primary approach of many theoretical studies of quantity of heat transferred.

Heat Transfer Methods

After defining and quantifying heat transfer and its effects on physical systems, we will discuss the methods by which heat is transferred. So many processes involve heat transfer, so that it is hard to imagine a situation where no heat transfer occurs. Yet every process involving heat transfer takes place by only three methods:

  • Conduction is heat transfer through stationary matter by physical contact. ( The matter is stationary on a macroscopic scale—we know there is thermal motion of the atoms and molecules at any temperature above absolute zero. ) Heat transferred between the electric burner of a stove and the bottom of a pan is transferred by conduction.
  • Convection is the heat transfer by the macroscopic movement of a fluid. This type of transfer takes place in a forced-air furnace and in weather systems, for example.
  • Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is the warming of the Earth by the Sun. A less obvious example is thermal radiation from the human body.

Heat as Energy Transfer

Heat is the spontaneous transfer of energy due to a temperature difference.

  • Identify SI and common units of heat

Consider two objects at different temperatures that are brought together. Energy is transferred from the hotter object to the cooler one, until both objects reach thermal equilibrium (i.e., both become the same temperature). How is this energy transferred? No work is done by either object, because no force acts through a distance. The transfer of energy is caused by the temperature difference, and ceases once the temperatures are equal. This observation leads to the following definition of heat: Heat is the spontaneous transfer of energy due to a temperature difference.

image

Heat Transfer and Equilibrium : (a) The soft drink and the ice have different temperatures, T1 and T2, and are not in thermal equilibrium. (b) When the soft drink and ice are allowed to interact, energy is transferred until they reach the same temperature T, achieving equilibrium. Heat transfer occurs due to the difference in temperatures. In fact, since the soft drink and ice are both in contact with the surrounding air and bench, the equilibrium temperature will be the same for both.

Where Is the Most Heat Lost? : Use movable thermometers to discover where a house has poor insulation.

Heat Transfer : A brief introduction to heat transfer for students.

Heat is often confused with temperature. For example, we may say the heat was unbearable, when we actually mean that the temperature was high. Heat is a form of energy, whereas temperature is not. The misconception arises because we are sensitive to the flow of heat, rather than the temperature.

Owing to the fact that heat is a form of energy, it has the SI unit of joule (J). The calorie (cal) is a common unit of energy, defined as the energy needed to change the temperature of 1.00 g of water by 1.00ºC —specifically, between 14.5ºC and 15.5ºC, since there is a slight temperature dependence. Another common unit of heat is the kilocalorie (kcal), which is the energy needed to change the temperature of 1.00 kg of water by 1.00ºC. Since mass is often specified in kilograms, kilocalorie is commonly used. Food calories (given the notation Cal, and sometimes called “big calorie”) are actually kilocalories (1kilocalorie=1000 calories), a fact not easily determined from package labeling in the United States, but more common in Europe and elsewhere. In some engineering fields, the British Thermal Unit (BTU), equal to about 1.055 kilo-joules, is widely used.

image

Figure 1 Equivalence of Heat and Work : Schematic depiction of Joule’s experiment that established the equivalence of heat and work

The total amount of energy transferred as heat is conventionally written as Q for algebraic purposes. Heat released by a system into its surroundings is by convention a negative quantity ( Q < 0); when a system absorbs heat from its surroundings, it is positive ( Q > 0).

Mechanical Equivalent of Heat

It is also possible to change the temperature of a substance by doing work. Work can transfer energy into or out of a system. This realization helped establish the fact 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 terms of the units used for these two terms, the best modern value for this equivalence is 1.000 kcal = 4186 J. We consider this equation as the conversion between two different units of energy.

Figure 1 shows one of Joule’s most famous experimental setups for demonstrating the mechanical equivalent of heat. It demonstrated that work and heat can produce the same effects, and helped establish the principle of conservation of energy. Gravitational potential energy (PE) (work done by the gravitational force) is converted into kinetic energy (KE), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. His contributions to the field of thermodynamics were so significant that the SI unit of energy was named after him.

Heat added or removed from a system changes its internal energy (a concept we will discuss in the following section) and thus its temperature. Such a temperature increase is observed while cooking. However, adding heat does not necessarily increase the temperature. An example is melting of ice; that is, when a substance changes from one phase to another. Work done on the system or by the system can also change the internal energy of the system. Joule demonstrated that the temperature of a system can be increased by stirring. If an ice cube is rubbed against a rough surface, work is done by the frictional force. A system has a well-defined internal energy, but we cannot say that it has a certain “heat content” or “work content.” We use the phrase “heat transfer” to emphasize its nature.

Internal Energy

The internal energy of a system is the sum of all kinetic and potential energy in a system.

  • Express the internal energy in terms of kinetic and potential energy

James Joule showed that both heat and work can produce the same change in the internal energy of a substance, establishing the principle of the mechanical equivalence of heat. Heat is emphatically a quantity that solely describes energy being transferred. It makes no sense to speak of the total ‘heat’ an object or system contains. However, a system does contain a quantifiable amount of energy called the internal energy of a system. The internal energy of a system is the quantity that changes with the addition or subtraction of work or heat. It is closely related to temperature.

The internal energy is the energy required to create a system, excluding the energy necessary to displace its surroundings. Internal energy has two components: kinetic energy and potential energy. The kinetic energy consists of all the energy involving the motions of the particles constituting the system, including translation, vibration, and rotation. The potential energy is associated with the static constituents of matter, static electric energy of atoms within molecules or crystals, and the energy from chemical bonds. The equation describing the total internal energy of a system is then:

\[\mathrm{U=U_{kinetic}+U_{potential}.}\]

We can also think of the internal energy as the sum of all the energy states of each component in the system:

\[\mathrm{U=∑_iE_i.}\]

At any finite temperature, kinetic and potential energies are constantly converted into each other, but the total energy remains constant in an isolated system. The kinetic energy portion of internal energy gives rise to the temperature of the system. We can use statistical mechanics to relate the (somewhat) random motions of particles in a system to the mean kinetic energy of the ensemble of particles, and thus the empirically measurable quantity expressed as temperature.

We can see that internal energy is an extensive property: it depends on the size of the system or on the amount of substance it contains.

In most cases, we are not concerned with the total amount of internal energy in the system, as it is rarely convenient or necessary to consider all energies belonging to the system. Rather, we are far more interested in the change in internal energy, given some transfer of work or heat. This can be expressed as:

\[\mathrm{ΔU=Q+W_{mech}+W_{other}.}\]

Q is heat added to a system and W mech is the mechanical work performed by the surroundings due to pressure or volume changes in the system. All other perturbations and energies added by other processes, such as an electric current introduced into an electronic circuit, is summarized as the term W extra .

We can calculate a small change in internal energy of the system by considering the infinitesimal amount of heat δQ added to the system minus the infinitesimal amount of work δW done by the system:

\[\mathrm{dU=δQ−δW.}\]

This expression is the first law of thermodynamics.

image

The Sun and Internal Energy : Nuclear fusion in the sun converts nuclear potential energy into available internal energy and keeps the temperature of the Sun very high.

  • Heat is a crucial concept that touches every aspect of our lives. James Clerk Maxwell set down important principles that couple into the definition of heat.
  • The quantity of heat transfer can be directly measured or estimated indirectly through the science of calorimetry.
  • There are three modes of heat transfer: conduction, convection, and radiation.
  • If two objects at different temperature are brought together, energy will transfer from the hotter object to the cooler one until both are at the same temperature. This transfer of energy is known as heat.
  • Heat should not be confused with temperature. Temperature describes the internal state of an object, while heat refers to the energy transferred to or from the object.
  • Since heat is a form of energy, its SI unit is the joule. Other common units of heat energy include the calorie and kilocalorie, equal to 4.186 and 4,186 joules, respectively.
  • Because heat and work both involve the transfer of energy, they can each produce the same effects. The concept of the mechanical equivalent of heat was instrumental in establishing the principle of conservation of energy.
  • While a system does not contain ‘ heat,’ it does contain a total amount of energy called internal energy.
  • The internal energy is the energy necessary to create a system, minus the energy necessary to displace its surroundings.
  • Most of the time, we are interested in the change in internal energy rather than the total internal energy.
  • The first law of thermodynamics, \(\mathrm{dU=ΔQ−ΔW}\), describes small changes in internal energy.
  • heat transfer : The transmission of thermal energy via conduction, convection, or radiation.
  • calorimetry : The science of measuring the heat absorbed or evolved during the course of a chemical reaction or change of state.
  • kilocalorie : A non-SI unit of energy equal to 1,000 calories or 4,186 joules; equal to the “calorie” or “Calorie” used in nutritional labeling. Symbol: kcal.
  • thermal equilibrium : Two systems are in thermal equilibrium if they could transfer heat between each other, but don’t.
  • mechanical equivalent of heat : The work needed to produce the same effects as heat transfer.
  • internal energy : The sum of all energy present in the system, including kinetic and potential energy; equivalently, the energy needed to create a system, excluding the energy necessary to displace its surroundings.
  • isolated system : A system that does not interact with its surroundings, that is, its total energy and mass stay constant.

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ScienceDaily

Physicists capture the first sounds of heat 'sloshing' in a superfluid

In most materials, heat prefers to scatter. If left alone, a hotspot will gradually fade as it warms its surroundings. But in rare states of matter, heat can behave as a wave, moving back and forth somewhat like a sound wave that bounces from one end of a room to the other. In fact, this wave-like heat is what physicists call "second sound."

Signs of second sound have been observed in only a handful of materials. Now MIT physicists have captured direct images of second sound for the first time.

The new images reveal how heat can move like a wave, and "slosh" back and forth, even as a material's physical matter may move in an entirely different way. The images capture the pure movement of heat, independent of a material's particles.

"It's as if you had a tank of water and made one half nearly boiling," Assistant Professor Richard Fletcher offers as analogy. "If you then watched, the water itself might look totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat goes back and forth, while the water looks totally still."

Led by Martin Zwierlein, the Thomas A Frank Professor of Physics, the team visualized second sound in a superfluid -- a special state of matter that is created when a cloud of atoms is cooled to extremely low temperatures, at which point the atoms begin to flow like a completely friction-free fluid. In this superfluid state, theorists have predicted that heat should also flow like a wave, though scientists had not been able to directly observe the phenomenon until now.

The new results, reported in the journal Science , will help physicists get a more complete picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars.

"There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars," Zwierlein says. "Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach."

Zwierlein and Fletcher's co-authors on the study are first author and former physics graduate student Zhenjie Yan and former physics graduate students Parth Patel and Biswaroop Mikherjee, along with Chris Vale at Swinburne University of Technology in Melbourne, Australia. The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).

Super sound

When clouds of atoms are brought down to temperatures close to absolute zero, they can transition into rare states of matter. Zwierlein's group at MIT is exploring the exotic phenomena that emerge among ultracold atoms, and specifically fermions -- particles, such as electrons, that normally avoid each other.

Under certain conditions, however, fermions can be made to strongly interact and pair up. In this coupled state, fermions can flow in unconventional ways. For their latest experiments, the team employs fermionic lithium-6 atoms, which are trapped and cooled to nanokelvin temperatures.

In 1938, the physicist László Tisza proposed a two-fluid model for superfluidity -- that a superfluid is actually a mixture of some normal, viscous fluid and a friction-free superfluid. This mixture of two fluids should allow for two types of sound, ordinary density waves and peculiar temperature waves, which physicist Lev Landau later named "second sound."

Since a fluid transitions into a superfluid at a certain critical, ultracold temperature, the MIT team reasoned that the two types of fluid should also transport heat differently: In normal fluids, heat should dissipate as usual, whereas in a superfluid, it could move as a wave, similarly to sound.

"Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it," Zwierlein says. "The character of the heat wave could not be proven before."

Zwierlein and his team sought to isolate and observe second sound, the wave-like movement of heat, independent of the physical motion of fermions in their superfluid. They did so by developing a new method of thermography -- a heat-mapping technique. In conventional materials one would use infrared sensors to image heat sources.

But at ultracold temperatures, gases do not give off infrared radiation. Instead, the team developed a method to use radio frequencyto "see" how heat moves through the superfluid. They found that the lithium-6 fermions resonate at different radio frequencies depending on their temperature: When the cloud is at warmer temperatures, and carries more normal liquid, it resonates at a higher frequency. Regions in the cloud that are colder resonate at a lower frequency.

The researchers applied the higher resonant radio frequency, which prompted any normal, "hot" fermions in the liquid to ring in response. The researchers then were able to zero in on the resonating fermions and track them over time to create "movies" that revealed heat's pure motion -- a sloshing back and forth, similar to waves of sound.

"For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth," Zwierlein says.

The experiments mark the first time that scientists have been able to directly image second sound, and the pure motion of heat in a superfluid quantum gas. The researchers plan to extend their work to more precisely map heat's behavior in other ultracold gases. Then, they say their findings can be scaled up to predict how heat flows in other strongly interacting materials, such as in high-temperature superconductors, and in neutron stars.

"Now we will be able to measure precisely the thermal conductivity in these systems, and hope to understand and design better systems," Zwierlein concludes.

This work was supported by the National Science Foundation (NSF), the Air Force Office of Scientific Research, and the Vannevar Bush Faculty Fellowship. The MIT team is part of the MIT-Harvard Center for Ultracold Atoms (an NSF Physics Frontier Center) and affiliated with the MIT Department of Physics and the Research Laboratory of Electronics (RLE).

  • Astrophysics
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Story Source:

Materials provided by Massachusetts Institute of Technology . Original written by Jennifer Chu. Note: Content may be edited for style and length.

Journal Reference :

  • Zhenjie Yan, Parth B. Patel, Biswaroop Mukherjee, Chris J. Vale, Richard J. Fletcher, Martin W. Zwierlein. Thermography of the superfluid transition in a strongly interacting Fermi gas . Science , 2024; 383 (6683): 629 DOI: 10.1126/science.adg3430

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COMMENTS

  1. 1.7: Mechanisms of Heat Transfer

    Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room, but much slower. Heat transfer by convection also occurs through cold air entering the room around windows and hot air leaving the room by rising up the chimney. Exercise 1.7.1 1.7. 1.

  2. Heat transfer physics

    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.

  3. Methods of Heat Transfer

    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.

  4. Heat transfer

    Explanation of heat transfer. See all videos for this article 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 ).

  5. 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 ...

  6. 13.4: Methods of Heat Transfer

    Example 13.4.1 13.4. 1: Calculating Heat Transfer by Convection: Convection of Air Through the Walls of a House. Most houses are not airtight: air goes in and out around doors and windows, through cracks and crevices, following wiring to switches and outlets, and so on. The air in a typical house is completely replaced in less than an hour.

  7. Heat transfer (video)

    Heat transfer (video) | Thermodynamics | Khan Academy High school physics - NGSS Course: High school physics - NGSS > Unit 4 Lesson 4: Thermodynamics Heat transfer Specific heat and latent heat of fusion and vaporization Specific heat capacity Understand: thermodynamics Apply: thermodynamics Science > High school physics - NGSS > Modeling energy >

  8. 11.2 Heat, Specific Heat, and Heat Transfer

    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.

  9. 1.6 Mechanisms of Heat Transfer

    Convection is the heat transfer by the macroscopic movement of a fluid. This type of transfer takes place in a forced-air furnace and in weather systems, for example. Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is ...

  10. 1.4 Heat Transfer, Specific Heat, and Calorimetry

    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.

  11. Heat transfer

    Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy ( heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes.

  12. 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

  13. The Physics Classroom Tutorial

    The thermal conductivity of the same area will be decreased to 0.0039 W/m/°C and the thickness will be increased to 16 cm. Determine the rate of heat transfer through this area of 2.16 m 2. The Physics Classroom Tutorial presents physics concepts and principles in an easy-to-understand language.

  14. 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 ...

  15. 1: Temperature and Heat

    1.S: Temperature and Heat (Summary) Thumbnail: Natural convection plays an important role in heat transfer inside this pot of water. Once conducted to the inside, heat transfer to other parts of the pot is mostly by convection. The hotter water expands, decreases in density, and rises to transfer heat to other regions of the water, while colder ...

  16. Thermal conduction, convection, and radiation

    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.

  17. Heat (Physics): Definition, Formula & Examples

    Heat is what scientists call the form of energy that is transferred between two materials of different temperature. This transfer of energy occurs because of differences in the average translational kinetic energy per molecule in the two materials. Heat flows from the material with higher temperature to the material with lower temperature until ...

  18. Heat Transfer

    Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics. The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal).

  19. Introduction to Heat and Heat Transfer Methods

    14.7 Radiation Discuss heat transfer by radiation. Explain the power of different materials. Energy can exist in many forms and heat is one of the most intriguing. Heat is often hidden, as it only exists when in transit, and is transferred by a number of distinctly different methods.

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

    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.

  21. 14.1: Heat

    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 terms of the units used for these two terms, the best modern value for this equivalence is. 1.000kcal = 4186J. (14.1.1) (14.1.1) 1.000 k c a l = 4186 J.

  22. PDF Heat Transfer Physics

    Heat Transfer Physics: Principal Energy Carriers • The macroscopic conduction, convection, and radiation heat transfer can be understood and analyzed using the four underlying atomic -level (principal) energy carriers. • Phonon (lattice vibration, cause of thermal conduction in electric insulators, and participant in

  23. MIT physicists capture the first sounds of heat "sloshing" in a

    In most materials, heat prefers to scatter. If left alone, a hotspot will gradually fade as it warms its surroundings. But in rare states of matter, heat can behave as a wave, moving back and forth somewhat like a sound wave that bounces from one end of a room to the other. In fact, this wave-like heat is what physicists call "second sound."

  24. 13.1: Introduction

    Heat Transfer and Equilibrium: (a) The soft drink and the ice have different temperatures, T1 and T2, and are not in thermal equilibrium. (b) When the soft drink and ice are allowed to interact, energy is transferred until they reach the same temperature T, achieving equilibrium. Heat transfer occurs due to the difference in temperatures.

  25. Physicists capture the first sounds of heat 'sloshing' in a superfluid

    For the first time, physicists have captured direct images of 'second sound,' the movement of heat sloshing back and forth within a superfluid. The results will expand scientists' understanding of ...