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Heat Transfer

by Ron Kurtus (revised 8 November 2014)

Heat transfer is the transfer of thermal energy from a higher temperature object or system to another system, raising its temperature. This process changes the thermal energy of both systems involved until thermal equilibrium is reached.

Thermal energy can be transferred from one end of a material to the other or from one material to another through conduction. It is also transferred indirectly by convection and radiation.

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Conduction

Thermal energy can be transferred from one substance to another when they are in direct contact. The moving molecules of one material can increase the energy of the molecules of the other. Heat can also travel along a material as one molecule transfers energy to a neighboring one. This type of heat transfer is called conduction.

(See Kinetic Theory of Matter for another explanation of this.)

Conduction is mainly seen with solid objects, but it can happen when any materials come into contact. When you put your hand in a container of warm water, you hand it heated by conduction from the water.

Some materials are better conductors of heat than others. For example, metals are good conductors of heat, while a material like wood isn't. Metal heated on one end will soon be hot on the other end too, while that is not true with a piece of wood. Good conductors of electricity are often good conductors of heat.

Since the atoms are closer together, solids conduct heat better than liquids or gazes. This means that two solid materials in contact would transfer heat from one to the other better than a solid in contact with a gas or a gas with a liquid.

Convection

When a gas or a liquid is heated, hot areas of the material flow and mix with the cool areas. This type of heating is called convection. Forced air heating and air conditioning are examples of heating (or cooling) by convection.

This is an effective way of bringing a hot (or cold) fluid to a different area. Convection transfers heat over a distance faster than conduction. But ultimately conduction must transfer the heat from the gas to the other object, though molecular contact.

Radiation

A third method to transfer heat is by radiation. A warm or hot object gives off infrared electromagnetic radiation, which can be absorbed in another object at some separation, heating it up. However, the heat transfer only works in one direction. You cannot cool an object by radiation, as you can with conduction or convection.

Electric heaters use radiation to heat an area. If a fan is added, the device uses both radiation and convection to heat the area.

Transfer of heat by radiation travels at the speed of light and goes great distances, even in a vacuum. We are heated from the Sun through radiation transfer of heat.

(See Infrared Radiation for more information on that subject.)

Summary

Thermal energy can be transferred from one end of a material to the other or from one material to another through conduction. It is also transferred indirectly by convection and radiation.

Atoms or molecules transfer kinetic energy to their neighbors through collisions. When objects are in contact this transfers heat by conduction. Liquids and gases can move high or low energy molecules to another region through convection. Atoms can radiate energy that can energize a distant atom, resulting in heat transfer by radiation.


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