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Explanation of Heat Transfer by Ron Kurtus - Succeed in Understanding Physics. Also refer to physical science, thermal energy, temperature, kinetic energy, conduction, convection, radiation, molecules, School for Champions. Copyright © Restrictions
Heat Transfer
by Ron Kurtus (revised 26 May 2006)
Heat is thermal energy that is transferred between two bodies due to a difference in temperature. Heat transfer is the process of increasing the kinetic energy of a material's particles from a material of high temperature to one of lower temperature. It can also be thought of in the opposite direction of cooling an object by slowing down its particles. Heat can be transferred to other materials through conduction, convection and/or radiation.
Questions you may have include:
- How is heat transferred by conduction?
- What is convection?
- How is heat transferred by radiation?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
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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 gasses. 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, heating it up. Electric heaters with the shiny reflector use radiation to heat. If they add a fan, they use both radiation and convection to heat an 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
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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Resources and references
The following resources provide information on this subject:
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Books
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books on Thermal Energy
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books on Heat Transfer
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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Heat Transfer