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CHAPTER 24THERMAL HEAT
TRANSFER
CONDUCTION
Conduction is the heat transfer through a material medium from places of higher temperature to places of lower temperature.Conduction is the main method of heat transfer in solids.Molecules in solids are not free to move from place to place.The material that conducts heat faster and is a bad insulator.Metals are good conductors of heat.The material that conducts heat slower and is a good insulator.Non-metals are poor conductors of heat.
CONDUCTION
The diagram shows that i. metal is a good conductor (heat spreads quickly through
the base of the pan, warming the food rapidly).
ii. wooden handle is a bad conductor (heat spreads slowly through the wooden handle so it feels cooler when it is held).
WHAT IS BAD OR POOR CONDUCTOR?
CONDUCTION
Molecular explanation
In metals
Metals are better conductors of heat than non-metals because they contain free electrons.
Due to these free electrons, metals are also good conductor of electricity.
So, heat is transferred from the hot end to the cold end with the help of these free electrons.
Watch: http://www.gcse.com/energy/conduction2.htm
WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
In metals
When the end P is heated, the free electrons there move faster, gaining more kinetic energy and move farther towards Q carrying the energy with them.
WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
As a result, heat energy is transferred from one place to another both by the vibrations of molecules as well as by the movement of free electrons.
P Q
In metals
Diagram below shows heat is conducted quickly by moving electrons:
WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
In non-metals and solids
Non-metals/solids do not have free electrons. This is why they do not conduct heat as well as metals.
When the end P of a solid/non-metal is heated, the molecules at P vibrate faster and gain kinetic energy.
As a result, the neighbouring molecules are affected to vibrate faster.
Finally, the molecules at the end Q also vibrate faster. In this way, heat is transferred from P to Q by
conduction.
WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
In non-metals and solids
Diagram below shows heat is conducted slowly by atoms bumping into each other:
WHY METALS ARE BETTER CONDUCTORS THAN NON-METALS?
P Q
WHY LIQUIDS AND GASES ARE POOR CONDUCTORS OF HEAT?
The molecules in a liquid are further apart than those in a solid.
The molecules of a gas are even further apart than those of a solid and a liquid.
Therefore, it is more difficult to transfer the heat energy or movement of the molecules to other molecules for liquids and gases.
However, heat can still move through them by convection.Watch: Video – water is a bad conductor of heat http://youtu.be/mBaaolWt3PM
Diagram below shows molecules of solids, liquids and gases:
WHY LIQUIDS AND GASES ARE POOR CONDUCTORS OF HEAT?
SOLID LIQUID GAS
CONDUCTION
The transfer of heat through a medium from one particle to another
without any flown of the medium
Different materials conduct heat at different rates
Good Conductors
Define as
Bad Conductors
Insulators
Only take place in Solid
Materials that conduct heat at
faster rate
Define asAlso known as
Materials that conduct heat at
faster rate
Define as
Metals
exam
ple
s
AluminiumCopper
IronBrass
Non-metals,Most liquids
(except mercury),
gases
exam
ple
s
PlasticsWoodWater
Air
Two mechanisms
Take place
through
Atomic or molecular molecules
Free electron diffusion
such as
such as
