# Heat Transfer

Brief overview of notes - AQA GCSE Physics Revision Guide P5-9

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• Created by: jenny100
• Created on: 19-06-12 08:42

## Heat Transfer

• Thermal (Infrared) Radiation is the transfer of heat energy by electromagnetic waves<-- that's why it can travel in a vacuum, because it has no particles
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• Conduction and Convection is the transfer of heat energy by particles

Amount of heat radiation depends on surface area:

This is why car engines have fins

The bigger the surface area exposed to the heat radiation the quicker it'll heat up

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Dark matt surfaces absorb heat radiation better than bright glossy surfaces and emit more heat

Silvered surfaces reflect nearly all heat radiation falling on them

That's why you get black solar panels and silver survival blankets

Experiments to denmonstrate this:

• Leslie's Cube
• Melting Wax Trick

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## Conduction

Main form of heat transfer in solids

Process where VIBRATING PARTICLES pass on their EXTRA KINETIC ENERGY to NEIGHBOURING PARTICLES

Metals conduct so well because of their free electrons:

• At the hot end electrons move faster and collide with other free electrons, transferring energy
• Because the electrons can move freely it's a much quicker way of transferring heat through the metal than slowly passing it between jostling neighbouring atoms

The further the electrons have to transfer energy the less efficient it is

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## Convection

Convection occurs when the more energetic particles MOVE from the HOTTER REGION  to the COOLER REGION - AND TAKE THEIR HEAT ENERGY WITH THEM

Liquid particles, when heated, start moving around faster meaning there's a greater distance between them ----> the water expands and becomes less dense

Convection currents are all about CHANGES IN DENSITY

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## Useful Heat Transfers - Thermos Flask

Why is a Thermos Flask a good insulator?

• Glass bottle is double-walled meaning it has a vacuum between the two walls <-- stops all convection and conduction through the sides