Heat Transfer - AQA GCSE Physics Core

Revision notes on convection currents, conduction and radiation.

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  • Created on: 03-01-12 16:54
Preview of Heat Transfer - AQA GCSE Physics Core

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Heat transfer:
Thermal radiation:
The sun emits radiation in all parts of the electromagnetic spectrum.
The Earth's atmosphere absorbs most of the radiation but doesn't block thermal radiation from the sun.
Radiation does not require a material medium. This mean it can travel through a vacuum (Space).
Infrared radiation is the transfer of energy by electromagnetic waves.
Surfaces and radiation:
Darker surfaces are better absorbers than lighter surfaces
Matt surfaces are better absorbers than shiny ones because it has cavities which trap and absorb the radiation.
Dark, matt surfaces are better emitters of radiation
Conduction:
This happens in metals and nonmetals but metals are better at conduction.
This is because metals contain electron that are very loosely attached the atoms.
When one end is heated, the free electrons at the hot end gain kinetic energy and move faster.
The electrons diffuse (spread out) and collide with other free electrons and ions (charged particles) in the
cooler part of the metal.
They transfer the kinetic energy to the other electrons and ions.
In nonmetals, all the electrons are held in the atoms. Therefore the energy transfer only happens when the
atoms vibrate and shake each other.
Energy is always transferred from hot to cold.
Convection:
This only happens in liquids and gases.
When a vessel of liquid is heated at the bottom, the liquid becomes less dense and therefore rises.
Its place is then taken by cooler, less dense liquid moving downwards.
The movement of fluid that is heated and cooled in this way is called convection currents.
Eg: Convection currents are used in hot water tanks. Hot water from the boiler rises into the hot water. The cool
water then replaces the warm water in the boiler, where it is heated.
Saving Energy:
Double Glazing ­ it has two panes of glass with dry air or a vacuum between them. This stops conduction
because dry air is a good insulator and convection because it can't work in a vacuum.
Aluminium foil ­ put it between the wall and the radiator because it reflects heat radiation away from the wall
and into the room.
Loft insulation (fibre glass) ­ it traps the air, which is a poor conductor. Also, because air can't move,
convection is reduced.
Cavity wall insulation ­ reduces conduction because it isn't a good conductor. reduces Convection because
there are no air circulating.
Other:
Pay back time ­ time it takes to recover costs paid for house insulation, with annual savings on energy bills.
A smaller dog would lose heater quicker than a bigger dog because smaller animals have a larger surface area
in relation to their volume. Therefore they lose heat more quickly.
The bigger the temperature difference, the greater the rate of heat lose.

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