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Heat radiation

We feel heat energy from the Sun.
In an eclipse of the Sun the heat and light are cut off at the same moment.
A greenhouse keeps the heat in it.
Electric fires have shiny reflectors behind them.
Houses in hot countries are often painted white.
Glass in greenhouses is often painted white in summer.
Artificial satellites are highly polished.
A highly polished stainless steel teapot will take longer to cool down than one of the same size with a rough dark surface.
Aluminium foil is sometimes put behind a radiator in a house.
A vacuum flask has shiny metal surfaces.
The fins on a motorcycle engine should be painted black.
You can control your TV with a remote controller.
Firemen search for people in smoke filled buildings with infra red detectors.

We can explain all these things it we know something about the transfer of heat energy by RADIATION. This radiation is invisible to the human eye but the radiation from hot bodies can be felt by your hand. There are various sensitive detectors but a very simple one is a thermometer with a blackened glass bulb.

The proper name for heat radiation is INFRA RED RADIATION.

All objects emit heat radiation. If we take two objects of the same material and the same size but at different temperatures the one which is hotter will emit more radiation. Two objects of the same material and the same temperature but of different sizes emit different amounts of heat radiation – the bigger one emitting more radiation than the smaller one.

Investigating heat radiation

Heat radiation can be studied with quite simple apparatus.

Using the heater as a source of heat radiation and your hand as a detector you can find out the following things:

1. Heat radiation travels through air.
2. Heat radiation travels very fast - if you put a book between your hand and the heater the heat stops at once.
3. Long wavelength heat radiation in the far infra red does not travel through glass.
4. Heat radiation is absorbed better by black surfaces than by shiny ones

You can show this last property by sticking a thin piece of aluminium foil on the back of your hand and testing the heat received from the heater. If you now paint the foil black (using carbon black dissolved in meths) and test the heat again you will find that it feels much hotter than without the black although the heater was giving out the same amount of heat radiation in both cases.

Some surfaces emit heat better than others. If you heat up a metal plate which has one side black and the other shiny you will find that if you test the heat emitted the black side emits much more heat than the shiny one. It is important to remember that both sides of the plate were at the same temperature.

You may find it interesting to measure the heat radiation emitted by each other's hands.

The results of the emission of radiation experiments can be summarised as follows:

Black surfaces are the best emitters of heat radiation.
Black surfaces are the best absorbers of heat radiation.
Shiny surfaces are the best reflectors of heat radiation.
Shiny surfaces are poor absorbers of heat radiation.
Glass will not transmit long wave infra red radiation.

These properties of heat radiation have the following effects and applications:

1. Radiators in houses do not actually radiate very well; they ought to be painted black. Car radiators usually are a dark color so that they can get rid of the heat efficiently.
2. Aluminium foil is sometimes put behind radiators to reflect the heat out into the room,
3. A vacuum flask has shiny surfaces on the vacuum side to prevent heat loss,
4. White clothes are worn in summer as they reflect the heat better than dark ones.
5. The reflector behind an electric fire should be kept clean so that it will reflect the heat well.
6. A greenhouse keeps warm inside because of the properties of the glass. The short wavelength radiation from the Sun can get in but because the plants and soil are cooler they emit longer wavelength radiation which cannot escape through the glass.
7. Spacecraft have shiny surfaces to reflect the radiation from the Sun.
8. Highly polished teapots will keep hot longer as their surfaces do not give out heat so welt.
9. White washed buildings will keep cooler in hot weather than darker ones.

Crookes Radiometer

This classic piece of apparatus was devised to show the difference between the absorption of heat radiation by black and silvered surfaces. When heat radiation falls on the vane the black sides absorb heat better than the shiny sides, this means that they hat up more quickly. The air next to the black sides heats up, expands and so pushes the vane round.

Some versions of this apparatus have a vacuum inside – see if you can find out how these work and which way the vane will rotate.

Leslie's cube

Leslie's cube is a metal box with four differently painted sides.
If it is filled with boiling water from a kettle you can use either your hands or a thermopile connected to a meter to try and decide which surfaces give out the most heat radiation.

Remember that all the sides of Leslie's cube are at the SAME TEMPERATURE and so are both sides of the metal plate although some surfaces give out more heat radiation than others.

These experiments should have proved to you that:

1. Heat radiation travels through air
2. It travels at the same speed as light
3. It will not travel through glass
4. Black surfaces are the best emitters of heat radiation
5. Black surfaces are the best absorbers of heat radiation
6. Shiny surfaces are the best reflectors of heat radiation
7. Heat radiation can travel through a vacuum – that’s why we can feel heat from the Sun

Heat radiation is rays or waves of heat energy. In spite of what some people think, wearing clothes of different colours does not make a lot of difference to how well we can loose heat. It is the layers of air between our bodies and the clothes that really matter. Also the colour of a car is a lot less important than whether it is shiny or not.

© Keith Gibbs 2012