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Infrared emission and absorption
The surface of an object is really important when it comes to how infrared radiation is absorbed or emitted. For example, if you're wearing a black t-shirt on a hot day, infrared radiation in sunlight will be absorbed very well by the black matt surface - you'll warm up very quickly!
Black matt surfaces are also very good emitters of infrared radiation once the object has become warm.
On the other hand, light shiny surfaces are poor absorbers and poor emitters of infrared radiation. They're also good reflectors. In hot countries, the houses are often painted in white or light colours. This is to keep the house cool as the white surfaces are poor absorbers (and good reflectors) of heat radiation.
Another example is a shiny teapot. The shiny light surface of the teapot is a poor emitter of infrared and so the teapot stays hot for longer.
Sometimes we need objects to lose their heat and remain cool. For example, the cooling fins at the back of a refrigerator. These are usually painted a black colour with a matt finish, so the fins can emit infrared heat quickly to keep the refridgerator cool.
Key Points and Some Examples...
Remember that heat energy will always transfer from a hotter object to a cooler object - never the other way round. Also, a hot object will emit more heat radiation than a similar cooler object.
Here are 4 possible combinations of teapots and surroundings:
1. A BLACK/MATT teapot that has HOT water in it on a cold winter's day:
- The black/matt surface will emit infrared heat radiation very well. This means that heat energy will be transferred quickly away from the teapot to the cooler surroundings and the teapot will get cooler quickly.
- The surroundings are at a lower temperature than the teapot and so do not emit as much infrared radiation to the teapot. So even though the teapot is a good absorber of radiation (black/matt), it will not absorb much radiation from the surroundings and so won't heat up much... Meanwhile the teapot is emitting a LOT of heat radiation and getting cooler.
Hope that makes sense so far?
2. What about if the BLACK/MATT teapot contains ICED water on a really hot day?
- <>The teapot is cold, so it won't be emitting much infrared radiation - even though it has a black/matt surface.
- The hotter surroundings emit a lot of infrared heat to the teapot which is an excellent absorber because it is black/matt. This means that heat energy transfers from the surroundings to the teapot.
3. Now for the more likely scenario where we have a WHITE/SHINY teapot full of HOT tea on a cold winter's day:
- The white/shiny teapot is a poor emitter of heat radiation. It is hotter than the surroundings, so heat will be transferred from the teapot to the surroundings, but only slowly because of its white/shiny surface. Therefore, it will take a longer time for the teapot to cool down compared to a black/matt teapot, thankfully - otherwise we'd have cold tea... 🙂
- The surroundings are much cooler than the teapot, and so only emit a small amount of heat radiation to the teapot (a lot of which is reflected away by the white/shiny surface anyway).
4. Finally, what about if we have a WHITE/SHINY teapot full of ICED water on a really hot day? In this case, the surroundings are at a higher temperature than then teapot, so...
- The white/shiny surface is a good reflector of infrared radiation and so even though the hotter surroundings are emitting a lot of infrared radiation to the teapot, only a little bit of this will be absorbed by the teapot. Therefore, it will take a long time for the iced water teapot to heat up to the same temperature as the surroundings.
A key point in all of this is that when the teapot has reached the same temperature as the surroundings, it will emit and absorb heat energy at the same rate. The surface colour and texture would make no difference and the same amount of heat is transferred in both directions. We could say that in this situation, the surroundings and the teapot are in 'thermal equilibrium'.
GCSE Physics Exam Keywords: Absorber, Emitter, Shiny/Matt, Light/Dark, Infrared radiation