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Thermal Physics

Heat Pumps

If a heat engine is operated in reverse, as described above, it has the effect of transferring internal energy from a body at a low temperature to one at a higher temperature. It is then called a “heat pump” (or a refrigerator depending on what it is used for).

A heat pump or fridge can be represented by a similar diagram to the one used for the heat engine but with the arrows representing energy flows reversed.

 

 

An explanation of the operation of a fridge requires consideration of cooling caused by evaporation.

The temperature of a body is a measure of the average kinetic energy of its particles. During evaporation, the molecules which are more likely to "escape" from liquid and become part of the vapour are the ones which have higher than average kinetic energy. Therefore, if you cause the rate of evaporation of a liquid to increase, without supplying energy, the temperature of the remaining liquid will decrease.

The rate of evaporation of a liquid can be increased by

i) decreasing the pressure acting on its surface
ii) blowing air over the surface (clothes dry more quickly on a windy day)
iii) increasing the surface area of the liquid (evaporation only occurs at the surface)
iv) increasing the temperature

The diagram below shows the main parts of a refrigerator.

In the tubes around the freezer compartment, the pressure is decreased by the pump (there is a small section of the tube which is narrower than the rest). Rapid evaporation takes place here and latent heat of vaporisation is taken in.

In the tubes outside the refrigerator, the vapour is compressed and then it condenses. Latent heat is given out as it condenses.

A favourite question about refrigerators:

A refrigerator is in a perfectly insulated room. Somebody leaves the door of the refrigerator open. Will the temperature of the room decrease, increase or stay the same?

The phase change, liquid to vapour (inside the fridge), occurs at low temperature. During this phase change, latent heat of vaporisation is taken in.

When the vapour is compressed (in order to change it back into a liquid) there is an increase in temperature. In other words, the phase change, vapour to liquid (outside the fridge), occurs at high temperature. Latent heat of vaporisation is given out but also heat will be lost as the liquid cools down to the same temperature as the surroundings.

Heat taken in = mLv

Heat given out = mLv + mcDELTA01T

Obviously, the heat given out is greater than the heat taken in. This answers the question but where does this extra energy come from?

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