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Can heat be transfer from low temperature body to high temperature body? Because according to second law of thermodynamics it can be transfered. But how?

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  • $\begingroup$ It will help us answer if you describe your understanding of how the second law allows this. Unless @KevinKostian has answered your question, in which case please accept his answer. $\endgroup$
    – garyp
    Commented Jan 15, 2015 at 15:11
  • $\begingroup$ i want to know how heat can be transferred from lower temp body to higher temp $\endgroup$ Commented Jan 15, 2015 at 15:20

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On a tiny scale there is energy is more likely to be transferred from hot to cold, but it occasionally goes the other way. "How" is simple: suppose a cold object is in contact with a hot gas. On average, the cold object's molecules have less energy, but once in a while an unusually slow gas molecule will happen to bump to an unusually fast object molecule and gain energy. However, on a macroscopic scale these fluctuations are average out so it is exceedingly unlikely to transfer heat the "wrong" way.

Heat pumps, such as air conditioning, have the overall effect of transferring heat from cold to hot (as well as consuming useful energy from electical mains). However, they do this by allowing a liquid to evaporate (which makes it colder than the indoor air) and then compressing the vapor back into liquid (which makes it hotter than the outdoor air).

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@Kevin is right, but let me try to give a simpler answer.

Just look at how a refrigerator works. It transfers heat from the cold box (making it colder) to the warm air outside (making it warmer).

It can do this because it is not a closed system. The way it is not closed is that it has energy input, which drives the pump. The air outside is doubly warmed - by the heat removed from the box, and by the heat of the energy that drives the pump.

This is an example of a heat engine. The earth itself is a heat engine, taking energy from the sun (hot) and radiating it to space (cold). It can use that energy to decrease entropy locally, driving natural systems.

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