# Extracting heat energy from a material

Does it violate any physical laws to take a portion of the energy out of a system and use it? Specifically I'm referring to heat. (Kinetic energy).

For example, if you have a material which has a lot of energy and has a high temperature, is it possible to take a portion of that energy and use it for something else, and leave the material cooler?

I realise it is impossible to cool it to absolute-zero because of the third law of thermodynamics.

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Think about this one next time you open the refrigerator =) – Matt Calhoun Mar 2 '11 at 7:44
@Matt, that seems like a different kettle of fish. The fridge is consuming energy to move heat from the inside to the outside. If it were possible to extract heat energy (and if we knew how to do it, technically), then the fridge could power itself. – Ozzah Mar 2 '11 at 7:50
Every "system" is also a "subsystem" in some larger system. – Matt Calhoun Mar 2 '11 at 8:05

Extracting thermal energy is possible, doesn't violate any laws of thermodynamics and is done routinely every day.

http://en.wikipedia.org/wiki/Heat_engine

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youtube.com/watch?v=si1iGuigAiQ – Janne808 Mar 2 '11 at 8:37

Yes, this is OK. BTW heat is not generally thought of as being kinetic energy - at a microscale it is, but because the kinetic energy is randomly distributed is cannot be directly, usefully and 100% harnessed as kinetic energy. So the concept of "heat" is a useful way of making this distinction.

Remember always though that heat only flows from a hotter body to a colder body.

If you imagine the heat from my warm hands flowing into a hot cup of tea that I am holding, you realize that this is not what actually happens in real life. This example does not violate any laws about conservation of energy, but it does violate the Second Law of Thermodynamics, which is why it doesn't happen.

As @Janne808 said, extracting heat energy and turning it into work is done all the time (e.g. automobile engine, boiler and steam turbine, thermoelectric junction etc). Heat engines are governed not only by the conservation of energy, but also by Carnot's Law or the Second Law of Thermodynamics (which places a theoretical limit on the fraction of heat that can be converted into work; the rest must be rejected as waste heat to a lower temperature region)

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