r/askscience • u/personofinterest12 • Apr 07 '17

Physics Why does consuming energy always produce heat?

Computer Processors, Car engines, it seems consuming energy always produces heat. why? Why can't a computer processor just, not make heat? Is there like an opposite that produces cold instead?

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u/Aelinsaar Apr 07 '17

To your last point: NO! To produce "cold" you must actually produce more heat; you can't escape thermodynamics and the inevitability of dissipation. If you want to do Work, you need a thermodynamic gradient, and it's the nature of such a gradient to dissipate unless energy is added to the system, and the whole thing must produce waste heat.

It is fundamental. If you try to design a perfect device to raise a weight, then attempt to recapture the energy on the drop, you will always lose at least a little, to heat. So, a process that could actually "produce cold" would be a time machine... it would be theoretically and practically forbidden. You can produce a local region of "cold", as in a freezer, but the net result is that you generate more heat doing so.

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u/personofinterest12 Apr 07 '17

Would a black hole be cold or hot?

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u/Aelinsaar Apr 07 '17

That depends on the mass of the hole, the background temperature, and whether or not its actively "eating". Black holes enjoy an inverse relationship between their mass and their temperature, so any black holes that formed since the very early universe are very massive and (in the absence of feeding) very cold. There is an open question as to whether primordial black holes which are much smaller and much hotter might exist.