# Re: Energy, heat

16 Dec 1999 03:40:38 +0100

Rob Harris <rob@hbinternet.co.uk> wrote:

> What exactly is the difference between energy and heat ?

This is one of the basics of thermodynamics, which is probably the least widely understood part of classical physics. (I'm clueless about it, too.)

In a pinch: Energy can be used to do work (mechanical, electrical, etc). Heat can't.

>From the viewpoint of energy conservation, heat is just another
kind of energy. If you do any work, some energy is invariably dissipated as heat.

(Which has unpleasant consequences for a closed system that starts out with a limited amount of energy. If you do work, eventually all the energy is converted into heat, and that's it.)

Other than serving to create the temperature of a particular physical environment, heat is a waste product. Converting heat back into useful energy is difficult. In order to do this, you need a heat flow from a warmer to a colder temperature zone. Even then, only a fraction of the total heat flow can be turned into energy (and thus work). The higher the temperature difference, the better the efficiency. This is why modern fossil fuel burning power plants or combustion engines try to burn their fuel at the highest achievable temperatures, and why geothermal sucks as a commercial energy source.

(In a closed system, all temperature differences are eventually leveled out. Along with the assumption that our universe is a closed system, that's what the "heat death of the universe" phrase is about. It's not about high temperature, but rather about the final state where all energy has been converted to heat, and the temperature is uniformly the same. That's a pretty sorry state to be in.)

> I can't think of one example where heat is a different entity
> from energy in concrete reality.

Part of the problem is that the term "energy" can exclude or include heat, depending on whether you're talking thermodynamics jargon or not. Thermodynamically speaking, you can convert energy into other kinds of energy (losing a bit as heat in the process in non-ideal systems) or into heat, but you can't just the same convert heat into energy.

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