From: Christian Weisgerber (naddy@mips.rhein-neckar.de)
Date: Wed Dec 15 1999 - 19:40:38 MST
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.
-- Christian "naddy" Weisgerber naddy@mips.rhein-neckar.de
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