Re: Why do we care about heat?

[Daniel Schroeder <DSCHROEDER@CC.WEBER.EDU>]

I'm not going to try to fight John M's whole battle for him, but
I just have to chip in my .02 worth...

> The conventional name of that experiment is "the mechanical equivalent of
> heat".  The notion that the result of this experiment was "not heat" is
> beyond unorthodox.  It's beyond unconventional.  It's bizarre.

I grew up with the understanding that physicists use heat in two very
different ways:  one formal, the other informal.  The phrase "mechanical
equivalent of heat" is an example of the informal usage, where "heat"
is more or less synonymous with what we more formally call "thermal
energy".  In this sense we *can* ask how much "heat" (thermal energy)
is in a system.  But the formal definition that I was taught was that
"heat" is the Q in the first law:  DeltaU = Q + W.  Here, "heat" is
energy in transit, distinguished from work only by the mechanism of
energy transfer, not by the end result.  At an elementary level,
we call it "heat" (rather than work) if the transfer of energy is
a spontaneous one caused by a difference in temperature between the
two objects.

What's clear to me is that we need two different terms for these
two uses of the word "heat".  I am quite comfortable with "thermal
energy" for the first use, reserving the word "heat" for the second
sense only.  For at least a generation, I believe that this has
been the usage in the vast majority of textbooks (though some use
"internal energy" or simply "energy" in place of "thermal energy").
Perhaps it would have been better to use the word "heat" for thermal
energy, and make up another term for energy transferred spontaneously
from a hot object to a cold object.  Serway's fourth edition calls
it "thermal energy transfer" (as distinguished from "thermal energy
content").  There seems to be a movement these days to call it
"energy transferred by heating".  Personally I don't think either
of these is an improvement.  Probably there is no terminology that
will make the confusion go away.  The subject is inherently confusing.

> What about inductive heating?  Suppose I shine a high-power microwave beam
> onto a chunk of butter.  I think it heats the butter.  Does anybody really
> think this should be described as work not heat?

In a word, yes.

Dan Schroeder
dschroeder@cc.weber.edu