Re: dW turns into dQ

[John Denker <jsd@MONMOUTH.COM>]

At 09:16 AM 11/1/99 -0600, Joel Rauber wrote:
> Instead of trying to decide if some activity was dW or not, lets decide if
> it is dQ or not.

A fine idea.  Let's see where it leads.

> dQ arises from that part of an energy transfer between two systems in
> contact

[pause]

> (not to be viewed as a flow, but rather as an accounting terminology)

I don't mind calling it a flow.  But an accounting is OK too.

> that occurs by virtue of a difference in temperature between the systems.

Whoa!  I cannot endorse such a narrow restriction on the meaning of dQ.
There exist of course certain restricted problems for which that is the
only form of dQ that need be considered -- but that is certainly not the
general case.

> The transfer of energy that arises from any other means is dW; that is, the
> complement to what is dQ.

Depending on how much one reads into that statement, it might lead to a
subtle misconception, namely that every energy-transfer process has a
definite way of partitioning the energy between dQ and dW.

There's an entire concept that seems to have been short-changed in this
discussion, namely the idea of a dissipative process that creates new heat
where there was none before.

As a pair of illustrative examples:

  a) Suppose I hook a battery to a motor.  It seems reasonable to say that
the energy that leaves the battery is not heat.  It has very low entropy.
This electrical energy is converted by the motor to mechanical forms of
low-entropy energy, i.e. work.

  b) Suppose in contrast I hook a battery to a resistor.  Once again the
energy leaves the battery in a low-entropy non-thermal form.  You can say
the battery does electrical work.  But what happens in the resistor?  The
energy is immediately thermalized.  There is no place in the resistor where
the result is anything other than heat, on any halfway relevant timescale.

  In this case (b), from the battery's point of view it is work, but from
the resistor's point of view it is heat.

I suppose somebody will accuse me of trying to "have it both ways" but I
stand by my analysis.  Heat is not conserved.  Work is not conserved.
Energy is conserved.  In this example, energy is dissipated -- i.e. work is
converted to heat.

In such a conversion process, asking whether "the" energy is heat or work
is asking an unanswerable underspecified question.

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com