Re: COLLISION 2

[Brian McInnes <bmcinnes@PNC.COM.AU>]

It shows how careful we have to be with students when we can so easily
misinterpret each other.

In replying to Leigh, John D wrote:
> > Change in entropy is not associated with an actual process;
>
> Huh?  I would have said just the opposite.  Entropy doesn't just
> change.  There has to be some actual process that changes it.

Leigh was not insinuating that there wasn't an actual process.  When
he wrote "the change in entropy" he was referring to the difference in
the final value of the entropy of a system and the initial value of
the entropy of that system after a process had occurred that resulted
in a change in entropy.  When he wrote "is not associated with an
actual process' he was pointing out that the value of the change, the
number of entropy units, depended on the initial and final states and
NOT on the actual process.  Leigh then went on to state how to
calculate the value of the entropy change, as follows:

> > it is associated with *any* hypothetical reversible process
> > capable of taking an identical system from the same initial to the
> > same final state as the system in question.
>
> Huh?  I wouldn't have said that at all.

Well, certainly I would, John, and to borrow a phrase from you:"a lot
of very fine physicists .. would say that"

>  Entropy changes occur in
> irreversible as well as reversible processes.  And entropy changes occur
> under lots of conditions that are not closed cycles.  And not hypothetical,
> either.

Of course entropy changes occur in all changes of state, no matter
what the process is: the key thing that Leigh is saying is that the
value of the change does not depend on the process but on the initial
and final states.

Note that if the process is a closed cycle then the change in the
entropy of the system (not the change in the entropy of the
surroundings) is zero.
Further, it is only for a cyclic reversible process (found only in an
ideal physics shop with massless strings and frictionless pulleys)
that the entropy changes of both the system and its surroundings ( or,
as sometimes, somewhat grandiosely, written "the entropy change of the
universe") are zero.  For a non-cyclic reversible situation, the
entropy change of the system and the entropy change of the
surroundings are equal in magnitude and opposite in sign.

Brian McInnes