Re: why pseudowork (NOT)

[John Denker <jsd@MONMOUTH.COM>]

At 12:28 PM 10/30/99 -0400, Robert A Cohen wrote:
> Could someone please tell me why we need to really
> understand the microscopic "goings-on" that produces friction in order to
> identify whether it does work or not?

That's exactly the question, isn't it!

I'd say there are two main viewpoints:

a) According to the first viewpoint, we do *not* need to understand the
microscopic goings-on.  We choose a macroscopic timescale which allows us
to define work in terms of the macroscopic "F dot dx" of the main part of
the sliding block and similarly for the stationary table.  This leads to a
consistent, natural, and conventional partitioning of the energy-exchange
into "work" and "heat".

b) According to an extreme version of the other viewpoint, we could choose
to examine the microscopic goings-on.  If we watch things closely enough,
the ultramicroscopic interactions obey nondissipative reciprocity
relationships, so there is no such thing as heat or entropy or temperature.
 According to this viewpoint, 100% of any energy exchange is work, not heat.


Remarks:

One might narrowly argue that viewpoint (b) is permssible because it does
not violate any of the classical equations of motion.

However, it does violate the quantum-measurement laws, because you simply
cannot watch a real block or table that closely.  There are classical
analogs to these laws which might be invoked also.

More broadly one can argue that viewpoint (b) is unconventional and (for
most problems) unhelpful.

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