Re: entropy

[palmer@sfu.ca (Leigh Palmer)]

My favorite example (and there are many) of the use of a term in
ordinary and scientific parlance is the term "in general". In
ordinary speech the use of this term implies the existence of
exceptions. The mathematical and physical meaning of the term is
exactly opposite the ordinary!

>    So, it is not unreasonable to use the everyday word of DISORDER
> to explain the meaning of the scientific word ENTROPY. The difficulty
> lies in the fact that the word DISORDER is not very well defined.

I see. It is reasonable to use the word after you have deprived it of
its ordinary meaning. Why use it at all, then, if you are just going
to use it to make entropy clearer? Why not define entropy directly?

>    Indeed, DISORDER refers really to the INFORMATION we have about
> the system. Example: If you take a deck of cards, and have no idea of
> its previous history, and you should guess which is the third card
> from the top, then you have no idea of the correct answer and any
> guess is as good as any other. AND THIS REMAINS TRUE even if the
> deck of cards is perfectly ordered as it came from the factory.
> This because you DON'T KNOW IT. So, don't tell me that everybody has
> a CLEAR IDEA of what ORDER and DISORDER means in common language.
> It is such a discussion which we should have with students when
> trying to make them understand the concept of entropy.

That won't wash. The entropy of a system does not depend one tiny bit
on "the INFORMATION we have about" it! If you believe that it does,
you believe in magic. The entropy of a physical system (apart from a
question about its zero point) is a function only of its thermodynamic
coordinates, and those depend not at all on human knowledge. My entire
problem with teaching this particular pernicious factoid is that it
tends to mystify physics. Students hear it and quite justifiably think
that physicists believe in magic.

Leigh