Re: [Phys-l] Entropy Increase Question

[Leigh Palmer <palmer@sfu.ca>]

Does that happen in a thermally isolated, sealed (no evaporation), constant volume system? I was unaware of that.

There is a problem that I think is serious in the teaching of the concept of entropy. The idea that entropy is somehow a measure of the disorder of a system is a dangerous one. That idea is really either wrong or else it is tautologous. In the latter case it is of little use; in the former case it is detrimental to understanding - and seductive to the lazy student.

If one construes disorder in the common sense then a shuffled pack of cards in arbitrary order is more disordered than a pack arranged in rank order within suits, as it emerges new from its box. All else being equal, however, the disordered pack has exactly the same entropy as the orderly pack. There is no contribution to the entropy of that system due to the sequence in which the cards are stacked. None. Not approximately zero, but there is exactly zero contribution to entropy due to disorder.

This idea of a gloriously faceted gem obviously raised my hackles, feeling as I do about this disorder thing. If this phenomenon does indeed occur, and I have no reason to doubt it, then I think I understand it as being a system (the irregular seed crystal and solution) in which thermodynamic equilibrium has not quite been reached initially.

Leigh

On 16 Mar 2010, at 09:00, Chuck Britton wrote:

> Excellent example!!
>
> In irregular seed crystal evolves into a gloriously faceted gem.

after I had written:

> Yes, there are situations in which the condition of a system can change without suffering an increase in entropy. In order for this to occur, the system must be initially in a state of thermodynamic equilibrium. It then can, in principle, undergo spontaneous changes to other states which have the same entropy. (Note that I have added a constraint to your question which, I expect, you failed to mention. If not, then of course any reversible adiabatic process carried out an the system will be isentropic.)
>
> A simple instance of such a change which might be observable would be in a system of solid crystals of a pure substance in contact with a saturated solution of the same material. The configuration of the crystals will change over time - spontaneously - without incurring a ticket for violation of the Second Law.