Re: [Phys-l] Arrow of Time Issue

[Bennett Sessa <bcsessa@tx.rr.com>]

If you have a very messy desk, the entropy of the system is x. If you clean up the desk area, then it would seem that the entropy would decrease, but you must do work to clean the desk, and I'd you do work then you are jostling the air particles, which causes friction (and other things) that result in a thermodynamic shift. An example of this is a Carnot engine and Beckenstein-Hawking black hole entropy.

Heat does flow from a warmer reservoir to a cooler one, but the system is "thrown out of" thermodynamic equilibrium, causing a change (increase) in entropy. An example is Clausius theorem.



On Mar 9, 2012, at 12:09 PM, John Denker <jsd@av8n.com> wrote:

> On 03/09/2012 05:39 AM, Bennett Sessa wrote:
> > Could you explain to me why entropy can decrease (without stating the
> > work (ie that experiment just mentioned) of someone else?
>
> It depends on what you mean by "the" entropy.  The obvious answers
> are:
> a) The entropy "S(i)" of the ith region can decrease, to the extent 
>  that entropy flows across the boundary into some neighboring 
>  region(s).  This sort of thing happens all the time.
> b) The entropy "S" of an entire isolated system does not decrease.
>
> In this context, on 03/09/2012 07:28 AM, carmelo@pacific.net.sg wrote:
> > > If you play a game of cards, you may get straight flush or full house  
> > > after some time.
> > > If there are 1 million cards in your house, and you can randomly  
> > > choose 1000 cards again and again. Would you repeatedly get the same  
> > > set of cards after some time?
>
> In context, that's a red herring.  That is to say, it's irrelevant
> and misleading.
>
> A particular hand, such as a full house, is a microstate.  Entropy is
> by definition a property of the macrostate, i.e. a property of the
> ensemble.
>
> Let's be clear:  
> -- If it's a property of the microstate, it's not the entropy.
> -- If it's the entropy, it's not a property of the microstate.
>
> This point is discussed at
> http://www.av8n.com/physics/thermo/entropy.html#card-cut-twice
> http://www.av8n.com/physics/thermo/entropy.html#sec-peek
> and especially
>  http://www.av8n.com/physics/thermo/entropy.html#sec-surprise-value
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