Re: [Phys-l] T dS versus dQ

[John Denker <jsd@av8n.com>]

On 02/12/2010 02:50 PM, Carl Mungan wrote:
> > If you push/pull on the gas with a sound transducer, you
> > can heat it up just fine, with no need for implausibly
> > high velocities or other heroic measures.  The heating
> > _per cycle_ will be small, but if you do it 10,000 times
> > per second for a long time, it adds up to something quite
> > significant.
>
> Fair enough, but is it really all that different in spirit from a 
> human arm? 

Not different speed-wise.  Just different frequency-wise.
Whether that counts as different spirit-wise is a matter
of interpretation.  I'll leave that up to you and your
spiritual advisor :-).

> Say I use a diaphragm of a loudspeaker (not exactly what 
> you had in mind, but perhaps you'll allow me to stretch to make the 
> point) with a peak-to-peak motion of 1/2 mm. At 10 kHz, that's going 
> to mean average speeds on the order of 10 m/s (and peak speeds higher 
> yet), comparable to what I'm going to do with the adiabatic apparatus 
> I'd guess. Granted you've got to run longer because your compression 
> ratio is much smaller ... but I was just thinking about the speeds. 

All in all, we've seen quite a few irreversible things that
can happen in our gas:
 -- thermal conductivity across a non-infinitesimal
  temperature difference (energy transport)
 -- viscous drag (momentum transport)
 -- sudden high-velocity movement of the piston
 -- cumulative effect of many small low-velocity movements
 -- et cetera

==================

I renew my warning that there are two or three different
yet well-established technical meanings for the word
"adiabatic".  You can use the word however you like ... 
just don't expect people to know what you mean by it, 
unless you explain it each time.

 -- adiabatic approximation as opposed to sudden approximation
  (having to do with entropy production inside the region)
 -- adiabatic meaning thermally insulated
  (having to do with the boundaries of the region)
 -- both of the above
  (which means the region will be isentropic)

I've been bitten by this bug more times that I care to
admit.  I've given up on the word.  My advice:
 -- If you mean gentle as opposed to sudden, say
  "gentle" or "non-sudden".
 -- If you mean thermally insulated, say "thermally
  insulated".
 -- If you mean isentropic, say "isentropic" or
  "constant entropy".  And even then you need to
  say _which_ entropy is being held constant.  The
  entropy of region 1?  The entropy of region 1 and
  region 2 together?
 -- If you mean reversible, say "reversible".