Re: [Phys-l] PV question

[Brian Whatcott <betwys1@sbcglobal.net>]

It's always good to see that Physics emeriti  are confused on some point
raised in an interest list:  welcome to MY club!  :-)

In this particular case, the note starting "Themo Question..." and 
continuing with "ASCII art below" seems to originate with an 
unattributed quote of a note from some other list from Carl M. rather 
than a continuation of the examination of reversible processes which 
preceded it. Carl then makes the same points as Leigh.

So the essence turns out to be:
if gas is compressed, is that work done on it, or negative work done by 
[or on] it?
To which there is general agreement on the appropriate response.

Perhaps there was an intention to trap some hapless engineer into taking 
the position that expansion provided by a gas is positive work? As Leigh 
notes, that was an older convention....     :-)


Leigh Palmer wrote:
> 3.The convention that work done by a gas is defined as positive comes from engineering, where the desired work is that done by an engine. I am old enough to have learned that convention first. The convention that is now in popular use among physicists is the opposite; we define the work done on a system as positive, and the increment of mechanical work is - PdV.
Just as the conventional polarity of work done by expanding gas came 
from engineering, I suppose it is the case that the topic of 
thermodynamics itself came from engineering.   But conventions change.....

Brian W