Re: [Phys-l] PV question

[Carl Mungan <mungan@usna.edu>]

> Themo question
> If I have a PV diagram where the process travels diagonally along a straight
> line from lower right to upper left (B to A), can a generic statement be
> made where this shows NEGATIVE work BY the gas because the volume decreases.
> ...regardless of the change in pressure?
>
> ASCII art below
>
>
>  2Po+\A
>     | \
>     |  \
>     |   \
>   Po+    \B
>     ++---+-
>      Vo  2Vo

Yes, assuming the process is done reversibly (that is, slowly and 
without dissipation, so that there is no bulk KE of piston or gas, 
and no turbulence in the gas or friction between the piston and 
cylinder).

The POSITIVE work done ON the gas is the area under the diagonal line 
(down to the V-axis at zero pressure) which is 1.5*Po*Vo in your 
diagram. You can call that negative work done by the gas if you like, 
although it sounds a bit artificial to speak that way. Is there some 
particular problem or point in mind?

--
Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/