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# Re: [Phys-l] PV question

• From: Carl Mungan <mungan@usna.edu>
• Date: Fri, 22 Jan 2010 12:51:18 -0500

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/