Re: [Phys-l] PV question

["Edmiston, Mike" <edmiston@bluffton.edu>]

Perhaps I don't understand the intent of the question.  Others have pointed out that the phrase "NEGATIVE work BY the gas" is not conventional wording because we consider work as work done *on* the system rather than work done "by" the system.

There certainly was a time when most physical chemistry textbooks defined work as work done by the system, and that meant PV-work would be calculated as (integral of PdV).  That's the way I learned it in college between 1968 and 1972.  Sometime shortly after I graduated from college the physical chemists started switching to the convention that anything going into the system should be positive, and this meant PV-work on a system would be calculated as (*negative* integral of PdV).  I was under the impression everyone now considers work with respect to a system as positive if work is going into the system.  Therefore, PV-work is (negative integral of PdV), and this will always come out positive if the system decreases in volume, and it will always come out negative if the system increases in volume.

So... a generic statement can be made that the conventional view of PV-work means the work will be a positive quantity if the volume of the system decreases.  That means one could indeed say that a decrease in volume shows *negative work by the gas*, but I would prefer that we not mix people up by sometimes talking about work as being done *on* the system and other times talking about work done *by* the system.

I went through all that confusion by being in graduate school when the chemists were changing, and half the physical chemistry textbooks had it one way, and half had it the other way.  When I was a TA, we actually had some of the profs using one convention and other profs using the other.  That meant some profs were writing the first law of thermodynamics as (U = q - w) and others were writing it as (U = q + w) and somehow the undergrads and their TAs were supposed to keep it straight from one class to the next.  That was not pleasant.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.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