Re: [Phys-l] PV question

[Brian Whatcott <betwys1@sbcglobal.net>]

Perhaps if I concede that I defined the process in question too narrowly
(after all the ascii diagram shows that pressure doubles while the 
volume halves, apparently)   then John will not demur if I deny that a 
pressure changing process is isobaric, nor suppose  a volume changing 
process is isochoric?   :-)

Brian W

John Mallinckrodt wrote:
> Biran Whatcott wrote:
>
>   
> > If one specifies a straight line on such a PV graph, then the curve
> > describing the pressure volume and temperature change during its
> > compression  will necessarily cut some isotherm (actually many of  
> > them)
> > TWICE.
> >     
>
> Not so.  Not in the case of an isobaric or isochoric process or, for  
> that matter, many other processes.  Consider, for instance, any  
> process for which P(V) = P_o + C (V - V_o) with C a positive  
> constant.  In fact, depending on starting and ending volumes, many  
> processes with negative C also satisfy the requirement.  All of these  
> processes can be carried out reversibly.
>
>   
> > The implication (for me) is that this straight line is not a  
> > description
> > of a reversible adiabatic. process.
> >     
>
> That's *certainly* true, but you don't need to and, in fact, can't  
> deduce that result from any of the foregoing observations.  A  
> reversible adiabatic process is described by a curved line on a PV  
> graph, period.
>
> John Mallinckrodt
> Cal Poly Pomona
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