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Re: isobaric expansion



Leigh, may I add a comment to yours? I assume we are assuming a standard
adiabatic cylinder with a piston on one end and a hot plate like thing on
the other. See my web page for a graphic.

What Leigh says is quite correct. Further, the original poster correctly
assumes that as the molecules strike a stationary piston elastically they
do not loose energy/"momentum"/"velocity".

BUT for a MOVING piston -- required for an expanding volume -- even though
the collision of the molecules with the piston maybe elastic, they DO loose
energy. One can see my web page for the calculation, but I think that my
team is in the process of updating it so It may not be totally legible --
read discombobled.-- but one can get the idea.

Jim Green

At 11:00 17 03 2000 , you wrote:
At 9:05 AM -0800 3/17/00, Savinainen Antti wrote:
>Hello,
>
>I have a question on isobaric expansion (this is taken from the paper
>published by Rozier and Viennot):
>îAn ideal gas is heated at constant pressure. Its volume and temperature
>both increase. Why?î
>
>The question is quite easy to answer from the point of view of energy.
>Heat transferred to the system is partially used to mechanical work (=
>P*deltaV) and partially to increase kinetic energy of the molecules and
>hence temperature.
>
>But what happens in a molecular level? The molecules bounce from a piston
>and exert a force on it. When temperature increases the molecules have
>greater momentum which results to greater force in collisions. Increase in
>volume decreases number of collisions per time unit. The overall effect is
>that average force per area, pressure, remains constant.
>
>Is there momentarily greater pressure exerted on the piston for instance
>in the beginning of the process? A student of mine asked this and I
>couldnít convince him that pressure is constant all the time (thatís why
>it is called isobaric!). Have I given the correct argument or am I missing
>something?
>
>Regards,
>
>Antti Savinainen
>Kuopio Lyseo High School/IB
>Finland

If you consider the details of the process then the pressure must differ
any time the piston is accelerating. Your explanation (which is very clear
and should be understood by any student) pertains to equilibrium states
through which the system passes during the process of isobaric expansion
or, alternatively, to comparison of two static equilibrium states. If the
piston is accelerating then clearly thermodynamic equilibrium does not
describe the system.

Leigh

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen