Ideal Gas Law and an Isobaric Process

[nothing123]

Hi,

So let's take the standard example of a gas in a container with a piston at the top. Charles' Law states that at constant pressure, an increase in temperature (kinetic energy of gas molecules) will increase the volume. This makes sense both conceptually and mathematically (per PV = nRT). However, in an isobaric process (pressure is constant again), the kinetic energy of the gas molecules is what is moving the piston so it must have lost some energy in doing so. Therefore, although initially Esystem = q that was added, it's net energy change would be Esystem = q - Pext*V.

So this is my problem, wouldn't this isobaric process be inconsistent with the ideal gas law? That is, using strictly the ideal gas law, woudn't the ending temperature not take into account the work done on the piston? Or are we assuming in using the ideal gas law that the work to keep the pressure constant is from an external source?

Thanks.

---

Members don't see this ad.

 

[engineeredout]

So this is my problem, wouldn't this isobaric process be inconsistent with the ideal gas law? That is, using strictly the ideal gas law, woudn't the ending temperature not take into account the work done on the piston? Or are we assuming in using the ideal gas law that the work to keep the pressure constant is from an external source?

Work is a function of changing temperature in an isobaric process. W = -R(T2 - T1). Work also equals -PdV. All of this is consistant with the ideal gas law.

I get what you're saying and it may seem confusing, but don't worry, the work is in there.

 

[nothing123]

Thanks for your reply but you're going to have to convince me a little more. Are you saying that the temperature given by E (of the original system) + Echange (q - Pext*V) would be equal to the temperature given by T = PV/nR?