How is isobaric process physically possible?

[Omar Nagib]

We have an ideal gas enclosed in cylinder whose top is covered by a piston of certain weight $mg$. At this stage, the piston is at rest which means the force by which the gas acts on the piston ($F=PA$ where $P$ is the gas pressure and $A$ is the cross sectional area of the cylinder) is equal to $mg$.

Now if this system was subject to an isobaric process, then its temperature and volume changes with its pressure held constant(suppose $T$ and $V$ increase). But this is confusing, since during the process the piston is continuously changing its position, which implies its being acted upon by a certain net force; Now the weight of the piston $mg$ is constant, so the force $F=PA$ by which the gas acts on the piston must have increased, but since $A$ is constant, therefore $P$ must have increased; therefore $P$ is not constant.

So How isobaric process is physically possible?

 

[Andrew Mason]

The external pressure (atmospheric pressure + mg/A) is constant. So the gas pressure just has to be a tiny bit greater than this in order to expand. You just have to have a process that is slow enough so that there is no significant acceleration (ie. keeping acceleration small so that adding ma/A<<mg/A). For example, if the process is an isobaric expansion caused by heat flow into the gas, the rate of heat flow must be sufficiently slowly so that temperature and volume increases slowly. That way, the gas pressure remains virtually identical to external pressure, which is constant.

AM