Generally, a piston is kept on top of a cylinder containing the gas. The gas is heated and it expands causing a change in the volume. I was wondering how might one keep the pressure constant. Websites on the Internet have illustrated this by keeping a single weight on the piston throughout all the measurements. Does this really guarantee a constant pressure? As the height of the piston increases (if the gas is expanding) then shouldn't the atmospheric pressure fall? My book mentions that if it happens slowly enough then the pressure can be considered as constant, however I don't understand what the speed of the piston's motion has anything to do with maintaining constant pressure. Please Help.
[Physics] How is pressure kept constant in Charles’ Law
pressurethermodynamics
Best Answer
If the weight and piston are motionless the weight will exert a constant downward force on the piston. This force is balanced by an equal upward force caused by the gas pressing against the inner surface of the piston.
If the piston accelerates, then the two forces will no longer balance because a net force is needed to make the piston and the weight on top of it change their motion. The imbalance lasts the entire time the piston and weight are accelerating.
If you imagine very slow motion that only accelerates for a brief period at the very start and finish, and proceeds at constant speed the rest of the time, then the situation comes very close to that when the piston doesn't move--the forces balance almost the whole time.