When we refer to constant pressure like in Charles law what is this pressure? The pressure of the gas or the pressure that the container (the piston maybe) exert on the gas? Also does atmospheric pressure is taking into account when we refer to constant pressure?

  • $\begingroup$ The pressure of the gas hence the pressure exerted on the walls of the container by the gas in which the gas is contained. $\endgroup$ – Farcher Dec 31 '18 at 13:38

It is all three things you mentioned. It is the force per unit area that the gas exerts on the wall of the container, the force per unit area that the wall exerts on the gas, and the force per unit area that each parcel of gas exerts on its neighboring parcels (and that they exert on it). If the container is totally isolated from the atmosphere, the atmospheric pressure is irrelevant to the gas pressure inside the container.

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  • $\begingroup$ What do you mean by the container being totally isolated from the atmosphere? If the container is a cylinder with a massless piston isn't the gas pressure atmospheric pressure? $\endgroup$ – Bob D Dec 31 '18 at 14:49
  • $\begingroup$ @Bob D Thanks. That's not what I meant. I meant a gas in a rigid container with no piston. Incidentally, I posted that thing that we were talking about. $\endgroup$ – Chet Miller Dec 31 '18 at 15:06

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