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Container with movable piston at top and valve near bottom is filled an ideal gas. Valve is opened and some gas slowly escapes. Valve is then closed and the piston appears to be at a lower position. System is in thermal equilibrium. Is final pressure of container, greater, less or equal?
Equal: If you find the sum of the forces, you will find that the final pressure and initial pressure are equal because the weight of the piston and the atmospheric pressure remain the same throughout the stages.