Two pistons are placed into a thermally insulated cylinder as shown. The top piston is heavy and does not conduct heat. The bottom piston is very light and conducts heat easily. Initially, the system is in thermodynamic equilibrium, and the distance between the pistons (x) is the same as the distance between the bottom piston and the bottom of the cylinder. Each “half” contains one mole of helium.

Then the amount of heat Q is slowly delivered to the gas. What must be the force of friction acting on the bottom piston in order for that piston to remain in place? The top piston can slide without friction.

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