1. The problem statement, all variables and given/known data One mole of an ideal gas is in a container with a moveable piston. A 100 N force moves the piston down 1 m; the compression is isothermal. Does 100 J of energy leave the gas or enter the gas? 2. Relevant equations W=Fd U=Q-W=Q-P(Vf-Vi) 3. The attempt at a solution W=P(Vf-Vi). Since Vf<Vi, Vf-Vi<0, so W<0. Hence U=Q-W=Q+W. Since the compression is isothermal, U=0, so 0=Q+W <=> Q=-W <=> Q=100 J. So 100 J of heat enters the gas.