I've gone through a some of our textbooks but none of them effectively explains this process clearly. For the isothermal process, according to the first law of thermodynamics, ΔU = 0, indicating that Q = -W. So if heat flows into the system, all the energy put in is converted to work which the gas does on the surroundings. However, microscopically, how do the molecules exert more force on the wall (i.e. more pressure) without a change in their kinetic energy (i.e. their internal energy)? Taking a container with a movable piston, if the gas moves the piston by work - doesn't that mean that the molecules have gained more kinetic energy to move it? Thanks in advance!