It is said that in an isobaric expansion of a gas pressure remains constant throughout the expansion process. Suppose we have a quantity of gas at initial pressure P1 and volume V2 in a piston cylinder arrangement.We heat it slowly such that it expands to obtain a state with pressure P2 and volume V2. Now if the process was isochoric P1=P2 and V2>V1.So the work done is said to be the displacement work done by the gas on the piston as ∫PdV=P(V2-V1) [as pressure remains constant]. But on the other hand for a system to expand there must be an imbalance in the external(surroundings) and internal(system)pressures.But in an isochoric process expansion takes place even if the pressure is said to be same at all instances of the process! That is, if P1=P2=P(system pressure) how does the gas push the piston back? Since the system was under equilibrium with the applied external pressure at state point 1 when external force was balanced by the internal pressure(force) by the gas on the piston multiplied by piston area.At state point 2 there is no change in pressure and hence internal pressure of the system is the same while the external pressure is still the same!Then how does the piston move back?