We know that dW = F.dxIn an isothermal system, pressure is being applied by the gas which moves the piston upwards.So dW = Pgas.A.dx = Pgas.dVBut in all books it is mentioned that dW=Pext.dV.Can you please clarify on this?

Here the pressure is external to the system and opposing the expansion, dw=Pext*dV

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regards

Ramesh

Sir, The work is done against external pressure, but the work is being done BY the gas, so we consider the force that the gas exerts. Work done by a body depends upon the force exerted BY the body, not on the force against which the work is being done, right?? For example, let us consider two men A and B pushing a block in opp. directions with force F and G by A and B resp. Then work done by A = F.x Then why are we considering the opposing factor in case of gases?