how in reversible condition P external is equal to P internal plus minus DP Share with your friends Share 0 Mehvash Shams answered this Dear student,if we have compress the gas then Pext > Pint.So Pext=Pint+ dp.Now if gas is expand then Pext < Pint,in this case Pext=Pint-dp..So pext=pint+/- dp.Regards. 0 View Full Answer