I was taking a course in thermodynamics, where the instructor started to derive the formula for work in case of isobaric process for an ideal gas It stated W = -∫ P_ext dV And then equated it with -∫PdV where P was the internal pressure. I am sure that it is not a misunderstanding because later this PdV was written as d(PV) (because P is stated to be constant) and further written as d(nRT) This brings me to my question.
Is it the external pressure or the internal pressure that is constant in a isobaric process? The above equations indicate that it is the internal pressure which us constant but for isobaric process, it is the external pressure vs volume graph that has a horizontal line
If it is internal pressure that is constant which seems to be the case to me, how is it, that we can equate internal pressure and external pressure?