Pressure in our system, if we assume that it was just so small In our process video, we had a bunch of pebbles. We had a situation where the ceiling was movable. Saw that a system could do work by expanding. Work = Area under the curve in a PV diagram, that's all there's to it. Sal initially used 'bigger' stairs to drive home the point that infinitesimally small quasistatic processes, we practically get a smooth slide, just like how the actual process should be. Now, if we use smaller and smaller 'stairs' to describe this process, i.e. So, he had to use stairs, because in each individual stair, the pressure is constant, in order to show us, the viewrs, that Work = the area under the curve. Instead, Sal simply went down in the pressure for every new beginning of a new quasistatic process, kinda like how you go down in stairs, 'abruptly', and not in a 'smooth' way, like in a slide.īut why did Sal choose to'go down in stairs' instead of a 'smooth slide'? Well, recall that Sal proved that W=Px∆V, and he used that fact to show the viewer that Work = area under the curve in a PV diagram. He just took the ∆P in incremental steps instead of constantly taking its change, and we can clearly see in the PV diagram that Sal didn't ignore the ∆P because if he had done so, our area under the curve would've been a simple quadrangular. In the video, Sal didn't actually ignore the ∆P as the volume changed. Your question is 8+ years old, and my answer is most probably not relevant to you, but it may be relevant to future readers.

Workdone in thermodynamics