[galpinj]

Hi all,

Quick question about the equation Work = -PΔV. I was watching a video on Khan Academy, and he mentioned that during a reversible process (point 1 to point 2), we need to calculate the change in volume x ext. pressure to find the work. However, it's clear in the video that infintesimal changes in volume are accomponied by infintesimal changes in pressure. Why can chemists ignore the change in pressure without ignoring the change in volume? Both variables are changing throughout, so I feel like ΔP would also need to be measured to find the work done.

Link:https://www.khanacademy.org/science/chemistry/thermodynamics-chemistry/internal-energy-sal/v/pv-diagrams-and-expansion-work
(at about the 7 minute mark)

Thank you

[galpinj]

Any ideas from the community? I still haven't been able to understand why pressure can be held constant when looking at U = Q - W --> U = Q -PΔV

[Borek]

In general - it can't. But that's how the math in infinitesimal changes work, change in P is so small we can ignore it when calculating PdV. When we move to PΔV we need to integrate though.