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In lecture today, Dr. Lavelle described that a gas expanding against a vacuum is reversible, and that as it expands, it does work against the piston (loses energy) that is then compensated by the heat (gain in energy). I'm a little confused, however, how could the gas be said to be doing work when it's expanding against a vacuum? Thank you so much!
I believe the piston had a string with mass attached to it on the outside of the chamber. Not fully sure, but I would recommend looking at the picture in your notes again.
As I recall from the book, a system does no expansion work when it expands into a vacuum because there is not an opposing force (pressure is 0). However, in Lavelle's lecture today, I believe he included a mass that was outside the vacuum chamber attached to the piston by a string. Because the expansion of the gas, though in a vacuum, raises the mass and work, as explained in the book is "the transfer of energy to a system by a process that is equivalent to raising or lowering a weight," work is being done.
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