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When would we use this equation -work=-nrt(ln(vf/vi))- versus w=-p(changeinv). I get that we use the first one when the system is in equilibrium, but would a problem explicitly say "this system is in equilibrium/ is reversible?"
The first equation is used to find the work done by a reversible process which will not be on the upcoming test. The second equation however, will be on the test as it is the work by expansion.
Specifically, the textbook says that the first equation is used for an isothermal, reversible expansion of a gas. As a gas expands, the pressure decreases according to Boyle's Law which is how the equation developed. The second equation is used for non expansion work, so volume change isn't involved.
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