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### Sign for compression vs expansion

Posted: Sat Mar 17, 2018 1:46 am
When using the equation for w=-P$\Delta V$ is it true that for compression the answer will be positive and for expansion it will be negative?

### Re: Sign for compression vs expansion

Posted: Sat Mar 17, 2018 1:49 am
Yes, compression is a negative delta V, which multiplied by negative P, gives you a positive value for work.
Same methodology for expansion, positive delta V.

### Re: Sign for compression vs expansion

Posted: Sat Mar 17, 2018 1:52 am
To determine the sign, think about if the work is done to the system or by the system. W is positive if work is done by the system, and negative if work is done on the system

### Re: Sign for compression vs expansion

Posted: Sat Mar 17, 2018 2:00 am
Yes, that is correct. You can assume pressure is 1 atm and then plug in values for delta V to check. If you compress something, Vf-Vi will be negative, so the two negative signs will combine to make a positive value. Delta V is positive for expansion, so the value will end up being negative.

### Re: Sign for compression vs expansion

Posted: Sat Mar 17, 2018 11:19 am
W=-P∆V. ∆V = V_f - V_i.