Sign for compression vs expansion

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Kassandra Molina 2B
Posts: 43
Joined: Wed Nov 16, 2016 3:03 am

Sign for compression vs expansion

Postby Kassandra Molina 2B » Sat Mar 17, 2018 1:46 am

When using the equation for w=-P is it true that for compression the answer will be positive and for expansion it will be negative?

GabrielGarciaDiscussion1i
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Joined: Fri Sep 29, 2017 7:04 am
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Re: Sign for compression vs expansion

Postby GabrielGarciaDiscussion1i » 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.

diangelosoriano
Posts: 49
Joined: Fri Sep 29, 2017 7:07 am

Re: Sign for compression vs expansion

Postby diangelosoriano » 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

Angel Gomez 1K
Posts: 36
Joined: Fri Sep 29, 2017 7:04 am

Re: Sign for compression vs expansion

Postby Angel Gomez 1K » 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.

melissa carey 1f
Posts: 53
Joined: Fri Sep 29, 2017 7:06 am

Re: Sign for compression vs expansion

Postby melissa carey 1f » Sat Mar 17, 2018 11:19 am

W=-P∆V. ∆V = V_f - V_i.


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