### W=-(Pex)(deltaV)

Posted:

**Fri Jan 19, 2018 11:22 am**Can someone explain what the negative sign means on this equation?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=130&t=25815

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Posted: **Fri Jan 19, 2018 11:22 am**

Can someone explain what the negative sign means on this equation?

Posted: **Fri Jan 19, 2018 11:27 am**

There is a negative sign in this equation because energy is being lost as work when the system is expanding and this causes the internal energy of the system to decrease.

Posted: **Fri Jan 19, 2018 1:27 pm**

As the volume increases and the system expands, energy is lost as work so there is a negative sign.

Posted: **Fri Jan 19, 2018 5:56 pm**

Wait, so is the work in perspective of the system?

Posted: **Sat Jan 20, 2018 12:24 pm**

Yes, when a system does work, work leaves the system -- resulting in a net loss of work from the "perspective of the system."

Posted: **Sun Jan 21, 2018 8:46 pm**

The negative sign is relative. Because we are calculating the work that the system performed on, say, a piston, the work represents the energy that was lost or gained by the system.

If the final volume increased (expansion), then the system did work and, hence, the system lost energy

If the final volume decreased (compression), then work was done on the system and, thus, the system gained energy

If the final volume increased (expansion), then the system did work and, hence, the system lost energy

If the final volume decreased (compression), then work was done on the system and, thus, the system gained energy