## W=-(Pex)(deltaV)

$\Delta U=q+w$

Liam Maxwell 2E
Posts: 53
Joined: Fri Sep 29, 2017 7:07 am

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

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

LMendoza 2I
Posts: 41
Joined: Sat Jul 22, 2017 3:00 am

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

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.

Katie Lam 1B
Posts: 52
Joined: Fri Sep 29, 2017 7:06 am

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

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

Michael Cheng 1C
Posts: 51
Joined: Thu Jul 13, 2017 3:00 am

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

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

Silvino Jimenez 1A
Posts: 52
Joined: Fri Sep 29, 2017 7:04 am

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

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

Diego Zavala 2I
Posts: 65
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

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

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