Work

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KarenaKaing_1D
Posts: 37
Joined: Fri Sep 25, 2015 3:00 am

Work

Postby KarenaKaing_1D » Wed Jan 13, 2016 8:45 pm

Work is equal to -PV. Can someone clarify why there is a negative sign please? Lavelle went over it during lecture.

Prina Patel 1H
Posts: 62
Joined: Fri Sep 25, 2015 3:00 am

Re: Work

Postby Prina Patel 1H » Wed Jan 13, 2016 8:48 pm

There is a negative sign because whenever a system does work it loses energy because its pushing against the pressure. Having a positive sign means its gaining energy which doesn't make sense.

Vivian Van Ngo 2A
Posts: 5
Joined: Fri Sep 25, 2015 3:00 am

Re: Work

Postby Vivian Van Ngo 2A » Wed Jan 13, 2016 11:47 pm

Work is negative because is the system is releasing energy to combat pressure.

Lovelyn Edillo 4F
Posts: 15
Joined: Fri Sep 25, 2015 3:00 am

Re: Work

Postby Lovelyn Edillo 4F » Thu Jan 14, 2016 3:26 am

The negative sign is there to signify that the system has lost energy, because it has to expend work to push against the external pressure on the system.

Sarah Noorani 3L
Posts: 30
Joined: Fri Sep 25, 2015 3:00 am

Re: Work

Postby Sarah Noorani 3L » Thu Jan 14, 2016 10:12 am

The negative sign in -PdeltaV must always be used when calculating work (even when work is done on the system). When work is done on the system, the gas will be compressed. This means that the final volume will be less than the initial volume, so the change in volume will be negative. A negative value for the change in volume multiplied by a negative pressure will result in a positive value for the work, which makes sense because the system is gaining energy through work. When work is done by the system (gas expands), the final volume is greater than the initial volume. This gives a positive value for the change in volume and therefore a negative value for work (because the positive change in volume is multiplied by the negative pressure). This makes sense because the system is losing energy when it does work, so the work must be negative.


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