8.17 Work done by a system


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Ashley Davis 1I
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Joined: Fri Sep 29, 2017 7:04 am

8.17 Work done by a system

Postby Ashley Davis 1I » Wed Jan 17, 2018 5:08 pm

I'm confused about work in general for a couple reasons. Why is it that when a system does work, its work value is negative? Also during condensation, how is it that work is done ON the system rather than by it?

William Satyadi 2A
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Joined: Sat Jul 22, 2017 3:00 am

Re: 8.17 Work done by a system

Postby William Satyadi 2A » Wed Jan 17, 2018 5:13 pm

When a work is done by a system the work is considered negative because the system is losing energy. For instance, if we considered our bodies as the system, whenever we perform work, move, or exercise we use up some of our energy reserves, subtracting from our total energy. Since we are losing energy, we consider the work as a negative. In the same way, if work is performed ON a system, energy would be entering, increasing the total energy, making it a positive work to the system.

Ya Gao
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Re: 8.17 Work done by a system

Postby Ya Gao » Wed Jan 17, 2018 7:31 pm

Hey, if a work is done by the system, it is considered negative because the system as a whole is losing energy. Since condensation requires energy, work has to be done to the system so that condensation can occur. Hope that helps.

Adrian Lim 1G
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Re: 8.17 Work done by a system

Postby Adrian Lim 1G » Tue Jan 23, 2018 4:47 pm

For 8.17, the solutions manual says delta E is 0 when change is occurring at a constant temperature. is deltaE just the same as deltaU?

Humza_Khan_2J
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Re: 8.17 Work done by a system

Postby Humza_Khan_2J » Tue Jan 23, 2018 4:55 pm

Since the system is at constant temperature, we can take into account the fact that q=-w. Therefore, deltaU must be 0, so I suppose that the solution equation deltaE and deltaU.

Yeyang Zu 2J
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Re: 8.17 Work done by a system

Postby Yeyang Zu 2J » Fri Jan 26, 2018 12:38 am

Work done by a system: the energy of system released to do work, which means the system loses ENG
Work done on the system: the energy done by the surroundings, like compressing the air, which means the internal energy of the system increases

Charles Ang 1E
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Re: 8.17 Work done by a system

Postby Charles Ang 1E » Fri Jan 26, 2018 5:27 pm

Work is negative because when we work with systems that have zero internal energy, the q and w cancel out. Because in the formula U=q+w they are both on the same side, we can manipulate q=-w when U=0.


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