Work on a system +/-


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zfinn
Posts: 106
Joined: Fri Aug 30, 2019 12:16 am

Work on a system +/-

Postby zfinn » Tue Feb 04, 2020 11:27 pm

i'm kind of lost on the concept of how to calculate if work is positive or negative in respect to different things, and also the concept of work being "done on" a system. for example, homework problem 4A 3b. thanks!

Shivam Rana 1D
Posts: 106
Joined: Fri Aug 09, 2019 12:16 am

Re: Work on a system +/-

Postby Shivam Rana 1D » Wed Feb 05, 2020 2:29 am

If work is being done on the system the free energy is increasing because the external environment is transferring energy to the internal system and when the system is doing work like expansion that energy is being transferred outside.

Justin Sarquiz 2F
Posts: 106
Joined: Fri Aug 30, 2019 12:15 am

Re: Work on a system +/-

Postby Justin Sarquiz 2F » Wed Feb 05, 2020 8:13 am

The negative sign in the formula takes into account that the system is doing work, so it is losing energy. For example in w=-P x delta V, if the volume increases (expansion), the system is doing work and is losing energy.

Connor Chappell 2B
Posts: 59
Joined: Wed Feb 20, 2019 12:16 am

Re: Work on a system +/-

Postby Connor Chappell 2B » Wed Feb 05, 2020 8:40 am

When work is done on the surroundings, work is negative. The negative sign is used to indicate that the system is losing energy to the surroundings through doing work (you get tired when you constantly have to push against something that is pushing in on you). When work is done on a system by the surroundings, work is positive. This is due to the fact that when work is done on a system, its internal energy increases (as well as its capacity to do work).

Eunice Nguyen 4I
Posts: 100
Joined: Sat Aug 17, 2019 12:17 am

Re: Work on a system +/-

Postby Eunice Nguyen 4I » Wed Feb 05, 2020 9:39 am

Work is negative if the system does work on surroundings; the system's internal energy decreases and energy is lost.
Work is positive is the surroundings does work on the system; the internal energy of the system increases.


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