Work on a system vs. work by a system


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Franca Park 3J
Posts: 64
Joined: Fri Sep 25, 2015 3:00 am

Work on a system vs. work by a system

Postby Franca Park 3J » Wed Jan 27, 2016 9:53 pm

Can someone explain the difference between work done on a system and work done by a system? If you were given data such as question 8.7 in which it states, "The internal energy of a system increased by 982 J when it absorbed 492 J of heat. Was work done by or on the system?," how would you figure it out?

Nikhil Davuluri 2A
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Work on a system vs. work by a system

Postby Nikhil Davuluri 2A » Wed Jan 27, 2016 11:21 pm

When a system does work it loses energy; therefore, its internal energy decreases.
Also, when a system absorbs heat it gains energy and its internal energy increases.
In this example the internal energy increases by 982 J. The energy gained from absorbing heat is only 492J so the system still needs to gain 490J. This energy comes from work, and since the system is gaining energy work must be done to the system (if the system did work then the system would lose energy). As a result, 490 J of work is done on the system.

704628249
Posts: 43
Joined: Fri Sep 25, 2015 3:00 am

Re: Work on a system vs. work by a system

Postby 704628249 » Thu Jan 28, 2016 11:15 am

Imagine a container with a piston on top. Work done on the system would be if someone pushed the piston down. This would be Positive Work. Work done by the system would be if the piston pushed up into the surrounding environment, increasing the volume of the system. This would be Negative Work.


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