Piston and value of W

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Samantha Chung 4I
Posts: 77
Joined: Wed Oct 03, 2018 12:16 am

Piston and value of W

Postby Samantha Chung 4I » Wed Jan 30, 2019 11:10 pm

Why is it that in a closed system, when you move the piston up, the W value is negative, but when you move the piston down, the W value is positive (what Dr. Lavelle mentioned in lecture today)?

Mona Lee 4L
Posts: 60
Joined: Fri Sep 28, 2018 12:16 am

Re: Piston and value of W

Postby Mona Lee 4L » Wed Jan 30, 2019 11:23 pm

When you move the piston up, the system is doing work on the piston so the system loses energy. Because the system loses energy, the W is negative. When you move the piston down, the opposite occurs.

Jessica Tsui 1H
Posts: 30
Joined: Fri Sep 28, 2018 12:21 am

Re: Piston and value of W

Postby Jessica Tsui 1H » Thu Jan 31, 2019 9:39 pm

Work done by the system is energy lost, so w is negative. Work done on the system is energy gained, so w is positive.

Madeline Motamedi 4I
Posts: 62
Joined: Fri Sep 28, 2018 12:23 am

Re: Piston and value of W

Postby Madeline Motamedi 4I » Fri Feb 01, 2019 1:15 pm

I think the best way to approach any one of these problems is to first identify what your system is. In this case, the air inside is the system so you must look to see what is being done to it or by it. If the gas's volume expanded it pushes the piston and thus it is the one doing work. On the other hand, if the piston moves it down, the piston is doing work to the gas within.


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