Compression (Question 8.3)

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Cassidy 1G
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Joined: Fri Sep 29, 2017 7:07 am

Compression (Question 8.3)

Postby Cassidy 1G » Wed Jan 17, 2018 7:28 pm

Question 8.3 is about compressing a bike pump. Part B asks if the work is positive or negative with respect to the air in the pump. The answer is positive, but why? To me it seems like it would be negative.

Audrey Goodman 1F
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Joined: Fri Sep 29, 2017 7:06 am

Re: Compression (Question 8.3)

Postby Audrey Goodman 1F » Wed Jan 17, 2018 8:44 pm

Since the work is done on the air (which is our system), the work is positive. The air in the pump is compressed.

Xin He 2L
Posts: 38
Joined: Fri Sep 29, 2017 7:05 am

Re: Compression (Question 8.3)

Postby Xin He 2L » Wed Jan 17, 2018 8:48 pm

Whenever work is being done on the system, in this case the air in the pump, it is positive. If, for example, the air becomes decompressed and does work on its surroundings, the work would be negative.

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Re: Compression (Question 8.3)

Postby aTirumalai-1I » Wed Jan 17, 2018 9:15 pm

Can someone explain what Dr. Lavelle was trying to clarify with the tire example and the compression of the bike pump he was talking about in class today? I don't think I completely followed it.

Thanks in advance!
Last edited by aTirumalai-1I on Wed Jan 17, 2018 9:18 pm, edited 1 time in total.

Wilson Yeh 1L
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Joined: Fri Sep 29, 2017 7:06 am

Re: Compression (Question 8.3)

Postby Wilson Yeh 1L » Wed Jan 17, 2018 9:17 pm

The answer is positive because when compressing a bike pump you're doing work and increasing the internal energy inside the pump. Basically, the energy is being transferred from you into the air inside the bike pump. As such, the energy is positive. It would be negative if energy was released in the case of say an expansion.

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