Change in Internal Energy

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Hellen Truong 2J
Posts: 50
Joined: Sat Jul 22, 2017 3:00 am
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Change in Internal Energy

Postby Hellen Truong 2J » Sat Jan 20, 2018 2:49 pm

When you compress a system, is that considered the system doing work or work being done on the system? How does that affect internal energy and how can I remember?

Ju-Wei Wang 1I
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

Re: Change in Internal Energy

Postby Ju-Wei Wang 1I » Sat Jan 20, 2018 3:09 pm

Compression is work being done on the system. Remember the equation is delta U = q + w. Internal energy, U, will increase because work (w) is a positive value.

Nishma Chakraborty 1J
Posts: 51
Joined: Fri Sep 29, 2017 7:04 am

Re: Change in Internal Energy

Postby Nishma Chakraborty 1J » Sat Jan 20, 2018 3:12 pm

Hi!

Compression of a gas means that work is being done on the system. When we look at deltaU= q + w, we are looking at it from the perspective of the system. So because the system is not losing any energy by doing work (its surroundings are instead), the sign of work will be positive. So, compression increases the internal energy of a system.

-w=when the system is doing work
+w=when the surroundings are doing work on the system

Hope this helps :)

Samantha Joseph 1F
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Re: Change in Internal Energy

Postby Samantha Joseph 1F » Sat Jan 20, 2018 3:15 pm

If it helps to remember, it's basically the first law of thermodynamics! If you think about the "compressing a piston" example, the person is clearly doing work by pushing on the piston, or exerting a force over a certain distance. And force x distance = work, which is a form of energy.


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