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Postby melissa_dis4K » Thu Jan 31, 2019 8:24 pm

How do you know when work is positive or negative. I know that when F and displacement are in the same direction work is positive but in the homework 8.3b.) 6th edition I got confused. Thanks!

Cynthia Aragon 1B
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Joined: Mon Apr 09, 2018 1:38 pm

Re: Work

Postby Cynthia Aragon 1B » Thu Jan 31, 2019 8:41 pm

According to my understanding In the first law of thermodynamics : Q +W ; W is positive if work is done by the system and negative if work is done on the system.

Millicent Navarro 1I
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Joined: Fri Sep 28, 2018 12:25 am

Re: Work

Postby Millicent Navarro 1I » Fri Feb 01, 2019 1:25 am

When given the term of work, I think of the amount of energy being transferred from one form to another in order to do something. Personally, some keywords that help when determining work are: work being done (positive), does/doing (negative).

Faith Fredlund 1H
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Joined: Fri Sep 28, 2018 12:18 am

Re: Work

Postby Faith Fredlund 1H » Fri Feb 01, 2019 10:34 am

w is positive when the system does work , and w is negative when work is being done on the system. The same goes for heat; Q is positive when heat is added, and Q is negative when heat is taken out or absorbed by the surroundings. Both of these can be seen in the equation
ΔU= w + Q

Danny Zhang 4L
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Joined: Fri Sep 28, 2018 12:26 am

Re: Work

Postby Danny Zhang 4L » Fri Feb 01, 2019 1:19 pm

I believe that the main things to consider about work in thermodynamics are that negative work is done onto the system(compression of gas) and that positive work is done by the system(expansion of gas).

Michael Novelo 4G
Posts: 64
Joined: Fri Sep 28, 2018 12:28 am

Re: Work

Postby Michael Novelo 4G » Fri Feb 01, 2019 6:34 pm

In addition to the previous statement we can think about it using the analogy Lavelle gave about using an airpump on a tire. We use energy when pumping air, the energy is lost and the system which is the bike gains the energy. We exert negative work and the bike has positive work.

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