## Chapter 8 #5 [ENDORSED]

Ariana Murillo 2O
Posts: 21
Joined: Tue Jul 19, 2016 3:00 am

### Chapter 8 #5

Why do you add the energies? I thought you have to subtract to calculate change.

Maggie Bui 1H
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Joined: Fri Jul 22, 2016 3:00 am
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### Re: Chapter 8 #5  [ENDORSED]

Usually we subtract a final quantity from an initial quantity to calculate a change, but in this case, we should be thinking about what is affecting the system and how it is affecting it. Whether to add or subtract a quantity depends on its sign, which is determined by how the system is affected by the quantity. Energy expended by the system is negative, but energy added to the system is positive.

Raymond_Guan_1H
Posts: 21
Joined: Fri Jul 15, 2016 3:00 am

### Re: Chapter 8 #5

Another way to think of it is to use DeltaU = q + w since the energy in the cylinder is used as heat and the piston is also exerting work on it. Thus, you can find the internal energy.

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