How to know when q is negative (7.11 from the Textbook)

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Mia-Kyla Coronel 2N
Posts: 7
Joined: Fri Sep 26, 2014 2:02 pm

How to know when q is negative (7.11 from the Textbook)

Postby Mia-Kyla Coronel 2N » Sun Jan 11, 2015 4:33 pm

7.11: In a combustion cylinder, the total internal energy change produced from the burning of a fuel is -2573 kJ. The cooling system that surrounds the cylinder absorbs 947 kJ as heat. How much work can be done by the fuel in the cylinder?

The solutions manual states that q=-947, making the end answer 1626 kJ.
How would we know this?

Thank you in advance!

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: How to know when q is negative (7.11 from the Textbook)

Postby Niharika Reddy 1D » Sun Jan 11, 2015 5:28 pm

The cylinder is the system and the cooling system makes up the surroundings. Since the surroundings are absorbing heat, the system (cylinder) must be giving it off, or releasing it. q is negative when heat is released.

Lauren Mendoza 3H
Posts: 2
Joined: Fri Sep 26, 2014 2:02 pm

Re: How to know when q is negative (7.11 from the Textbook)

Postby Lauren Mendoza 3H » Wed Jan 14, 2015 10:07 pm

Just a tip for you: I think it helps to always remember that q regards to the heat of the system you are focused on. The question is phrased to seem like heat is gained, but since it is the surroundings that gained heat, the system is actually losing heat. Another thing that can reveal losing heat is the fact that the system is doing work, and therefore losing heat to the surroundings.


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