Sign of ΔS

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Sign of ΔS

Postby RyanTran2F » Sat Jan 27, 2018 3:29 pm

On #9.1A why did the answer key say ΔS = -qrev/T and then have the answer as a positive value? When do we use a negative sign in the formula? I thought it was when entropy decreases but then on #9.5 the answer key also used a negative in 1 value but not the other? So how do you know if we should make it negative or positive? Thanks :)

Gevork 2E
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Re: Sign of ΔS

Postby Gevork 2E » Sat Jan 27, 2018 6:26 pm


so the idea here is what you are looking at, either your system or your surroundings. So for the questions with the negative, note: "At what rate does your body heat generate
entropy in your surroundings, taken to be at 20. C? I believe that this is the case. Same with question 9.5. You need to see specifically what is being affected and where transfers of heat are occurring.

Hope this helps.

Masih Tazhibi 2I
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Re: Sign of ΔS

Postby Masih Tazhibi 2I » Sun Jan 28, 2018 3:49 pm

The sign for the change in entropy comes from the q value in the numerator. Thus, if your system is releasing heat, the change in entropy of the system is decreasing, and vice versa.

Nicole Anisgard Parra 2H
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Re: Sign of ΔS

Postby Nicole Anisgard Parra 2H » Wed Jan 31, 2018 10:23 pm

The q value in the numerator indicates whether heat is flowing to the system or being released by the system. If your system is releasing heat, the entropy in the system decreases and the entropy in your surroundings increase.

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