How do I know the sign convention for $W$ and $Q$ in thermodynamics?

Asked Mar 07 '20 at 05:51

Active Mar 07 '20 at 06:32

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Can someone correct me if I am wrong.

For example,

43 kJ work energy has been done on the system, sign convention is +43kJ, because work is entering into the system

62 kJ work energy has been generated from the system, sign convention is -62kJ, because work is entering out of the system

21 kJ of heat energy supplied to the system from its surroundings, sign convention is +21kJ, because heat energy is entering into the system.

I apologize for my unclear phrasing. The actual question is here:

I got mixed up with the sign convention.

edited Mar 07 '20 at 06:29

Qmechanic

asked Mar 07 '20 at 05:51

kkksrct

1

You can't know what the convention is without any context. That's what makes it a convention -- it's up to whoever is writing it what the sign is. – knzhou Mar 07 '20 at 05:57
What is unambiguous is that is $X$ joules of work is done on the system, then $-X$ joules of work is done by the system. If somebody just tells you "the work is $W = 1$ J", you don't know they mean $X = 1$ or $-X = 1$. Because it's up to convention. – knzhou Mar 07 '20 at 05:58
Possible duplicates: https://physics.stackexchange.com/q/37904/2451 , https://physics.stackexchange.com/q/39568/2451 and links therein. – Qmechanic Mar 07 '20 at 06:31

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