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The first law of thermodynamics is formulated as: the change in internal energy of a system is equal to the heat added to the system minus the work done by the system:

$dU = Q - W$

Then for what reason do many people, like to use the alternative with a plus sign instead:

$dU = Q + W$

Joe
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1 Answers1

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Chemists define work as the work done by a system on it's environment. Physicists define work as the work done on a system by it's environment. So what a physicist would call 120 J of work, a chemist would call -120 J. It's the same thing, it's just a sign convention.

Chemist:

$dU = Q - W$

$W = -120 J$

$dU = Q + 120 J$$

Physicist:

$dU = Q + W$

$W = 120 J$

$dU = Q + 120 J$$

Six of one, half dozen of the other.

Sam Spade
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