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18 November 2009

Mailbag -- thermodynamics sign convention?

From Jonathan Kirby, an Atlantan:

"I have a quick question for you. We are just getting into thermodynamics, and I was wondering if I should teach "ΔU = Q-W" where W is the work done BY the system, or if I should teach "ΔU = Q+W" where W is the work done ON the system. Which way would be better (if either) for the AP Test?"

Answer:
The AP test changed to the ΔU = Q+W route in about 2002. Don't even mention the other way, unless your textbook does, in which case, good luck. :-)

(When I've used such a textbook, I've just repeated the correct definition over and over, and prayed.)

GCJ

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