I keep getting “First law of thermodynamics” questions wrong in Thermodynamics & Kinetic Theory. What's the trap?
getting the sign of W wrong (work done by vs on the gas).
The usual trap: getting the sign of W wrong (work done by vs on the gas). How to avoid it: heat in is +Q; work done by the gas is +W and lowers ΔU. Go back to the definition — energy conservation for gases: ΔU = Q − W, where W is work done BY the gas.
Key point
First law of thermodynamics
Common mistake
getting the sign of W wrong (work done by vs on the gas).
Memory tip
heat in is +Q; work done by the gas is +W and lowers ΔU.
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