Ah, the first law: ∆U = q + w. The universal sign convention is that *q is positive when heat is absorbed by the system*, and *w is positive when work is done on the system*. The single mistake that flips every answer is mixing up the system's perspective—thinking from the surroundings' point of view instead.
I drill this into my students because I see the same confusion in my recordings: if you don't know whether the mic is receiving sound or emitting it, your whole mix is ruined. My life is all about keeping the system—my little home studio—straight, while the landlord's rent hike feels like work being done *on* me.
#exams#study
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