In the same equation ΔU = Q - W, the net work W also carries a sign convention. According to the source, when is W positive?
AWhen work is done on the system by an outside force
BWhen work is done by the system on its surroundings
CWhen the system's volume stays perfectly constant
DWhen the system loses heat to its surroundings
Answer & Solution
Correct answer: B. When work is done by the system on its surroundings
1. The first law's work term W is positive when the system itself performs work on the outside world.
2. This is stated directly: work done by a system, either against an external force or by expanding its volume, counts as positive W.
3. Work done on the system by an external force is work put in, which is negative W in this convention, ruling out option A.
4. Constant volume and heat loss describe other quantities entirely, not the sign of W, so options C and D are irrelevant here.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.1 The First Law of Thermodynamics_
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