In the first law of thermodynamics, ΔU = Q - W, the net heat transfer Q into a system is assigned a sign. What sign convention does the source use for Q?
AQ is positive when heat transfer occurs into the system
BQ is positive when heat transfer occurs out of the system
CQ is positive only when no heat transfer occurs at all
DQ is positive whenever the system does positive work
Answer & Solution
Correct answer: A. Q is positive when heat transfer occurs into the system
1. The first law is written ΔU = Q - W, where Q is the net heat transfer for the system.
2. The sign convention is stated directly: heat transfer into the system counts as positive Q.
3. Heat leaving the system would make Q negative, not positive, so that rules out option B.
4. Q's sign tracks heat direction only; it says nothing about whether work is being done, so option D confuses two different quantities.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.1 The First Law of Thermodynamics_
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