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In the IUPAC sign convention used in modern NCERT, the first law of thermodynamics is written as $\Delta U = q + W$. Then:
ABoth $q$ and $W$ are always positive for any spontaneous process
B$q$ is positive when heat is released by the system; $W$ is positive when work is done by the system
C$q$ is positive when heat is absorbed by the system; $W$ is positive when work is done **on** the system
D$W$ is positive when work is done by the system, irrespective of direction
Answer & Solution
Correct answer: C. $q$ is positive when heat is absorbed by the system; $W$ is positive when work is done **on** the system
The modern NCERT sign convention: $q > 0$ for heat absorbed by the system, $W > 0$ for work done **on** the system (compression). $W < 0$ for work done by the system (expansion). With this convention, isothermal compression by an external pressure $p_\text{ext}$ on a gas gives $W = -p_\text{ext}\Delta V > 0$ since $\Delta V < 0$.
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