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For an adiabatic process, the first law of thermodynamics reduces to:
A$\Delta U = q$, since $W = 0$
B$\Delta U = 0$ irrespective of $W$
C$\Delta U = W$, since $q = 0$
D$q = -W$, so $\Delta U = 0$
Answer & Solution
Correct answer: C. $\Delta U = W$, since $q = 0$
In an adiabatic process no heat is exchanged with the surroundings, $q = 0$. The first law $\Delta U = q + W$ therefore becomes $\Delta U = W$. So an adiabatic compression ($W > 0$) increases internal energy and hence temperature; an adiabatic expansion ($W < 0$) decreases both.
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