Discussion
If 10 calories of heat is added to a system which does not change its specific volume, then as per the first law of thermodynamics the change in internal energy of the system will be :
Answer: B
The change in internal energy is positive.
The first law of thermodynamics is \(dQ = dU + p\,dV\). If the specific volume does not change, \(dV = 0\), so no work is done and all the heat added goes into internal energy:
\[ \begin{aligned} dU &= dQ - p\,dV \\ &= 10 - 0 = +10\ \text{cal} \end{aligned} \]
The internal energy rises by 10 calories, which shows up as a rise in temperature. It cannot be zero, since heat was added and none left as work, and it cannot be negative, since that would need the system to lose energy. So 'either zero or negative' is also wrong.
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