NEET 2026 (Phase 1) · PhysicsPrevious Year Question
An electric heater supplies heat to a system at a rate of $100$ W. If the system performs work at a rate of $75$ J/s, the rate at which internal energy increases is
- A.$75$ W
- B.$100$ W
- C.$125$ W
- D.$25$ W✓
Correct Answer
(D) $25$ W
Solution & Explanation
\textbf{Answer:} (d) $25$ W \textbf{Solution:} The first law of thermodynamics in rate form is $$\frac{dQ}{dt}=\frac{dU}{dt}+\frac{dW}{dt},$$ where $\dfrac{dQ}{dt}$ is the rate of heat supplied, $\dfrac{dW}{dt}$ is the rate of work done by the system, and $\dfrac{dU}{dt}$ is the rate of change of internal energy. Given $\dfrac{dQ}{dt}=100$ W and $\dfrac{dW}{dt}=75$ J/s, $$\frac{dU}{dt}=\frac{dQ}{dt}-\frac{dW}{dt}=100-75=25\text{ W}.$$ This matches option (d).
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