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The power dissipated in the circuit shown in the figure is 30 W. The value of $R$ is:

1. 20 Ω
2. 15 Ω
3. 10 Ω
4. 30 Ω

Correct answer: 10 Ω

Solution

The total power dissipated is related to the circuit current and equivalent resistance by $P=I^2R_{eq}$ or $P=V^2/R_{eq}$. Using the resistance combination in the figure, the value of $R$ comes out to be $10\,\Omega$.

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