A choke coil (inductor) and a resistor are connected in series across an ac source supplying 130 V. If the voltage across the resistor is 50 V, what is the potential difference across the choke coil?

80 V
120 V
100 V
90 V

Correct answer: 120 V

Solution

Because V_R and V_L are in quadrature, V² = V_R² + V_L². Thus V_L = sqrt(130² - 50²) = sqrt(16900 - 2500) = sqrt(14400) = 120 V.

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