A 50 Hz AC source of 20 V (rms) is connected across a resistor R and capacitor C in series. The voltage across R is measured as 12 V. What is the voltage across C?

8 V
16 V
10 V
not possible to determine unless values of R and C are given

Correct answer: 16 V

Solution

In a series RC circuit, V_R and V_C are perpendicular phasors. The source voltage is the hypotenuse: Vₛ² = V_R² + V_C². Thus 400 = 144 + V_C², giving V_C = 16 V.

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