In a series L-R AC circuit, the potential differences across the inductor and the resistor are 12 V and 16 V respectively. Find the total potential difference across the circuit.

20 V
28 V
4 V
14 V

Correct answer: 20 V

Solution

Because VL leads the current by 90 degrees while VR is in phase with the current, the two voltages add as perpendicular vectors. V = sqrt(VR² + VL²) = sqrt(16² + 12²) = sqrt(256 + 144) = sqrt(400) = 20 V.

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