A lamp marked 40 W at 230 V is connected. What is its resistance?

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Multiple Choice

A lamp marked 40 W at 230 V is connected. What is its resistance?

Explanation:
Power dissipated by a resistor relates to voltage and resistance through P = V^2 / R. For a device rated 40 W at 230 V, its operating resistance is R = V^2 / P = 230^2 / 40. Calculating, 230^2 = 52,900, and 52,900 / 40 = 1,322.5 ohms, about 1.32 kΩ. So the lamp’s resistance at its operating voltage is roughly 1.32 kΩ. (In practice, incandescent lamps’ resistance varies with temperature, but this uses the hot operating value corresponding to the rating.)

Power dissipated by a resistor relates to voltage and resistance through P = V^2 / R. For a device rated 40 W at 230 V, its operating resistance is R = V^2 / P = 230^2 / 40. Calculating, 230^2 = 52,900, and 52,900 / 40 = 1,322.5 ohms, about 1.32 kΩ. So the lamp’s resistance at its operating voltage is roughly 1.32 kΩ. (In practice, incandescent lamps’ resistance varies with temperature, but this uses the hot operating value corresponding to the rating.)

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