First slide

Electromagnetic radiation and spectra

Question

An electric bulb is rated 220 volt -100 watt. The power consumed by it when operated on 110 volt will be

Moderate

A

75 watt
B

40 watt
C

25 watt
D

50 watt

Solution

$R = \frac{V_{rated}^{2}}{P_{rated}} = \frac{220 \times 220}{100} = 484 ohm Power consumed at 110 V = \frac{110 \times 110}{484} = 25 W$ Power is voltage squared over resistance, or P= $\frac{V^{2}}{R}$ . Likewise, resistance is voltage squared over power.

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