A bulb is rated 100 W-220 V. It is being used at 110 V supply. The power consumed by bulb is:

1. 100 W
2. half of 100 W
3. nearly 25 W
4. none of the above

nearly 25 W

Reason — Given,

Power (P) = 100 W

Voltage (V) = 220 V

We know that,

Power (P) = VI

Substituting the values in the formula above, we get,

$100 = 220 \times I \\[0.5em] \Rightarrow I = \dfrac{100}{220} \\[0.5em] \Rightarrow I = 0.45 A$

Hence, current through the lamp = 0.45 A

Power consumed when voltage is 110 V = ?

Resistance of lamp (R) = $\dfrac{V^2}{P}$

Substituting the values in the formula, we get,

$R = \dfrac{220^2}{100} \\[0.5em] \Rightarrow R = 484 Ω$

From relation,

P = $\dfrac{V^2}{R}$

Substituting the values in the formula, we get,

$P = \dfrac{110^2}{484} \\[0.5em] \Rightarrow P = 25 W$

Hence, power consumed is 25 W