Question

Refer above figure.

An electric bulb 'P' rated 220 V, 60 W and, another identical bulb 'Q' is connected across the mains as shown here.
The power consumed is _ _ _ _ _

• A. 20W
• B. 40W
• C. 30W
• D. 60W

Answer 1

Answer: (C)

30W

Given: Rating of $P-220V,60W$

Also, resistance of P and Q are identical

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

$R=\frac{V^2}{P}=\frac{220\times 220}{60}$...............(1)

Now, total resistance $R+R=2R$

$=\frac{220\times 220}{60}\times 2$

$V=I{R}_{net}$ $I=current$

$\therefore$ Current = $\frac{V}{R_{net}}=\frac{220\times 60}{220\times 220\times 2}=\frac{30}{220}$

$\Rightarrow$ Current (I) $=\frac{3}{22}$

$\therefore Power=I^2{R}_{net}=\frac{3}{22}\times \frac{220\times 220\times 2}{60}\times \frac{3}{22}=30W$

Option C is correct.