Question

A 25 watt, 220 volt bulb and a 100 watt, 220 volt bulb are connected in series across 440 volt line. Then ,
( Breaking Voltage of both the bulbs is 300 V )

• A. Only 100 watt bulb will fuse
• B. Only 25 watt bulb will fuse
• C. None of the bulbs will fuse
• D. Both bulbs will fuse

Answer 1

Answer: (B)

Only 25 watt bulb will fuse

$P_1=\frac{V^2}{R_1}$

$\Rightarrow R_1=\frac{220\times 220}{25}$

$P_2=\frac{V^2}{R_2}$

$R_2=\frac{220\times 220}{100}$

$\Rightarrow \frac{R_1}{R_2}=\frac{4}{1}$ P.D across bulb $\alpha$ Resistance of bulb

Let the P.D across bulb 2 be $x$

then across bulb 1 will be $4x$

$\Rightarrow$ P.D. of 1 + P.D. of 2 $=$ 440

$\Rightarrow$ 5x $=$ 440

x $=$ 88

$\Rightarrow$ P.D across bulb 1 $=$ 88 x 4 $=$ 352 which is > 300 so the bulb will fuse

P.D across bulb 2 = 88 which is < 300 so the bulb will not fuse

$\therefore$ Bulb 1 which is of 25 W will fuse