Question

If two bulbs of wattages 25 W and 100 W respectively each rated by 220 V are connected in series with the supply of 440 v·then which bulb will fuse?

Options

• 100 W bulb

• 25 W bulb

• None of these

• Both of these

Answer 1

25 W bulb

Explanation:

Resistance of 25 W bulb

`"R"_1 = "V"^2/"P" => (220)^2/25` = 1936 Ω

Resistance of 100 W bulb

`"R"_2 = "V"^2/"P" => (220)^2/100` = 484 Ω

Series resistance = R₁ + R₂

= 1936 + 484 = 2420 Ω

Current through series combination will be

I = `440/2420 = 2/11`

`"V"_2 = "R"_2 xx "I" => 484 xx 2/11 => 88 "V"`

`"V"_1 = "I" xx "R"_1 = 1936 xx 2/11 = 352 "V"`

Thus the bulb of 25 W will be fused because it can tolerate only 220 V while the voltage across it is 352 V.