Question

if two bulbs rated 2.5 watt 110 volt and 100 watt 110 volt are connected in series to a 220 volt supply tthen
a) 100W bulb will fuse
b) 2.5W bulb will fuse

c) both will fuse
d) Non will fuse
e) none of these

Answer 1

answer : option (b) 2.5 W bulb will fuse.

explanation : first of all, find resistance of each bulb, R = V²/P

for first bulb ( P = 2.5 watt, V = 110 volts)

so, R1 = (110)²/(2.5) = 12100/2.5

= 4840 ohm.

for second bulb (100 watt, 110 volts)

so, R2 = (110)²/(100)

= 121 ohm

both bulbs are joined in series combination. so, Req = R1 + R2

= 4840 + 121

= 4961 ohm

so, current passing through circuit , I = (220 volts)/(4861 ohm)

= 220/4861 ≈ 0.045 A

now current passing through first bulb, I1 = P1/V1 = (2.5 watt)/110 volts

≈ 0.02272 A

and current through second bulb, I2 = P2/V2 = (100 watt)/110 volts

≈ 0.909A

here I > I1

The total current is bigger than the allowed value on the first bulb, so the 2.5watt bulb will fuse.