Question

What will happen when a 40 watt, 220 volt lamp and 100 watt, 220 volt lamp are connected in series across 40 volt supply

• A. 100 watt lamp will fuse
• B. 40 watt lamp will fuse
• C. Both lamps will fuse
• D. Neither lamp will fuse

Answer 1

Answer: (D)

Neither lamp will fuse

Answer 2

Bulb (I) : Rated current $I_{1} = \frac{P}{V} = \frac{40}{220} = \frac{2}{11}amp.$

Resistance $R_{1} = \frac{V^{2}}{P} = \frac{(220)^{2}}{40} = 1210\Omega$

Bulb (II) : Rated current $I_{2} = \frac{100}{220} = \frac{5}{11}amp$

Resistance $R_{2} = \frac{(220)^{2}}{100} = 484\Omega$

When both are connected in series across 40 V supply

Total current through supply

$I = \frac{40}{P_{1} + P_{2}} = \frac{40}{1210 + 484} = \frac{40}{1254} = 0.03A$

This current is less than the rated current of each bulb. So neither bulb will fuse.

Short Trick : Since V_Applied < V_Rated, neither bulb will fuse.