Question

A 100watt, 110 volt and a 50 watt, 110 volt lamps are connected in series across 220 volt d.c. source. If the resistances of two lamps are assumed to remain constant, the voltage across 100 watt lamps is

Answer 1

Resistance of lamp $R=\frac{V^2}{P}$

Resistance of 100 watt-110-volt lamp $R_1=\frac{{110}^2}{100}=121\Omega$

Resistance of 50 watt-110volt lamp $R_2=\frac{{110}^2}{50}=242\Omega$

Series connection :

Voltage across 100 watt lamp, $V_1=\frac{R_1}{R_1+R_2}V=\frac{121}{121+242}\times 100=33.33volts$