Question

Two bulbs one of 200 volts, 60 watts & the other of 200 volts, 100 watts are connected in series to a 200 volt supply. The power consumed will be:

• A. 37.5 watt
• B. 160 watt
• C. 62.5 watt
• D. 110 watt

Answer 1

The correct option is A 37.5 watt

Here, two bulbs one of 200 volts, 60 watts & the other of 200 volts, 100 watts are connected in series to a 200 volt supply.

For bulb 1,

$I=\frac{V}{P}=\frac{60}{200}=\frac{6}{20}A$

Hence,

$R=\frac{V}{I}=\frac{200\times 20}{6}=\frac{4000}{6}\Omega$

Similarly, for bulb 2,

$I=\frac{V}{P}=\frac{100}{200}=\frac{1}{2}A$

Hence,

$R=\frac{V}{I}=\frac{200\times 2}{1}=400\Omega$

When bulbs are connected in series their resistances will added up

$R^{\prime }=\left(\frac{4000}{6}\right)+400$

$R^{\prime }=\frac{6400}{6}$

Hence, current is

$I^{\prime }=\frac{V}{R^{\prime }}=\frac{200\times 6}{6400}=\frac{12}{64}A$

Hence, $P^{\prime }=VI=200\times \frac{12}{64}$

$P^{\prime }=37.5W$