Question

A bulb of rating 100 watt (R = 4$\Omega$ ) is connected to cell of EMF 20 volt & internal resistance r = 4 $\Omega$, then

• A. Bulb dissipates power of 100 watt.
• B. If bulb of same internal resistance and rating of 75 watt would have been used, then it would have dissipiated its rated power.
• C. If bulb of same internal resistance and rating of 50 watt would have been used, then it would have dissipiated its rated power.
• D. If bulb of same internal resistance and rating of 25 watt would have been used, then it would have dissipiated its rated power.

Answer 1

The correct option is D If bulb of same internal resistance and rating of 25 watt would have been used, then it would have dissipiated its rated power.

As internal resistance is equal to external resistance, therefore we can apply formula of maximum power dissipiated through an external resistance which is given by -
$P_{max}$ $=\frac{e^2}{4r}=\frac{20\times 20}{4\times 4}=25$ watt.
Bulb cannot dissipiate more then 25 watt power.