Question

Three in incandescent bulbs of 100 watt each are connected in series in electric circuit. In another circuit, another set of three bulbs of the same wattage are connected in parallel to the same source. Will the bulb in the two circuit glow with the same brightness? justify answer.

Answer 1

1. No the bulbs won't glow with the same brightness in both circuits. The bulb in parallel circuit will glow brighter than when connected in series.

Power of each bulb (P) = 100W at 220V

Hence, Resistance of each bulb (R) = $V^2/P$

Now, when they are connected in series,

$R_{eq}=R+R+R=3R$

Let the source voltage be V'

hence, $I=V^{\prime }/3R$

Hence, power through each bulb = $I^{2}R=V^{\prime 2}/9R$

In Parallel, bulbs are connected to the same source i.e. voltage = V'

Hence, power through each bulb = $V^{\prime 2}/R$

Power when connected parallel > power when connected in series

Since brightness is directly proportional to power, hence bulbs when connected in parallel would glow brighter.