First slide

Electrical power

Question

A 25 watt ,220 V and a 100 watt ,220V bulbs are joined in series and connected to the mains. Which bulb will glow brighter

Moderate

A

25 watt
B

100 watt
C

both will glow with same brightness
D

first 25 watt and then 100 watt

Solution

Resistance of 25 watt bulb
$R_{1} = \frac{V^{2}}{P} = \frac{(220)^{2}}{25} = 1936 Ω$
Resistance of 100 watt bulb
$R_{2} = \frac{V^{2}}{P} = \frac{(220)^{2}}{100} = 484 Ω$
Brightness of bulb is directly proportional to $i^{2} R .$ Since same current is flowing through the bulbs ,
25 watt bulb will glow brighter.

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