First slide

Electrical power

Question

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

Easy

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 1oo watt bulb
$R_{2} = \frac{V^{2}}{P} = \frac{(220)^{2}}{100} = 484 Ω$
Series resistance $= R_{1} + R_{2} = 2420 Ω$
Current through combination
$i = \frac{200}{2420} = \frac{1}{11}$
Voltage across 25 watt bulb
$= R_{1} \times i = (\frac{1936}{11}) = 176 volt$
Voltage across 100 watt bulb
$= R_{2} \times i = (484 / 11) = 44 volt$
Hence 25 watt bulb will glow brighter

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