First slide

Electrical power

Question

Fig. shows three similar lamps L₁, L₂, and L₃
connected across a power supply. If the lamp L₃. fuses, how will the light emitted by L₁, and L₂, change ?

Easy

A

no change
B

brilliance of L₁, decreases and that of L₂ increases
C

brilliance of both L₁ and L₂ increases
D

brilliance of both L₁ and L₂, decreases

Solution

Let R be the resistance of each lamp. If E be the applied the circuit I, is given by
$i_{1} = \frac{E}{R + (R / 2)} = \frac{2 E}{3 R}$
Current flowing through L₂ or L₃
$= \frac{1}{2} [\frac{2 E}{3 R}] = \frac{E}{3 R}$
When L₃ is fused, the whole current flows through L₁ and L₂. Thus
$i_{2} = \frac{E}{2 R}$
So, current through L₁ decreases and through L₂increases.

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