Question

Figure `7.44` shows three similar lamps `L_(1) , L_(2), and L_(3)` connectged across a power supply. If the lamp ` L_(3)` fuses, how will the light emitted by `L_(1) and L_(2)` change?

A. no change
B. brillance of `L_(1)` decreases and that of `L_(2)` increases
C. brilliance of both `L_(1) and L_(2)` increases
D. brillance of both `L_(1) and L_(2)` decreases

Answer 1

Correct Answer - B
Let `R` be the resistance of each lamp. If `E` is the applied emf, then the current in the circuit `I_(1)` is given by
`( E ) /(R + ( R //2)) = 2 E// 3R`
Current fllowing through `L_(2) or L_(3)` is
`(1)/( 2) [( 2E)/( 3 R )] = ( E)/( 3 R)`
When `L_(3)` is fused , the whole current flows through `L_(1) and L_(2)`. Thus `I_(2) = E// 2 R`. So current through `L_(1)` decreases and the current through `L_(2)` increases.