Question

The given figure shows three similar lamps $L_1,L_2,L_3$, which are connected across a power supply. If the lamp $L_3$ fuses then the light emitted by $L_1$ and $L_2$ will change as:

• A. no change
• B. brilliance of $L_1$ decreases and that of $L_2$ increases
• C. brilliance of both $L_1$ and $L_2$ increases
• D. brilliance of both $L_1$ and $L_2$ decreases

Answer 1

Answer: (B)

brilliance of $L_1$ decreases and that of $L_2$ increases

$P=I^{2}R$
When $L_3$ fuses, no current passes through it

$\Rightarrow$ Resistance of the circuit increases $($ $\because$ at first they were in parallel $)$. and after fusing of third lamp they becomes in series.

The current passing through $L_1$ decreases and current passing through $L_2$ increases.

$\Rightarrow P_1$ decreases and $P_2$ increases.