A) Brightness of P increases but that of R decreases.
B) Brightness of P remains the same but that of R decreases.
C) Brightness of both P and R decreases.
D) Brightness of P increase but that of R remains same.
Correct Answer: A
Solution :
If switch is closed, bulbs get connected in parallel to bulb R, due to which the effective resistance of the circuit decrease and hence effective current of the circuit increase. Since brightness of \[\operatorname{bulb}\,\,\propto {{(current)}^{2}}\] \[\therefore \] brightness of bulb P bulb increase. Now current of bulb R is shared by bulb Q, \[\therefore \] Brightness of bulb R decreases.
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