Question

`B_1,B_2 and B_3` are three identical bulbs connected as shown in (fig. 3.46). When all the three bulbs glow, a current of `3 A` is recorded by the ammeter `A`.
(i) What happens to the glow of the other two bulbs when the bulb `B_1` gets fused ?
(ii) What happens to the readings of `A_1,A_2,A_3 and A` when the bulb `B_2` gets fused ?
(iii) How much power is dissipated in the circuit when all the three bulbs glow together ?
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Answer 1

(i) Since bulbs `B_1,B_2 and B_3` are in parallel, when bulbs `B_1` gets fused, the pd across `B_2 and B_3` does not change. Thus, the glow of bulbs `B_2 and B_3` remains unchanged.
(ii) When the bulbs `B_2` gets fused, no current flows through `A_2` and as such it shows zero reading. Since the total current through `B_1 and B_3` in parallel id `2 A` (as 3 A is the total current which flows when all the three bulbs, i.e., `B_1,B_2 and B_3` are in parallel), current through `B_1 or B_3` is `1 A` (as their resistances are equal).
Thus, `A_1` shows `A,A_2` show zero, `A_3` shows `1 A and A shows 2 A`.
(iii) When all the three bulbs are glowing together, `I = 3 A and V = 4.5 V`.
Power dissipated in the circuit, `P = VI = (4.5 V)(3 A) = 13.5 W`.