Question

Three incandecent bulbs of `100 W` each are connected in series in an electric circuit. In another circuit another set of three bulbs of the same wattage are connected in parallel to the same source.
(a) Will the bulbs in the two circuits glow with the same brightness ? Justify your answer.
(b) Now let one bulb in both the circuits get fused. Will the rest of the bulbs continue to glow in each circuit ? Give reason.

Answer 1

(a) No, the bulbs in parallel combination glow more brightly. This is clear from the following discussion. If `R` is the resistance of each bulb and `V` is the voltage of the source, current through each bulb (in series combination, `I_s = (V)/(3 R)`
Total current through all the bulbs (in parallel combination),
`I = (V)/(R//3) = (3 V)/( R)` (total resistance of `3` bulbs in parallel is `R//3`)
Current through each bulb (in parallel combination), `I_p = I//3 = (3 V//R)/(3) = (V)/( R)`
Clearly, `I_p = 3 I_s`, The glow of a bulb is due to Joule heating `(I^2 R)`.
(b) (i) When one of the bulbs in series circuit gets fused, the circuit is broken. Since no current passes through the bulbs, they stop glowing.
(ii) When one of the bulbs in the parallel circuit, gets fused, the other two bulbs keep glowing with the same brightness as their circuits are not broken.