In an electric circuit three bulbs of 100 W each are connected in series to a source. In another circuit set of three bulbs of the same wattage are connected in parallel to the same source.
Generated by claude-sonnet-4-6 · 2026-06-15 06:47 · grounding rag
Model Answer
(i) Brightness in series vs parallel:
No, the bulbs will not glow with the same brightness.
- In the series circuit, the total resistance is 3R (where R is resistance of one bulb). The current through each bulb is small (I = V/3R), so each bulb glows dimmer than normal.
- In the parallel circuit, full source voltage V appears across each bulb. Each bulb draws its rated current independently, so all three glow at full rated brightness.
Thus, bulbs in the parallel circuit glow much brighter than those in the series circuit.
(ii) Effect of one bulb fusing:
- Series circuit: If one bulb fuses, the circuit is broken (open). No current flows anywhere, so the remaining two bulbs go out completely.
- Parallel circuit: If one bulb fuses, only that branch breaks. The other two branches remain complete, each still receiving full voltage. So the remaining two bulbs continue to glow normally, unaffected.
Source: Chapter 11, Section 11.6.1 and 11.6.2
---
Explanation
- Examiners award marks for: (i) correctly stating "no", explaining series has higher resistance → less current → dim; parallel has full voltage → full brightness. (ii) Series → circuit breaks → all off; Parallel → other branches unaffected → continue glowing.
- The textbook explicitly states: "Another major disadvantage of a series circuit is that when one component fails the circuit is broken and none of the components works." Quote or paraphrase this for full marks.
- Power $P = V^2/R$ (parallel, full V) vs $P = I^2R$ with reduced I (series) — you can mention this to strengthen part (i) if space allows.