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Science — CBSE Class 10 board question

Q1. [1]
The minimum number of identical bulbs of rating 4V; 6W, that can work safely with desired brightness, when connected in series with a 240 V mains supply is :
  1. (a) 20
  2. (b) 40
  3. (c) 60
  4. (d) 80
Previously asked in CBSE board exam
2025 31/4/1 Q14
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer

Each bulb rating: 4V, 6W. Resistance of each bulb = V²/P = 16/6 = 8/3 Ω.

For safe operation, voltage across each bulb must not exceed 4V.

Number of bulbs needed = Total supply voltage ÷ Voltage per bulb = 240/4 = 60.

(c) 60

Explanation

The key idea: bulbs in series share the supply voltage equally. Each bulb is rated for 4V, so the number of bulbs = 240V ÷ 4V = 60. This ensures each bulb gets exactly its rated voltage (4V) and operates at desired brightness without damage. The power/resistance calculation is not required here — just the voltage division in series.

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