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A bulb of 100 W rating is connected with a 220 V supply. The resistance of the bulb is _____.

Asked in GUJCET 2020 · Rated bulbs and appliances

• (1) 2.2 Ω
• (2) 484 Ω m⁻¹
• (3) 484 Ω
• (4) 2.2×10⁻³ Ω m⁻¹

Step-by-step solution

Why the other options are wrong

• (1) 2.2 Ω is V/P = (220)/(100), which is not a resistance: V W⁻¹ is A⁻¹. Use R = (V²)/P.
• (2) The number 484 is right, but Ω m⁻¹ is resistance per unit length, not resistance.
• (4) No single slip gives 2.2×10⁻³, and Ω m⁻¹ is the wrong unit for a resistance anyway. Sanity check: R = (220²)/(100) = 484 Ω.

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