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An electric bulb is rated as 200 W. What will be the peak magnetic field at 4 m distance produced by the radiations coming from this bulb? Consider this bulb as a point source with 3.5% efficiency.

Asked in JEE Main 24th June 2nd Shift 2022 · Point source, efficiency and power on an area

• (1) 1.19 × 10⁻⁸ T
• (2) 1.71 × 10⁻⁸ T
• (3) 0.84 × 10⁻⁸ T
• (4) 3.36 × 10⁻⁸ T

Step-by-step solution

Why the other options are wrong

• (1) 1.19 × 10⁻⁸ T is B₀/√2, i.e. the rms field — the question asks for the peak
• (3) 0.84 × 10⁻⁸ T is half the answer; you used I = 1/4ε₀cE₀², the electric share alone
• (4) 3.36 × 10⁻⁸ T is twice the answer — you used u = ε₀E₀² instead of ε₀E₀²/2

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