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Two coaxial circular loops of radii R and 2R carry equal currents in the same sense. The ratio of the magnetic fields at their common centre due to the two loops is
A1 : 2
B1 : 4
C4 : 1
D2 : 1
Answer & Solution
Correct answer: D. 2 : 1
1. Field at the centre of a single circular loop is B = μ₀I/(2R), inversely proportional to R.
2. Loop of radius R contributes B₁ = μ₀I/(2R).
3. Loop of radius 2R contributes B₂ = μ₀I/(4R).
4. Ratio B₁ : B₂ = (1/R) : (1/2R) = 2 : 1.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", §4.5_
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