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

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_