Two concentric coplanar circular loops of radii a and 3a carry currents I₁ and I₂ respectively. What is the condition for the magnetic induction at the center of the loops to be zero?
Correct: D
The magnetic field at the center of a loop is μ₀I/(2R). For concentric loops with radii a and 3a, the fields oppose each other if currents are in opposite directions. For net zero field, μ₀I₁/(2a) = μ₀I₂/(2*(3a)). Solving gives I₁/I₂ = 1/3. Reversing direction for one loop to cancel the other satisfies the condition.