All straight wires are very long. Both AB and CD arc area of the same circle, both subtending right angles at the centre O. Then the magnetic field at O is

Question

G1 Physics · Accepted Answer

Use B from an arc = μ₀Iφ/(4πR) for each circular arc and B=μ₀I/(4πR) for each relevant semi-infinite straight part. Adding the directed contributions shown gives B at O=μ₀i/(2πR). Hence C.