Two wire loops PQRSP formed by joining two semicircular wires of radii -R1, and R2, carries a current as shown in fig. The magnitude of the magnetic induction at the centre C is

Magnetic inductions at C due to straight portions PQ and RS will be equal to zero because they are on the same line of centre. Magnetic fields due to semi-circular portions will be just in opposite direction as current through them are opposite.∴ B2=μ0i(1/2)2R2 and B1=μ0i(1/2)2R1∴ B=B1−B2=μ0i41R1−1R2