A wire carrying current `i` has the configuration as shown in figure. Two semi-infinite straight sections, both tangent to the same circle, are connected by a circular arc of central angle theta, along the circumference of the circle, with all sections lying in the same plane. What must be for `B` to be zero at the centre of the circle?

Explanation

The field due to the straight parts of the wire
`B_(1)=2[(mu_(0))/(4pi)i/R]odot`
The field due to curved part of the wire `B_(2)=(mu_(0)(theta/(2pi))i)/(2R)`, into the plane of the wire.
The resultant field at the centre to be zero `B_(1)=B_(2)` or `2[(mu_(0))/(4pi)i/R]=(mu_(0)theta/(2pi))/(2R)rArrtheta=2` rad