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?

Answer

A, B, D

Explanation

Two straight wires produces outward magnetic field by arc of circle produces inward magnetic field.
Due to straight wires,
`B_1=2[mu_0/(4pi)i/R (sintheta+sin90^I@)]`
`=mu_0/(2piR) (outwards)
`Due to circle arc `B_2=theta/(2pi)((mu_0i)/(2R))`(inward)
For net to be zero
`B_1=B_2`
`or theta=2 rad`