An equilateral triangle of side length $I$ is formed from a piece of wire of uniform resistance. The current $I$ is fed as shown in the figure. Then, the magnitude of the magnetic field at its centre $O$ is

A straight thin conductor is bent as shown in the adjoining figure. It carries a current $i$ ampere. The radius of the semicircular arc is $r$ metre. The magnetic induction at the centre of semicircular arc is

An infinitely long conductor is bent into a circle as shown in figure. It carries a current $I$ ampere and the radius of loop is $R$ metre. The magnetic induction at the centre of loop is

A current $I$ flows through a closed loop shown in figure. The magnetic field at the centre $O$ is

Magnetic field induction at the point $O$ due to current flowing in an infinite wire shaped as shown in the figure is

The wire loop $PQRSP$ formed by joining two semicircular wires of radii $R_1$ and $R_2$ carries a current $I$ as shown. The magnitude of magnetic induction at the centre $C$ is