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

Two long thin wires $ABC$ and $DEF$ are arranged as shown. The magnitude of the magnetic field at $O$ is

A circular conductor, of uniform resistance per unit length, is connected to a battery of $4 \mathrm{V}$. The total resistance of the conductor is $4 \mathrm{\Omega }$. The net magnetic field at the centre of the conductor is