An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current $\text{'}I\text{'}$ along the same direction is shown in Fig. Magnitude of force per unit length on the middle wire$\text{'}B\text{'}$ is given by

A long wire carrying a steady current is bent into a circular loop of one turn. The magnetic field at the centre of the loop is $B$ . It is then bent into a circular coil of n turns. The magnetic field at the centre of this coil of $n$ turns will be

A wire carrying current $I$ has the shape as shown in adjoining figure. Linear parts of the wire are very long and parallel to $X-$axis while semicircular portion of radius $R$ is lying in $Y-Z$ plane. Magnetic field at point $O$ is