The magnetic field due to a straight conductor of uniform cross section of radius $a$ and carrying a steady current is represented by

In the figure,the magnetic induction at the centre $O$ due to the current in the straight portion $AB$ will be:

Two concentric coplanar circular loops of radii $r_1$ and $r_2$ carry currents $i_1$ and $i_2$ in opposite directions (one clockwise and the other anticlockwise). The magnetic induction at the center of the loops is half that due to $i_1$ alone at the center. If $r_2 = 2r_1$,find the value of $\frac{i_2}{i_1}$.

The magnitude of the force per unit length acting on a thin wire carrying a current $I=8 \text{ A}$ at a point $O$, if the wire is bent as shown in the figure with a radius $R=10 \pi \text{ cm}$, is (in $\mu \text{N/m}$)

The magnetic induction at point $O$ for the current-carrying wire shown in the figure is:

The magnetic field at the centre $O$ in the given figure is