Magnetic field intensity $H$ at the centre of a circular loop of radius $r$ carrying current $I$ in e.m.u. is

$A$ and $B$ are two concentric circular conductors with center $O$,carrying currents $i_1$ and $i_2$ as shown in the figure. If the ratio of their radii is $1:2$ and the ratio of the magnetic flux densities at $O$ due to $A$ and $B$ is $1:3$,then the value of $i_1/i_2$ is:

$A$ current-carrying wire in its neighborhood produces:

Two long straight parallel wires $A$ and $B$ separated by $5 \ m$ carry currents $2 \ A$ and $6 \ A$ respectively in the same direction. The resultant magnetic field due to the two wires at a point $P$ at a distance of $2 \ m$ from wire $A$ in between the two wires is:

In a wire of radius $1 \ mm$,a steady current of $2 \ A$ uniformly distributed across the cross-section of the wire is flowing. Then the magnetic field at a point $0.25 \ mm$ from the centre of the wire is (in $\mu T$)

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 $\frac{i_2}{i_1}$.