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

Two circular loops having same radius $[ R =10\, cm ]$ and same current $\frac{7}{2} A$ are placed along same axis as shown. If distance between their centre is $10\, cm$, find net magnetic field at of point $P.$

A plastic disc of radius $R$ has a charge $q$ uniformly distributed over its surface. If the disc is rotated at an angular frequency $\omega$ about it axis, the induction at the center of the disc is :-

What will be the resultant magnetic field at origin due to four infinite length wires. If each wire produces magnetic field '$B$' at origin

A coil of $50\, turns$ and $4\,cm$ radius carries $2\,A$ current then magnetic field at its centre is......$mT$

An electron revolves around nucleus with rotational frequency $'f'$ in circular orbit, due to this magnetic induction produced at nucleus position is $'B'$ then radius of circular orbit is directly proportional to