If an electron revolves around a nucleus in a circular orbit of radius $R$ with frequency $n$,then the magnetic field produced at the centre of the nucleus will be

$A$ charge $q$ coulomb moves in a circle at $n$ revolutions per second and the radius of the circle is $r$ metre; then the magnetic field at the centre of the circle is

Two similar coils, which are separated by $2 \ m$, have a radius of $1 \ m$ and $80$ turns, and share a common axis. Calculate the strength of the magnetic field in microtesla at a point midway between them on their common axis when the current is $0.2 \ A$.

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

Two thick wires and two thin wires,all of the same material and same length,form a square in three different ways $P, Q,$ and $R$ as shown in the figure. With the correct connections shown,the magnetic field due to the current flow at the centre of the loop will be zero in the case of:

The magnetic induction at any point due to a long straight wire carrying a current is