$A$ long wire carries a steady current. It is bent into a coil of one turn such that the magnetic induction at the centre is $B$. If the same wire is bent to form a coil of smaller radius with $n$ turns,then the new magnetic induction $B^{\prime}$ at the centre is:

Two infinitely long wires each carrying the same current $i$ and pointing in $+y$ direction are placed in the $xy$ plane,at $x=-2 \ cm$ and $x=1 \ cm$. An electron is fired with speed $U$ from the origin making an angle of $+45^{\circ}$ from the $x$-axis. The force on the electron at the instant it is fired is [$B_0$ is the magnitude of the field at the origin due to the wire at $x=1 \ cm$ alone].

$A$ current of $I$ $A$ is passed through a straight wire of length $2.0$ $m$. The magnetic field at a point in air at a distance of $3$ $m$ from either end of the wire and lying on the axis of the wire will be:

The magnetic field at a perpendicular distance '$r$' from a long straight wire carrying current '$I$' is '$B$'. The magnetic field at a perpendicular distance '$2r$' from the same wire is:

In an ionised sodium atom,an electron is moving in a circular path of radius $r$ with angular velocity $\omega$. The magnetic induction in $Wb/m^2$ produced at the nucleus will be

For the adjoining figure,the magnetic field at point $P$ will be: