Three rings,each having equal radius $R$,are placed mutually perpendicular to each other,and each has its centre at the origin of the coordinate system. If current $I$ is flowing through each ring,then the magnitude of the magnetic field at the common centre is

$A$ coil having $N$ turns is wound tightly in the form of a spiral with inner and outer radii $a$ and $b$ respectively. Find the magnetic field at the centre,when a current $I$ passes through the coil.

Two infinite length wires carry currents $8 \ A$ and $6 \ A$ respectively and are placed along the $X$ and $Y$ axes respectively. The magnetic field at a point $P(0, 0, d)$ will be:

The adjoining figure shows a conductor carrying a current $i$. The magnetic field at the origin is

The direction of magnetic lines of force produced by passing a direct current in a conductor is given by

$A$ non-conducting disc of radius $R$ has a surface charge density which varies with distance from the centre as $\sigma(r) = \sigma_0 \left[1 + \sqrt{\frac{r}{R}}\right]$,where $\sigma_0$ is a constant. The disc rotates about its axis with angular velocity $\omega$. If $B$ is the magnitude of magnetic induction at the centre,then $\frac{B}{\mu_0 \sigma_0 \omega R}$ will be