Among the following,the unit of permeability is $NOT$ represented by

Two concentric thin circular rings of radii $50 \text{ cm}$ and $40 \text{ cm}$,each carry a current of $3.5 \text{ A}$ in opposite directions. If the two rings are coplanar,the net magnetic field due to the two rings at their centre is

As shown in the figure,a current of $2\,A$ flows in an equilateral triangle of side $4 \sqrt{3}\,cm$. The magnetic field at the centroid $O$ of the triangle is:
(Neglect the effect of the earth's magnetic field.)

The magnitude of the magnetic field at the center of an equilateral triangular loop of side $1 \ m$ which is carrying a current of $10 \ A$ is $.... \mu T$.

$A$ long thin hollow metallic cylinder of radius '$R$' carries a current '$i$'. The magnetic induction '$B$' at a distance '$r$' from the axis varies as shown in:

Write the formula for the magnetic field due to a circular current-carrying loop having $N$ turns and $R$ radius at a point on the axis of the loop at a distance $x$ from the center.