$A$ circular coil of radius $R$ carries an electric current $I$. The magnetic field due to the coil at a point on the axis of the coil located at a distance $r$ from the centre of the coil,such that $r \gg R$,the magnetic field at that point is proportional to

The magnetic field at the centre $O$ of a square loop of side $a$ carrying current $I$ as shown in the figure is:

$A$ coil having $2000$ turns is wound tightly in the form of a spiral with inner and outer radii $1 \,cm$ and $3 \,cm$, respectively. When a current $\frac{1}{\pi} \,mA$ passes through the coil, the magnetic field at the centre is calculated to be $K \ln 3 \times 10^{-6} \,T$. The value of $K$ is

If the unit of magnetic flux is in weber,then the unit of magnetic field is . . . . . . .

In the figure,two parallel infinitely long current-carrying wires are shown. If the resultant magnetic field at point $A$ is zero,then determine the current $I$ (in $A$).

The dimensions of the ratio of magnetic flux $(\phi)$ and permeability $(\mu)$ are