The source of magnetic field is . . . . . . and source of electric field is . . . . . . .

$A$ closely wound flat circular coil of $25$ turns of wire has a diameter of $10\, cm$ and carries a current of $4\, A$. Determine the magnetic flux density at the centre of the coil.

$A$ current of $5 \; A$ flows through a square loop of side length $\frac{1}{\sqrt{2}} \; m$. The magnitude of the magnetic field $B$ at the centre of the square loop is $p \times 10^{-6} \; T$. Find the value of $p$. [Take $\mu_0 = 4 \pi \times 10^{-7} \; T \cdot m \cdot A^{-1}$].

In the following figure,a wire bent in the form of a regular polygon of $n$ sides is inscribed in a circle of radius $a$. The net magnetic field at the centre will be

The magnetic field at the centre $O$ due to the given structure is:

State and explain the Biot-Savart law for the magnetic field produced by a current element. Give the direction of the magnetic field and define its unit.