State Biot-Savart's law.

Explanation

The magnitude of magnetic field d`vec(B) ` at a point P at a distance r from the small elemental length taken on a conductor carrying current varies
(i) directly as the strength of the current I
(ii) directly as the magenitude of the length element d`vec(l)`
(iii) directly as the sine of the angle (say, `theta`) between d `vec(l) and hat(r)` .
(iv) inversely as the square of the distance between the point P and length element d `vec(l)`. this is e`xx`pressed as .
`d B prop ("I dl")/(r^(2)) sin theta`