Question

A current of one ampere is passed through a straight wire of length 2 metre. The magnetic field at a point in air at a distance of 3 m from one end of the wire but lying on the axis of the wire will be

Answer 1

By Biot-Savart Law

$dB=\frac{\mu_o I}{4\pi}(\overrightarrow {dl} \times \overrightarrow{r})$

$\overrightarrow {dl} \times \overrightarrow{r}=0$ as they are collinear.

Hence, the magnetic field is zero