Explanation

(a) Magnetic field due to a long straight wire of radius $a$ carrying current $I$ at a point distant $r$ from the centre of the wire is given as follows
$B = \frac{\mu_{0}Ir}{2\pi a^{2}}$ for $r < a$
$B = \frac{\mu_{0}I}{2\pi r}$ for $r = a$
$B = \frac{\mu_{0}I}{2\pi r}$ for $r > a$
The variation of magnetic field $B$ with distance $r$ from the centre of wire is shown in the figure
[image:3_image167.emf]