Find the intensity of magnetic field at a distance r/2 from a straight current carrying wire, if the intensity of magnetic field at a point situated at a distance r from the same wire is B.

The intensity of magnetic field at a distance r = B

Let the intensity of magnetic field at a distance r/2 = B₂

The magnetic field at a point at distance r from an infinite wire is given by

 $B=\frac{{\mu }_{0}i}{2\pi \mathrm{r}}$

Since current flowing is same in the wire

$\text{ so, }B\propto \frac{1}{r}$

$\frac{{\mathrm{B}}_1}{{\mathrm{B}}_2}=\frac{{\mathrm{r}}_2}{{\mathrm{r}}_1}$

$\frac{B}{B_2}=\frac{\frac{r}{2}}{r}$

$B_2=2B_1$