Question

The strength of the magnetic field at a point $r$ near a long straight current carrying wire is $B$. The field at a distance $\frac{r}{2}$ will be

• A. $\frac{B}{2}$
• B. $\frac{B}{4}$
• C. $2B$
• D. $4B$

Answer 1

Answer: (C)

$2B$

The strength of magnetic field at a distance $r$ from a long straight current carrying wire is inversely proportional to the distance of the point

$B\propto \frac{1}{r}$

$\Rightarrow \frac{B_1}{B_2}=\frac{r_2}{r_1}$

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

$\Rightarrow B_2=2B$