Question

A very long straight wire of radius $r$ carries current $I$. Intensity of magnetic field $B$ at a point, lying at a perpendicular distance '$a$' from the axis is $\propto$ ______.

• A. $a^{2}$
• B. $\frac{1}{a^{2}}$
• C. $\frac{1}{a}$
• D. $a$

Answer 1

Answer: (D)

$B \times 2 \pi a=\mu_{0} \times \frac{I}{2 \pi r^{2}} \times \pi a^{2}$
$B=\frac{\mu_{0} I}{2 \pi r^{2}} \times a$
$B \propto a$