Q. A straight conductor carrying current $I.$ If the magnetic field at a distance $r$ is 0.4 T, then magnetic field at a distance $2r\ $will be
A) 0.4 T
B) 0.1 T
C) 0.8 T
D) 0.2 T
Explanation
(d) $B = \frac{\mu_{0}}{4\pi}\frac{2I}{r}$
or $B \propto \frac{1}{r}\ $
$\therefore\frac{B^{'}}{B} = \frac{r}{2r}$
or $B^{'} = B \times \frac{r}{2r} = 0.4 \times \frac{1}{2} = 0.2\ T$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.