Q. An electric current passes through a long straight copper wire. At a distance 5 cm from the straight wire, the magnetic field is $B.$ The magnetic field at 20 cm from the straight wire would be
A) $\frac{B}{6}$
B) $\frac{B}{4}$
C) $\frac{B}{3}$
D) $\frac{B}{2}$
Explanation
(b) Magnetic field, $B = \frac{\mu_{0}I}{2\pi r}$
or $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B \propto \frac{1}{r}$
or $\frac{B_{2}}{B_{1}} = \frac{r_{1}}{r_{2}}$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{B_{2}}{B} = \frac{5}{20} = \frac{1}{4}$
or $B_{2} = \frac{B}{4}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.