Question

An electric current passes through a long straight copper wire. At a distance $5cm$ from the straight wire, the magnetic field is $B$. The magnetic field at $20cm$ from the straight wire would be

• A. $\frac{B}{6}$
• B. $\frac{B}{4}$
• C. $\frac{B}{3}$
• D. $\frac{B}{2}$

Answer 1

The correct option is B $\frac{B}{4}$

By Ampere's circuit law, we have $B\left(2\pi r\right)={\mu }_{0}I$.

In this scenario, $I$ is the enclosed current and it remains same. Only, the radius is increased 4 times.

So, the magnetic field becomes one-fourth.