Question

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

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

Answer 1

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

In a long straight wire,
$B=\frac{{\mu }_{0}i}{2\pi r}$ and $B\propto \frac{1}{r}$
$\therefore \frac{B}{B_1}=\frac{200m}{50m}\Rightarrow \frac{B}{B_1}=4\Rightarrow B_1=\frac{B}{4}$