Question

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 $25 cm$ from the straight wire would be:

• A. $\frac{B}{6}$
• B. $\frac{B}{5}$
• C. $\frac{B}{3}$
• D. $\frac{B}{2}$

Answer 1

Answer: (B)

The magnetic field due to the straight wire at a distance $r$ is given by
$B=\frac{\mu_{0} i}{2 \pi r}$
or $B \propto \frac{1}{r}$ or $\frac{B_{2}}{B_{1}}=\frac{r_{1}}{r_{2}}$
$\therefore \quad \frac{B_{2}}{B}=\frac{5}{25}=\frac{1}{5}$ or $B_{2}=\frac{B}{5}$