Question

The magnetic field at a distance $r$ from a long wire carrying current $I$ is $0.4T$. What is the magnetic field at a distance $2r$ ?

• A. $0.1T$
• B. $0.2T$
• C. $0.8T$
• D. $1.6T$

Answer 1

Answer: (B)

$0.2T$

$B\propto \frac{1}{r}$
$\frac{B_1}{B_2}=\frac{r_2}{r_1}\Rightarrow \frac{4}{B_2}=\frac{2r}{r}$
$B_2=0.2T$
$\therefore B_2$ or magnetic field at a distance $2r$ is $0.2T$