Question

The magnetic field at a distance $r$ from a long wire carrying current $i$ is 0.4 Tesla. The magnetic field at a distance $2r$ is :

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

Answer 1

The correct option is A 0.2 T

$B_1=0.4T$;
Magnetic field at a distance r for a long wire carrying current is,

$BB=\frac{{\mu }_{0}i}{2\pi r}\Rightarrow B\alpha \frac{1}{r}$

$\therefore \frac{B_1}{B_2}=\frac{2r}{r}\frac{0.4}{B_2}=2$
$B_2=0.2T$