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 tesla
• B. 0.8 tesla
• C. 0.1 tesla
• D. 0.6 tesla

Answer 1

Answer: (A)

0.2 tesla

Answer 2

Magnetic field due to long wire,
$B=\frac{\mu_{0}i}{2\pi r}$ or $B\,\propto\frac{1}{r}$
When $r$ is doubled, the magnetic field becomes half, i.e., now the magnetic field will be $0.2\, T$.