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  4. 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

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Q. 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

VITEEEVITEEE 2019

Solution:

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 $0.2\, T$.