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

0.2 T

The correct answer is Option (1) → 0.2 T

Given:

Magnetic field at distance r from a long straight current-carrying wire is 0.4 T.

Formula: Magnetic field due to a long straight wire is given by:

$B = \frac{\mu_0 I}{2\pi r}$

This implies $B \propto \frac{1}{r}$.

So, if distance is doubled ($r \rightarrow 2r$), then magnetic field becomes:

$B' = \frac{0.4}{2} = 0.2\ \text{T}$