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
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 0.2\,\mathrm{T} \)
The magnetic field of a long straight wire falls off inversely with the distance from the wire.
\( \displaystyle B = \frac{\mu_0 I}{2\pi r} \;\Rightarrow\; B \propto \frac{1}{r} \)
When the distance doubles, the field becomes half.
\( \displaystyle B' = \frac{\mu_0 I}{2\pi (2r)} = \frac{B}{2} = \frac{0.4\,\mathrm{T}}{2} = 0.2\,\mathrm{T} \)