A long straight conducting wire carries a current of 100 A. At a distance of 0.4 m from the wire, the magnetic field produced by the current is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 5\times10^{-5} \) T
By Ampere’s circuital law (or the Biot–Savart law), the field of a long straight wire at distance r is \( B = \mu_0 I/2\pi r \).
\( \displaystyle B = \frac{\mu_0 I}{2\pi r} = \frac{4\pi\times10^{-7}\,\mathrm{T\,m\,A^{-1}} \times 100\,\mathrm{A}}{2\pi \times 0.4\,\mathrm{m}} \)
\( \displaystyle B = \frac{2\times10^{-7} \times 100}{0.4}\,\mathrm{T} = 5\times10^{-5}\,\mathrm{T} \)
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
