A solenoid of length \( 0.4 \) m, radius of \( 1 \) cm and \( 400 \) turns carries a current of \( 2 \) A. The magnetic field produced inside the solenoid is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( \sim 2.51\times10^{-3} \) T
Inside a long solenoid the field is uniform and depends only on the number of turns per unit length n and the current, not on the radius.
\( \displaystyle B = \mu_{0}nI, \qquad n = \frac{N}{l} \)
\( \displaystyle n = \frac{400}{0.4\ \mathrm{m}} = 1000\ \mathrm{m^{-1}} \)
\( B = (4\pi\times10^{-7}\ \mathrm{T\,m\,A^{-1}}) \times (1000\ \mathrm{m^{-1}}) \times (2\ \mathrm{A}) \)
\( B = 8\pi\times10^{-4}\ \mathrm{T} \approx 2.51\times10^{-3}\ \mathrm{T} \)
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
The Solenoid – …
