A 25 cm long solenoid with 100 turns carries a current of 3 A. The flux density at the center of the solenoid is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( 1.51\times10^{-3}\,\mathrm{Wb/m^{2}} \)
Near the centre of a long solenoid the magnetic field is uniform, with flux density \( B = \mu_0 n I \), where \( n \) is the number of turns per unit length. The solenoid is treated as long.
\( \displaystyle n = \frac{N}{l} = \frac{100}{0.25\,\mathrm{m}} = 400\,\mathrm{m^{-1}} \)
\( \displaystyle B = \mu_0 n I = 4\pi\times10^{-7} \times 400 \times 3 = 1.51\times10^{-3}\,\mathrm{T} \)
Since \( 1\,\mathrm{T} = 1\,\mathrm{Wb/m^{2}} \), \( B = 1.51\times10^{-3}\,\mathrm{Wb/m^{2}} \).
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 – …
