A solenoid has a core of a material with relative permeability 400. The windings of the solenoid are insulated from the core and carry a current of 1 A. If the number of turns is 800 per metre, the magnetic field inside solenoid will be
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 0.40\,\mathrm{T} \)
Inside a long solenoid the field in vacuum is \( \mu_0 n I \). A core of relative permeability \( \mu_r \) multiplies the field by \( \mu_r \).
\( \displaystyle B = \mu_r \mu_0 n I \)
\( \mu_r = 400 \)
\( n = 800\,\mathrm{m^{-1}} \)
\( I = 1\,\mathrm{A} \)
\( \mu_0 = 4\pi\times10^{-7}\,\mathrm{T\,m\,A^{-1}} \)
\( \displaystyle B = 400 \times 4\pi\times10^{-7} \times 800 \times 1 = 0.402\,\mathrm{T} \approx 0.40\,\mathrm{T} \)
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):
Magnetisation and Magnetic Intensity – …
