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A long solenoid of radius 1 mm has 100 turns per mm. If 1 A current flows in the solenoid, the magnetic field strength at the centre of the solenoid is

Asked in NEET 2022 · Field of a bar magnet and earth's field

Answer: (2) 12.56×10⁻² T

Step-by-step solution

Given: n=100 turns per mm, I=1 A. The radius is not needed — the field inside a long solenoid does not depend on it.

Convert the turn density to SI: n=(100)/(10⁻³ m)=10⁵ turns per metre.

Idea: well inside a long solenoid, B=μ₀ nI.

B=4π×10⁻⁷×10⁵×1.

=4π×10⁻² T.

=12.56×10⁻² T.

Why the other options are wrong

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