The magnetic field inside a current carrying very long solenoid is

The correct answer is A, Uniform. Inside a long current-carrying solenoid the magnetic field has the same strength and direction at every point, shown by parallel, evenly spaced field lines.

A solenoid is a long coil of many closely wound turns of insulated wire. When current flows, the fields of the turns add up inside the coil and almost cancel outside it. For a very long solenoid the field inside is B = μ₀nI, where n is the number of turns per unit length and I is the current; it does not depend on the position inside or on the radius. Placing a soft iron core inside turns the solenoid into a strong electromagnet, used in electric bells, cranes and relays.

B is wrong because the field becomes non-uniform only near the ends of the solenoid, where it falls to about half its inside value.

C is wrong because the field is nearly zero outside a long solenoid, not inside it.

D is wrong because the field is the same all along the inside, not greatest at the middle.

Exam tip: long solenoid, B = μ₀nI inside and uniform, nearly zero outside, half at the ends.