A long solenoid carries a steady current. Which of the following stat…

The correct answer is Option (4) → The field inside is uniform, and the field outside is nearly zero.

A long solenoid is a wire wound as a closely spaced helix. Its length is large compared with its radius. Inside it, the fields of all the turns add up.

Option (1) The field is strongest close to the turns and weakest on the axis. — Incorrect. On the axis, every turn gives a field along the axis. These fields add, so the field on the axis is strong. Inside, the field has the same value at every point.

Option (2) For the same turns per unit length and current, a thinner solenoid has a stronger field inside. — Incorrect. The field inside is \( B=\mu_0nI \). It depends only on the turns per unit length \( n \) and the current \( I \). The radius does not appear in it, so a thinner solenoid has the same field. The radius enters only the field at the centre of a circular coil, \( \mu_0NI/2R \).

Option (3) The field lines inside are circles drawn around the axis. — Incorrect. Circles around a line are the field lines of a long straight wire. Inside a solenoid the lines run straight along the axis.

Option (4) The field inside is uniform, and the field outside is nearly zero. — Correct. Inside, the field lines are straight, parallel to the axis and evenly spaced, so the field is uniform. Outside, the field becomes very weak and is taken as zero.

NCERT (2026-27):
The Solenoid – “… we see that the field at the interior mid-point P is uniform, strong and along the axis of the solenoid.”

The Solenoid – “The field outside the solenoid approaches zero. We shall assume that the field outside is zero. The field inside becomes everywhere parallel to the axis.”