The magnetic field inside a long straight solenoid-carrying current _______?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Is uniform inside the solenoid
A solenoid is a long coil of many close turns of insulated wire. When a current flows, the fields of all the turns add up inside the coil, and the magnetic field lines run inside it as parallel straight lines. Parallel, evenly spaced field lines mean that the field has the same strength and direction at all points inside, that is, it is uniform.
This is why a solenoid is used to make an electromagnet: a soft iron core placed inside is magnetised strongly and evenly.
Why the other options are incorrect
Option (1) Is zero inside the solenoid — Incorrect. The field is strongest inside the coil; it is outside a long solenoid that the field is very weak.
Option (3) Increases as we move towards its end — Incorrect. The field lines spread out at the ends, so the field does not become stronger there.
Option (4) Decreases as we move towards its end — Incorrect. The field weakens only at the open ends, where the lines spread out; along the inside of a long solenoid it stays the same.