The magnetic field inside a long straight solenoid-carrying current _…

The correct answer is Option (2) → Is uniform inside the solenoid

For a long straight solenoid carrying steady current, the magnetic field inside is nearly uniform (constant in magnitude and parallel to the axis) throughout most of the interior, except near the ends where fringe effects occur. This is derived from Ampere's law: applying it to a rectangular Amperian loop with one side inside the solenoid, the enclosed current is $N I$ (for N turns), and the field outside is zero, leading to $B = \mu_0 n I$ (where n is turns per unit length) uniformly inside an ideal long solenoid.

Why the others are incorrect:

• Is zero inside: No, it's $\mu_0 n I > 0$.
• Increases as we move towards its end: Actually, it slightly decreases near the ends due to fringing.
• Decreases as we move towards its end: This is partially true near the ends but not the primary characteristic; the key feature is uniformity in the central region.

Thus, the correct answer is Is uniform inside the solenoid.