The magnetic field inside a long straight solenoid with n turns per unit length carrying current I is approximately:

1. Ampère's law applied to a long solenoid gives B = μ₀ n I inside (uniform). 2. Outside the solenoid, B ≈ 0 (far from the ends). 3. Solenoids are common laboratory tools for producing uniform B. _Source: NCERT Class 12 Physics Ch 4 §4.8 Solenoid_