Consider an infinite solenoid with a rectangular cross section instead of a circular cross section.
-(b)The solenoid's magnetic field must still be zero everywhere outside the solenoid.

Question

Quizplus · Accepted Answer

The magnetic field outside an ideal infinite solenoid is zero regardless of the cross-sectional shape, as the field lines are contained within the solenoid due to symmetry and the infinite length assumption.

Consider an Infinite Solenoid with a Rectangular Cross Section Instead