Solenoid A long straight coil of wire can be used to generate a nearly uniform magnetic field similar field similar to that of a bar a bar magnet. magnet. Such coils, called solenoids, have an enormous number of practical applications. The field can be greatly strengthened by the addition of an iron core. core. Such cores are typical in electromagnets electromagnets..
In the above expression for the magnetic field B, n is the number of turns per unit length, sometimes called the "turns density". The expression is an idealiation to an infinite length solenoid, but provides a good approximation to the field of a long solenoid.
Solenoid Field from Ampere's Ampere's Law Ta!ing a rectangular path about which to evaluate Amperes #aw such #aw such that the length of the side parallel to the solenoid field is L gives a contribution BL inside the coil. The field is essentially perpendicular to the sides of the path, giving negligible contribution. If the end is ta!en so far from the coil that the field is negligible, then the length inside the coil is the dominant contribution.
This admittedly idealied case for Amperes #aw gives
This turns out to be a good approximation for the solenoid field, particularly in the case of an iron core solenoid.
