Solenoid

A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar 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. Such cores are typical in electromagnets.

In the above expression for the magnetic field B, n = N/L is the number of turns per unit length, sometimes called the "turns density". The magnetic field B is proportional to the current I in the coil. The expression is an idealization to an infinite length solenoid, but provides a good approximation to the field of a long solenoid.

Index

Magnetic field concepts

Currents as magnetic sources

Solenoid Field from Ampere's Law

Taking a rectangular path about which to evaluate Ampere's Law 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 taken so far from the coil that the field is negligible, then the length inside the coil is the dominant contribution. Index

Magnetic field concepts

Currents as magnetic sources

Solenoid Magnetic Field Calculation

For a solenoid of length L = m with N = turns,
the turn density is n=N/L= turns/m.

If the current in the solenoid is I = amperes

and the relative permeability of the core is k = ,

then the magnetic field at the center of the solenoid is

B = Tesla = gauss.

The Earth's magnetic field is about half a gauss.

The relative permeability of magnetic iron is around 200.

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Index

Magnetic field concepts

Currents as magnetic sources