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.

Derive field expressionCalculate fieldField of current loop
The solenoid as an inductor
Index

Magnetic field concepts

Currents as magnetic sources
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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 solenoic 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.

This admittedly idealized case for Ampere's Law gives

This turns out to be a good approximation for the solenoid field, particularly in the case of an iron core solenoid.

Solenoid discussionCalculate field
Index

Magnetic field concepts

Currents as magnetic sources
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Solenoid Magnetic Field Calculation

At the center of a long solenoid

Magnetic field = permeability x turn density x current

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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Solenoid discussionDerive field expressionRelative permeability
Index

Magnetic field concepts

Currents as magnetic sources
  HyperPhysics***** Electricity and Magnetism Go Back