Eddy current

From HvWiki

Currents induced in any conducting material in a changing magnetic field.


If a material is conductive (with conductivity \sigma\, and immersed in a time-varying magnetic field and/or is in motion with respect to the source of the magnetic field, the current density \textbf{J}\, in the material is written in terms of the apparent electric field as

\textbf{J} = \sigma\textbf{E}\,

where \textbf{E}\, is found using Faraday's Law. This is the so-called point form of Ohm's Law (described here for lumped circuits).

Eddy currents are usually undesirable because they introduce losses and expel magnetic flux from electrically conductive magnetic materials (a form of Lenz's Law). These effects can be reduced by using cores with insulated laminates, wires or powdered media like ferrite and powdered iron.

Sometimes, eddy currents can produce a desired effect. The Thompson Coilgun relies on eddy currents induced in the projectile to generate force. Also, eddy current effects are sometimes used in magnetic braking systems found on trams and trains.

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