Foucault currents

Foucault currents are electric currents induced within conductors by a changing magnetic field in the conductor. These circulating currents have inductance and therefore induce magnetic field which counteract primary magnetic field in accordance with the Lorentz force law. (If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force F. For any produced force there will be an opposite reactive force.)


The swirling current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion. Hence, they veer to their right, or left, depending on the direction of the applied field and whether the strength of the field is increasing or declining. The resistivity of the conductor acts to damp the amplitude of the Foucault currents, as well as straighten their paths. Lenz's law states that the current swirls in such a way as to create an induced magnetic field that opposes the phenomenon that created it. In the case of a varying applied field, the induced field will always be in the opposite direction to that applied. The same will be true when a varying external field is increasing in strength. However, when a varying field is falling in strength, the induced field will be in the same direction as that originally applied, in order to oppose the decline.