Induced Electric Field & Eddy Currents

If the loop isn’t moving, the magnetic force can’t start its electrons going. Something else must: an electric field produced by the changing magnetic field itself.

The induced electric field#

Hold a loop still in a steady field, then let the field change. A current starts. But the electrons were at rest, and a magnetic field cannot push charges at rest, so it must be an electric field that sets them moving. It is not made by charges (Coulomb’s law has nothing to say about it); it is made by the changing magnetic field.

• It is non-electrostatic and non-conservative: no potential can be defined for it.
• Its field lines are closed loops, with no beginning or end.
• It exists whenever B changes, loop or no loop. A conducting loop merely lets its free electrons respond, giving a current.

The work per unit charge round a loop is the emf, so Faraday’s law becomes a statement about the field:

Eddy currents#

A solid metal plate moving into a field has no single wire to guide the current, but every closed path drawn on it has a changing flux. Currents swirl round many such paths at once: eddy currents.

They heat the plate, and that energy comes out of its kinetic energy: the plate slows down. This is electromagnetic damping. Cutting slots in the plate breaks up the current paths and greatly reduces it.