Eddy currents dissipate energy as heat in bulk conductors

Electromagnetic Induction — AQA A-Level Physics

Key Definition

Eddy currents — Circulating currents induced in the bulk of a conductor when it moves through a non-uniform magnetic field, or when a changing magnetic field passes through it.

Eddy currents are the same physics as any induced currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). — Faraday's lawThe magnitude of the induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is proportional to the rate of change of magnetic flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns).. applies. The difference is that instead of currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flowing in a neat loop of wire, it swirls around inside a solid piece of metal.
These circulating currents dissipate energyThe capacity to do work. Measured in joules (J). as heat due to the resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). of the metal. The kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. or magnetic energy that caused the flux change is converted to thermal energy.
By Lenz's lawThe direction of an induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is such that it opposes the change in magnetic flux that produced it. This is a consequence of conservation of energy., eddy currents always flow in a direction that opposes the change producing them. A metal plate swinging through a magnetic field slows down because the eddy currents create a braking force.

Applications and examples

Electromagnetic braking: a metal disc or plate moving through a magnetic field is slowed by eddy-current-induced forces. Used in trains, roller coasters, and laboratory balances. No physical contact needed, so no wear.
Induction heating: eddy currents in a metal pan on an induction hob heat the pan directly. The cooktop surface stays relatively cool because only the metal pan has eddy currents.
Metal detectors: an alternating current in the search coil creates a changing magnetic field. Eddy currents in a nearby metal object create their own changing field, which induces a signal in a receiver coil.
Transformers: eddy currents in the iron core waste energy as heat, reducing efficiencyThe ratio of useful energy output to total energy input, expressed as a fraction or percentage.. The fix is laminations.

Laminations reduce eddy currents

Transformer cores are made from thin sheets (laminations) of iron, each insulated from the next by a layer of varnish or oxide.
The insulating layers break up the paths that eddy currents would normally follow. Smaller loops mean smaller currents and less energy wasted as heat.
The laminations are oriented parallel to the magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area. (Wb)., so they don't significantly reduce the flux through the core.
Thinner $laminations = less eddy current loss = higher transformer efficiencyThe ratio of useful energy output to total energy input, expressed as a fraction or percentage.$.

Examiner Tips and Tricks

When explaining why laminations reduce eddy current losses, you must say that the insulating layers increase the resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). of eddy current paths (or break up the loops).
Just saying 'laminations reduce eddy currents' without explaining the mechanism won't get the mark.