Eddy currents
Currents that swirl inside metal
This topic is beyond the current NCERT syllabus, but it is useful, especially for competitive exams, and it explains many machines around you.
So far, induced currents flowed in wires and coils. But a solid piece of metal can also carry induced currents. When the flux through a bulk conductor changes, currents circulate inside it, in closed loops.
Their swirling paths look like eddies in a stream of water, so they are called eddy currents. They were discovered by the French physicist Foucault in 1895.
Here is a classic demonstration. A copper plate hangs like a pendulum and swings between the pole pieces of a strong magnet.
As the plate enters the field, the flux through part of it increases. As it leaves, the flux decreases. Each change drives loops of eddy current in the copper.
By Lenz’s law, these currents oppose the change, so they oppose the motion of the plate. The swing dies away after a few oscillations. This is electromagnetic damping.
Where does the energy of the swing go? Into heat. The eddy currents flow through the resistance of the copper, and warm it up.
Now cut slots across the plate. The swing lasts much longer. The slots break the big current loops into small ones, so the eddy currents are much weaker.
This tells us how to reduce eddy currents when we do not want them: break up the metal so that the currents have no wide closed paths to flow around.
Unwanted heat, useful brakes
In the iron core of a transformer or an electric motor, the flux changes all the time. Eddy currents flow in the core, waste energy as heat, and warm it up.
To cut this loss, the core is not made of one solid block. It is built from thin sheets, called laminations, each coated with an insulating layer such as lacquer.
The sheets lie parallel to the magnetic field, so the flux passes easily along them. But the eddy currents would have to cross from sheet to sheet, through the insulation. Their paths are cut, and the currents are small.
Eddy currents can also be useful. Some electric trains use magnetic brakes. Strong electromagnets sit just above the rails. Switch them on, and eddy currents in the rails oppose the motion of the train. The braking is smooth.
In a moving-coil galvanometer, the coil is wound on a metal core. When the coil swings, eddy currents in the core damp the motion, so the needle settles quickly at its reading instead of oscillating.
An induction furnace passes a high-frequency alternating current through a coil around a container of metal. The eddy currents in the metal heat it until it melts. It is used to make alloys.
Older electricity meters in homes have a shiny aluminium disc that turns slowly. It is driven by eddy currents induced in it, and its turns count the energy used.
So the same physics that wastes energy in a transformer brakes a train, steadies a meter needle and melts steel. It is all Faraday’s law and Lenz’s law in a solid piece of metal.
Try it: the damped pendulum
- Release the solid copper plate. The swirling arrows are eddy currents, strongest where the plate enters or leaves the field.
- Now choose the slotted plate and release it again. The dashed curve keeps your previous run, so you can compare the two.
- Turn the magnet off: only air resistance remains, and the plate swings on and on.
- Halve the field strength. The damping force depends on B², so the swing lasts about four times longer.
Slots in the plate
A solid copper plate and an identical plate with slots cut in it swing as pendulums through the same magnetic field. What happens?
- Both stop at the same rate, since both are copper
- The slotted plate stops faster, because it is lighter
- The solid plate stops faster, because eddy currents in it are larger
- Neither is affected, since copper is not magnetic
Answer
C. Copper is not attracted by the magnet, but it conducts, so a changing flux drives eddy currents in it. In the solid plate they flow in large loops and strongly oppose the motion. The slots break the current paths, so the eddy currents and the damping are much smaller.
Why laminate the core?
Why is the iron core of a transformer made of thin insulated sheets instead of one solid block?
- To increase the magnetic flux through the core
- To reduce eddy currents and the heat they produce
- To make the core lighter and cheaper
- To increase the resistance of the transformer’s coils
Answer
B. The changing flux in the core would drive large eddy currents through a solid block, wasting energy as heat. Thin sheets coated with insulation, lying parallel to the field, still carry the flux, but they cut the paths of the eddy currents, so the loss is much smaller.
Summary
- Cause: changing flux through the metal:
ε = −dΦ_B/dt - Energy lost as heat:
P = I²R
- Eddy currents are induced currents that circulate in closed loops inside a solid conductor when the flux through it changes.
- They oppose the motion (Lenz’s law) and turn kinetic energy into heat: electromagnetic damping.
- They are reduced by breaking the current paths: slots, or laminated cores of thin insulated sheets.
- Uses: magnetic brakes in trains, damping in galvanometers, induction furnaces, and the discs of electricity meters.