1. Concept Overview
Eddy currents are circular currents induced in bulk pieces of metal when the magnetic flux passing through them changes. Instead of flowing through a wire loop, these currents swirl inside the metal like small whirlpools of electricity. That is why they are called “eddy” currents — just like water eddies in a river.
Eddy currents appear whenever a conductor is placed in a changing magnetic field. They can be useful in some situations and unwanted in others, depending on how strong they are and where they form.
1.1. One-line idea
Changing magnetic flux in a solid conductor → circulating induced currents inside it.
2. How Eddy Currents Are Produced
When a solid metallic plate or block is exposed to a changing magnetic field, different parts of the metal experience changing flux. According to Faraday’s law, induced emf is produced. Since the metal is continuous, the emf causes currents to flow in closed circular paths.
These circulating currents form loops inside the metal, just like tiny circles of flowing charge.
2.1. Conditions needed for eddy currents
- A conductor (usually a metal block or sheet)
- A changing magnetic field (due to motion or alternating current)
- Closed internal paths for current to swirl
3. Nature of Eddy Currents
Eddy currents always oppose the change that produces them. This comes directly from Lenz’s law. Whether the magnetic flux increases or decreases, the eddy currents produce their own magnetic fields that resist the change.
3.1. Opposition explained
If a metal sheet enters a magnetic field, eddy currents form in such a direction that they oppose the motion. If the sheet leaves the field, eddy currents reverse and still oppose the change.
4. Mathematical View is Not Required
There is no simple formula for eddy current magnitude at this level because it depends on the shape, conductivity, thickness and motion of the metal. What matters most is understanding how eddy currents behave, not calculating their exact values.
4.1. Key takeaway
Eddy currents are induced by changing flux and always oppose the change, just like any induced current described by Lenz’s law.
5. Effects of Eddy Currents
Eddy currents can have both helpful and harmful effects depending on the situation. They can produce strong magnetic forces, cause heating, and resist motion.
5.1. Heating effect
Eddy currents produce heat because the swirling charges face electrical resistance. This heating effect is used in induction cookers and induction furnaces.
5.2. Magnetic damping effect
When a conductor moves in a magnetic field, eddy currents oppose motion and slow it down. This slowing effect is called magnetic damping.
5.3. Mechanical resistance
If a metal plate swings between magnetic poles, eddy currents create a force that resists the swing. The swing quickly dies out because energy is lost due to eddy current heating.
6. Reducing Eddy Currents
In electrical machines like transformers, motors and generators, eddy currents cause unwanted heating. To reduce them, metal parts are designed to discourage swirling currents.
6.1. Laminated cores
Thick solid metal blocks are replaced by thin laminated sheets separated by insulating material. This breaks the large loops of eddy currents into small loops, greatly reducing them.
6.2. Using high-resistance materials
Materials like silicon steel increase electrical resistance and reduce eddy currents naturally.
6.3. Slotting and shaping
Making slots or cuts in the metal core breaks circular current paths and reduces eddy current strength.
7. Useful Applications of Eddy Currents
Although eddy currents often cause losses, they are very useful in devices where controlled heating or smooth braking is required.
7.1. Induction cookers
Rapidly changing magnetic fields induce strong eddy currents in the metal base of the cooking vessel. These currents heat the vessel quickly.
7.2. Eddy current brakes
Used in trains, roller coasters and exercise bikes. Eddy currents oppose motion and provide a smooth, wear-free braking force.
7.3. Metal detectors
Changing magnetic fields induce eddy currents in hidden metal objects. These currents disturb the detector’s field and help locate the metal.
7.4. Energy meters
Eddy currents in rotating aluminum discs help produce a braking torque that controls rotation speed in old-style energy meters.
8. Classic Demonstrations
There are simple demonstrations that clearly show eddy currents in action.
8.1. Falling magnet through a copper tube
A magnet falls slowly through a copper tube because eddy currents oppose its motion. The tube never touches the magnet, yet it slows the fall dramatically.
8.2. Swinging metal plate in a magnetic field
When a metal plate swings between strong magnetic poles, eddy currents damp the swing and quickly bring it to rest.