Eddy Currents: The Intuition
Imagine you have a solid metal plate — say, copper or aluminium — and you bring a strong magnet near it. If you move the magnet quickly, something strange happens: the metal plate seems to resist the motion. It feels like the plate is trying to push back against the magnet. And if you keep moving the magnet, the plate gets warm.
Why? The metal is a conductor. When the magnetic field through any part of it changes, that changing field induces an electric field inside the metal itself. That induced electric field pushes the free electrons in the metal, making them flow in closed loops — like tiny whirlpools of current. These are eddy currents.
The name comes from the way they swirl, like eddies in a river. They are not confined to a wire; they circulate freely within the bulk of the conductor.
The Precise Statement
Eddy currents are loops of electrical current induced within a conductor by a changing magnetic field, due to Faraday's law of electromagnetic induction. They flow in closed paths perpendicular to the direction of the magnetic flux.
The key points:
- They arise only when the magnetic flux through the conductor changes — either because the magnet moves, the conductor moves, or the field itself changes with time.
- They are not limited to a specific path; they spread throughout the conductor, following the path of least resistance for the induced electric field.
- Their magnitude depends on three things: the rate of change of flux, the conductivity of the material, and the thickness of the conductor.
Why They Matter: Two Major Effects
1. Heating (Joule Heating)
Eddy currents are real currents, so they encounter electrical resistance. As they flow, they dissipate energy as heat. This is why the metal plate gets warm when you move a magnet near it.
In many devices — like transformers and electric motors — eddy currents are wasteful. They convert useful electrical energy into unwanted heat, reducing efficiency. Engineers fight this by laminating the core: slicing it into thin sheets insulated from each other, which breaks the path of the eddy currents and shrinks them.
2. Electromagnetic Damping
Remember the feeling of resistance when you moved the magnet? That is Lenz's law in action. The eddy currents create their own magnetic field that opposes the change that caused them. So if you try to move a magnet toward a metal plate, the induced eddy currents produce a field that pushes the magnet back. If you try to pull it away, they pull it back.
This is electromagnetic damping — a non-contact braking force. It is used in:
- Magnetic brakes in trains and roller coasters (no friction, no wear)
- Galvanometers (a metal former around the coil damps the pointer's oscillations)
- Energy meters (a rotating aluminium disc is slowed by a permanent magnet)
A Simple Experiment to See It …