Eddy Currents
Pendulum between magnets. Builds on Lenz's Law.
You swing a solid aluminum pendulum bob (a flat plate) between the poles of a powerful electromagnet. The magnet is turned off at first, and the pendulum swings freely. Then you switch the magnet on.
What happens to the pendulum when the magnet is turned on?
The pendulum stops dead in just one or two swings! As the conducting plate moves through the magnetic field, the changing flux induces swirling eddy currents in the metal. These currents create their own magnetic field that opposes the plate’s motion. The braking is dramatic — and completely contactless. Cut slots in the plate, and the braking almost disappears because the eddy current loops are disrupted.
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Watch the solid pendulum swing into the magnet’s field — it stops dead. Notice the swirling eddy current arrows on the plate. Now try the slotted plate — the slots break the current loops and the pendulum swings much more freely!
F ∝ σvB²
The braking force grows with speed, field strength squared, and conductivity. At zero speed, the force is zero — eddy currents can slow you down but never fully stop you (in theory).
High-speed trains use eddy current brakes for smooth, wear-free deceleration. The braking force naturally increases with speed, providing self-regulating braking without any physical contact.
At the end of the ride, conductive fins on the car pass between permanent magnets. Eddy currents provide smooth, reliable braking that never wears out and works even if the power fails.
Transformer cores are made of thin laminated sheets instead of solid iron. The insulating layers between sheets break up eddy current loops, reducing energy loss and heating.
Metal detectors work by inducing eddy currents in nearby metal objects. The eddy currents create their own field that the detector picks up — revealing hidden metal without physical contact.
“Move a conductor through a magnetic field and invisible currents swirl inside it, fighting the motion. That’s eddy currents — Lenz’s law made visible in the dramatic death of a pendulum’s swing.”
Eddy Current Lab
Swing pendulums made of different materials through a magnetic field. Compare solid vs slotted plates. Adjust magnet strength and material conductivity. Watch the energy dissipation in real time.
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Start with a solid copper plate and strong magnets — watch it stop instantly. Now try the slotted plate: the slots break the eddy current loops and the pendulum swings through! Increase conductivity and watch the braking get stronger. Try a non-conductive plate — no braking at all. Can you find the combination where the pendulum just barely makes it through?
Eddy currents are loops of induced current in bulk conductors. They create forces that always oppose relative motion between the conductor and the magnetic field. Slotting or laminating the conductor disrupts the current loops and reduces the braking effect — a critical technique in electrical engineering.