Eddy Currents and Magnetic Damping

When a conductor moves through a magnetic field, induced currents called eddy currents circulate within the material. According to Lenz’s law, these currents oppose the motion that produced them, creating drag known as magnetic damping. This effect is demonstrated with a pendulum swinging between magnet poles and has applications in braking, balances, and induction technologies.

1. Suspend a flat metal plate as a pendulum and position it so that it swings between the poles of a strong magnet. Observe that its motion is quickly damped.
2. Replace the solid plate with a slotted metal plate. Swing it again and note that damping is much weaker because eddy currents are restricted.
3. Replace the plate with a nonconducting material (e.g., plastic or wood) and observe negligible damping, since almost no eddy currents are induced.
4. Ask students to predict current directions using Faraday’s law and Lenz’s law: as the plate enters the field, induced currents oppose the change in flux; as it leaves, currents reverse but still oppose motion.
5. Discuss how slots or laminated construction reduce eddy currents and are used in real devices.

Faraday's Law Demo: Eddy Pendulum - Physics Demos:

Eddy Currents and Magnetic Braking of a Pendulum Caused by Electromagnetic Induction - Electric and Magnetic Fields:

• Test plates of different metals (copper, aluminum, steel) to compare conductivity and damping strength.
• Try thicker versus thinner plates to show how plate geometry affects damping.
• Use a rotating conducting disk in a magnetic field to model how induction brakes work.
• Demonstrate induction heating by placing a ferromagnetic cooking pot on an induction cooktop and explaining the role of eddy currents.

• Keep fingers clear of the pendulum swing and strong magnets.
• Handle strong rare-earth magnets with care to avoid pinching or sudden attraction.
• Do not place magnets near electronic devices, bank cards, or medical implants.
• Ensure pendulum setup is stable so the magnet does not tip or fall.

• Why do eddy currents oppose the motion of the plate? (Lenz’s law: the induced current creates a magnetic field opposing the change in flux.)
• Why does a slotted plate reduce magnetic damping? (Slots break up current loops, reducing their size and effectiveness.)
• Why is there no damping when the plate is fully inside the uniform magnetic field? (No flux change occurs, so no current is induced.)
• Why does an insulating plate experience negligible damping? (No mobile charges exist to form currents.)
• How does magnetic damping help improve sensitive balances? (It reduces oscillations quickly without adding friction.)
• Why can eddy currents slow but not fully stop a moving object, like a train? (The damping force decreases with speed and approaches zero as motion slows.)