Lenz's Law
Drop magnet through copper tube. Builds on Faraday's Law.
You hold two identical strong neodymium magnets at the top of two vertical tubes — one made of copper (an excellent conductor) and one made of plastic (an insulator). You release both magnets at the same time.
What happens to the magnet falling through the copper tube?
The magnet drifts down the copper tube in slow motion — taking 5–10 times longer than freefall! Copper isn’t magnetic, so it won’t attract or stick to the magnet. But the falling magnet’s changing flux induces swirling eddy currents in the copper, and those currents create a magnetic field that opposes the magnet’s motion. It’s electromagnetic braking in action.
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Watch the side-by-side comparison: magnet in copper vs plastic tube. Notice the swirling eddy currents that appear on the copper tube walls. The induced B-field pushes back against the falling magnet.
ε = -dΦ/dt
The negative sign IS Lenz’s law. Nature opposes the change — always. If it didn’t, energy conservation would be violated.
Many roller coasters use magnetic braking at the end of the ride. Conductive fins on the car pass between strong magnets, and the induced eddy currents provide smooth, reliable braking — no friction or wear.
Precision measuring instruments use Lenz’s law to prevent the needle from overshooting. A conducting frame around the needle has eddy currents that damp oscillations, making it settle quickly.
Rapidly changing magnetic fields induce massive eddy currents in metal, heating it to melting point. Lenz’s law currents do the work of heating the metal directly.
Heavy vehicles use electromagnetic retarders for braking on long downhills. Rotating discs between electromagnets experience eddy currents that resist rotation — providing braking force without brake pad wear.
“Nature hates change. Push a magnet toward a coil, and the coil pushes back. That one-line summary of Lenz’s law is really conservation of energy wearing a magnetic disguise.”
Lenz’s Law Lab
Drop magnets through tubes of different materials and watch Lenz’s law in action. Adjust magnet strength, tube conductivity, and tube thickness to see how each affects the electromagnetic braking force.
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Start with a strong magnet in a thick copper tube — watch it drift down slowly. Now switch to plastic: it falls instantly. Try increasing tube conductivity — the braking gets stronger. Decrease the tube thickness and watch the braking weaken. Can you find the combination that makes the magnet take the longest to fall?
Lenz’s law says induced currents always oppose the change that created them. A magnet falling through a conductor induces eddy currents that create a braking force. Stronger conductor, stronger magnet, or thicker walls all increase the braking effect.