Lenz's Law
Drop a magnet through a copper tube and watch it float down as if through honey. Changing magnetic flux induces eddy currents in the tube wall, which create their own magnetic field that opposes the magnet's motion. The left tube contains a conductor; the right is just air for comparison. The magnet in the tube reaches a terminal velocity far slower than free fall.
ε = −dΦB/dt | Faraday's law: induced EMF opposes flux change
Magnetic Flux Through Tube Cross-Section
Lenz's law
Lenz's law states that an induced current flows in a direction such that the magnetic field it creates opposes the change in flux that induced it. This is the negative sign in Faraday's law: ε = −dΦB/dt. When a magnet falls through a conducting tube, the changing flux through each horizontal ring of the tube induces circular eddy currents. These currents create their own magnetic field that pushes back against the falling magnet, dramatically slowing its descent.
The physics of braking
The braking force is proportional to the magnet's velocity, the square of the magnetic field strength, and the tube's conductivity: Fbrake ∝ σB²v. This is why the magnet reaches a terminal velocity where gravitational force exactly balances the electromagnetic braking force: mg = σB²vterminal. At terminal velocity, the magnet drifts down at constant speed — sometimes taking 10× longer than free fall to traverse the tube.
Eddy currents
The induced currents form closed loops (eddies) in the tube wall. Above the magnet, where flux is increasing, the eddy currents create a field that repels the magnet (opposing the increase). Below the magnet, where flux is decreasing, the currents create a field that attracts the magnet (opposing the decrease). Both effects combine to slow the fall.
Material comparison
Copper has the highest conductivity among common metals (5.96 × 107 S/m), producing the strongest braking. Aluminum has about 61% of copper's conductivity, so the magnet falls faster. An ideal conductor (infinite conductivity) would stop the magnet completely — the braking force becomes infinite, and the magnet hovers.
Real demonstrations
This is one of the most popular physics demonstrations. A neodymium magnet dropped through a thick copper pipe takes several seconds to traverse a meter, while a non-magnetic object of the same size falls in a fraction of a second. The effect is dramatic and never fails to surprise audiences, because there is no visible mechanism — just an invisible electromagnetic embrace slowing the fall.