Lenz's Law
Drop a magnet through a copper tube and watch it fall in slow motion. Eddy currents induced by the changing magnetic flux create a braking force that opposes the magnet's motion — a vivid demonstration of electromagnetic induction and energy conservation.
About this lab
In 1831, Michael Faraday discovered that a changing magnetic field induces an electric current in a nearby conductor — the phenomenon of electromagnetic induction. Three years later, Heinrich Lenz formulated the rule that bears his name: the direction of an induced current is always such as to oppose the change that produced it. This is not merely a sign convention; it is a direct consequence of the conservation of energy. If the induced current aided the change instead of opposing it, the system would spontaneously generate energy from nothing, violating the first law of thermodynamics.
When a strong magnet falls through a copper tube, the changing magnetic flux through the tube walls induces circular eddy currents. These eddy currents generate their own magnetic field, which opposes the motion of the falling magnet. The faster the magnet moves, the greater the rate of flux change, the stronger the induced currents, and the greater the braking force. This velocity-dependent drag causes the magnet to reach a terminal velocity — a speed at which the magnetic braking force exactly balances gravity. The result is striking: a powerful neodymium magnet can take ten or twenty times longer to fall through a copper tube than a non-magnetic object of the same mass.
Eddy current braking is not just a physics demonstration — it is widely deployed in engineering. Regenerative braking systems in electric trains and hybrid vehicles convert kinetic energy back into electrical energy through electromagnetic induction, simultaneously slowing the vehicle and recharging its batteries. Roller coasters use permanent-magnet eddy current brakes for their final stops because they are completely fail-safe: they require no power and cannot malfunction in a way that removes braking force. Industrial metal detectors and electromagnetic sorting systems rely on the same physics to identify and separate conductive materials.
At the deepest level, Lenz's law is connected to one of the most profound results in theoretical physics: Noether's theorem, proved by Emmy Noether in 1918. The theorem states that every continuous symmetry of nature corresponds to a conservation law. The conservation of energy arises from the time-translation symmetry of physical laws — the fact that the laws of physics are the same today as they were yesterday. Lenz's law, as an expression of energy conservation in the electromagnetic domain, is thus ultimately a consequence of the temporal homogeneity of the universe. When you watch the magnet slow down inside the tube, you are watching the universe enforce its own symmetry.