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Experiments to Demonstrate Lenz's Law

A simple experiment can be used to help students develop an understanding of the induction of eddy currents and Lenz's Law. If a magnet is moved past a conductive material, two things happen. First, the moving magnetic field cuts through the conductor and induces eddy currents in the conductor. This was discovered by the English scientist, Michael Faraday. Next, the eddy currents in the conductor generates their own magnetic field, which opposes the magnetic field of the magnet. In 1834, Russian physicist Heinrich Lenz discovered this directional relationship between the induced magnetic fields and current, which is known as Lenz's Law.

Lenz's Law states:
An electromagnetic field interacting with a conductor will generate electrical current that induces a counter magnetic field that opposes the magnetic field generating the current.

Arago's Swinging Magnet Experiment

A French scientist, Dominique Arago, developed a simple experiment to demonstrate Lenz's Law.

Hang a magnet from a string over the surface of a conductive, but nonmagnetic material such as copper. The magnet should come very close to the surface when it swings back and forth, but it should not touch the surface. Swing the magnet and observe what happens. Remove the conductive material and compare what happens when the magnet swings in absence of the conductor.

It can be seen that the swing of the magnet is dampened when the conductor is in close proximity. The metal has no magnetic attraction to the magnet, so what force is acting on the magnet?

As the magnet swings and passes over the conductive material, the magnet's magnetic field cuts through the conductor and induces an electrical current (Remind the students about induction and how current is always generated in a conductor in the presence of a changing magnetic field). The current in the conductor generates its own magnetic field, which according to Lenz's Law, opposes the magnetic field that caused the current. Therefore, it is the opposing magnetic field from the induced current (eddy currents) that slows the swing of the magnet.

Want A More Exciting Experiment?

A more elaborate experiment that demonstrates the same scientific principles can be done with a solenoid wrapped around a pop can. When the switch is closed, a capacitor in the circuit is discharged through the solenoid. Since the current varies in time, the magnetic field in the solenoid and the magnetic flux defined in the solenoid will vary in time. This variable flux also passes through the can inside the solenoid, inducing in it a current in opposite direction of the current through the solenoid (Lenz's Law). The two antiparallel currents repel each other, and since the solenoid is fixed the can will be crushed.

Teaching Tips

Signal Interpretation
DAC Curve

Eddy Current
Depth of Penetration
Lenz Law

Backscatter Radiation
Film Handling Artifacts
Focal Spot Size
Lead Screens
Source/Object Orientation