Lenz law

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Some easy experiments that technicians can set up can be used to help develop an understanding of the induction of eddy currents and Lenz's Law. If a magnet is moved past a conductive material, like Copper, 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 an electrical current that induces 
a counter magnetic field that opposes the magnetic field generating the current

Simple experiment to demonstrate Lenz's Law.

  • You will need a standard magnet, a piece of copper wire and thin thread.

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.

Lenz law.jpg
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. As the magnet swings and passes over the conductive material, the magnet's magnetic field cuts through the conductor and induces an electrical current. 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.


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.

Details of the experimental setup can be found here:Colorado University Webpage

Note that the device,as described, is charged to 3kV and the current from a capacitor would exceed safe working levels. If it is intended to be built for school use significant attention should be paid to isolating the unit. Of course high voltage capacitors must be used as capacitors used beyond their working limits have a tendancy to go "phutt"!.
This probably not suited to a school environment.
Caution.jpg The dangers, as highlighted in the web article, suggest that this only be tackled by someone with a thourough understanding of electrical safety and good understanding of a.c. theory. The coil will act as an inductor and the circuit will require tuning for best action.

Another elegant display of Lenz’s law is where a rare earth magnet is dropped through a copper or aluminium pipe (such as plumbers use). Purpose made kits are available but can be made cheaply with a short length of piping only slightly larger than the diameter of the magnet.
The magnet is dropped through the tube and the descent time is far greater than would be expected. This is due to the eddy currents induced as the magnet falls creating a magnetic field opposing the falling magnet.

It is useful to either have a glass tube (of identical cross section as the pipe) or a ball bearing of similar dimensions as the magnet to illustrate that the effect is due to the magnet.

Finally, a method of illustrating how to reduce eddy currents is able to be demonstrated with two metal blades acting as pendula. These are sometimes described as sectors in catalogues. If making your own a thick plate of copper or aluminium should be used and two identical wedges should be cut. A pivot should be located toward the thin end of the wedges. This can be a simple hole with an optics pin into a cork.
One wedge should have multiple vertical slots snipped into (i.e. the slits run toward the pivot). When placed in a magnetic field the slotted pendulum does not stop so quickly as the complete pendulum

--Ssmith 21:24, 14 July 2008 (BST)
Notes added--D.B.Ferguson 21:28, 16 July 2008 (UTC)

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