neodymium(NdFeB) magnets, samarium cobalt(SmCo5, Sm2Co17)magnets, Alnico magnets, permanent rare earth magnets.

lenz's law

Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

A practical experiment involves dropping a magnet down a copper pipe. The inner diameter of the copper pipe should be only slightly greater than the diameter of the magnet to give the best effects. We suggest using our EP656 D12mm x 3mmA magnets with 15mm diameter copper pipe and our EP372 D18mm x 3mmA magnets with 22mm diameter copper pipe. Dropping the magnet outside of the pipe will show the speed at which the magnet falls due to gravity alone – this can be timed, typically less than a second for 0.5m fall. If the magnet is then dropped down the copper pipe, the magnet does not fall as quickly – it takes much longer, around 5 to 6 seconds. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

Magnetic fields from the magnet cause a change in field in the copper pipe, inducing currents. These currents oppose the change in magnetic field by creating opposing magnetic fields and this slows the fall of the magnet. Magnetic fields slow the fall of the magnet as the eddy current magnetic fields work against the magnet’s magnetic fields , reducing the rate of fall due to gravity. When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

The effect can be seen by placing the green magnetic viewing film around the outside of the copper pipe and then observing the field pattern on the copper pipe as the magnet passes by – a band of magnetic fields created by the eddy currents appears on the viewing film before the magnet appears. Experimenting with different numbers of magnets will give slightly different fall times (the strength of magnetic field versus the weight of magnets). Different thicknesses of pipe will also change the amount of eddy currents that can be produced. Alternative experiments could involve sliding magnets down slopes made from copper and aluminium (both will give eddy currents)

When a magnet is moved into a coil of wire, changing the magnetic field and magnetic flux through the coil, a voltage will be generated in the coil according to Faraday's Law. In the example shown below, when the magnet is moved into the coil the galvanometer deflects to the left in response to the increasing field. When the magnet is pulled back out, the galvanometer deflects to the right in response to the decreasing field. The polarity of the induced emf is such that it produces a current whose magnetic field opposes the change that produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. This inherent behavior of generated magnetic fields is summarized in Lenz's Law.

  • the magnets will need to have their pole face against the material. Thicker materials will allow more eddy currents and may show slower rates of fall (this can be used to introduce the concept of using laminated steels in motor and generator designs - laminations with electrical insulation between the laminations limits magnetic fields to being only within the planes of the laminations and thinner laminations reduce eddy currents which improves motor and generator efficiency).

    For note, the direction of induced current could also be explained by Fleming's Right-Hand Rule (mainly used in Generator current prediction).

    To assist with demonstrating Lenz’s Law, we offer both the magnets to fit 15mm and 22mm diameter copper pipe plus kits consisting of magnets plus copper pipe.

    We also offer a Magnets in Motion kit (this is part of our Science Discovery Kit range). Containing plenty of magnets and accessories, this professionally boxed kit allows young scientists to create electric currents with falling magnets, learn how subway and roller coaster brakes work and explore the theory of Lenz's Law. The Magnets in Motion Kit includes: Aluminum, copper and plastic tubes, paperclips, Neodymium magnets, ceramic magnets, steel plug, rubber bumpers, vinyl tubing, iron filings, compass, large nail and an activity book including 5 experiments and 5 projects.

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