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Faraday’s Law is used to describe the magnitude of an induced electromotive force (emf) when a magnetic field passes through a copper wire or coil. It does not state the direction of the current flow created. The direction of the current flow is given by Lenz’s Law which states that when a change in magnetic field produces an induced current in an electrically conductive wire (e.g. copper wire), the direction of that induced current is such that the magnetic field created by the induced current opposes the original change in magnetic field.
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. Magnetic fields from the magnet cause a change in field in the copper pipe, inducing currents. These currents (known as “eddy currents”) oppose the change in magnetic field by creating opposing magnetic fields and this slows the fall of the magnet (this is, in principle, how magnetic eddy brakes work). Magnetic fields slow the fall of the magnet as the eddy current magnetic fields work against the magnet’s magnetic fields (magnetic forces interacting), reducing the rate of fall due to gravity. 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) – 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.
|Part No.||Size||Grade||Pull (kg)|
|EP656||12mm dia x 3mmA||N38 - NiCuNi plated||1.90kg|
|EP372||18mm dia x 3mmA||N42 - NiCuNi plated||3.40kg|
|Lenz-15||15mm dia x 500mm Copper Pipe c/w 1 pack of EP656 NdFeB 12mm dia x 3mmA||Copper Pipe and N42||N/A|
|Lenz-22||22mm dia x 500mm Copper Pipe c/w 1 pack of EP372 NdFeB 18mm dia x 3mmA||Copper Pipe and N42||N/A|
|731103||Magnets In Motion Kit||Science Discovery Kit||N/A|
|MVF50X50||50mm x 50mm||Green Viewing Film - Laminated in a clear pouch||N/A|
|MVF150X150||150mm x 150mm||Green Viewing Film - Plain material||N/A|
|MVF300X300||300mm x 300mm||Green Viewing Film - Plain material||N/A|