A voltage is placed on the primary coil which surrounds a twelve-inch long, one-and-a-half-inch diameter iron core. A conducting ring surrounding this core will have a current induced in it. The force on this current due to magnetic field lines fringing out from the core will cause the ring to jump twenty to fifty centimeters in the air.
The point of this experiment is to demonstrate Faraday's Law and Lenz's Law by showing the effects of changing fluxes (in the primary coil) on conducting rings.
Further information on Theory, Apparatus, Procedure, and Helpful Hints is available. The equipment for this demo is stored in Rockefeller rooms 302 and 302B.
This experiment is an application of Lenz's Law. Lenz's Law states that an induced current in a closed conducting loop will appear in such a direction that it opposes the change that produced it. The actual explanation of what happens with the ring is as follows: we see an increase in magnetic flux in the primary coil which induces a current in the ring. The induced current opposes this change, and sets up its own magnetic field. This opposition is in effect a repulsion (two like poles facing one another) and the reason that the ring jumps off. At a more fundamental level, Lenz's Law gives the direction of current flow, and the j X b force due to the radial component of the magnetic field gives the lift to the ring. From this latter statement, you can see that, with a very long magnetic core and an iron return path, there would be very little fringing field, and therefore very little force, even though large currents would flow in both the primary circuit and the secondary ring.
The equipment needed for this demo is located in room 302- Cabinet D:
The use of a Variac is strongly recommended. Set it to about 50 volts initially, and show the class that the ring climbs up the core a short distance. This is the principle used in "MAGLEV" trains. Then shut the switch off, turn the Variac to 110 volts or higher, and flip the switch on. The surprise factor is always a key factor here.
You may want to have a ring with a break in it available. This will further prove Lenz's Law by showing that the conducting loop must in fact be closed for the current to appear in opposition to the change in flux. The voltage across the break can be read to check Faraday's Law. You can also put a multi-turn coil around the primary coil with a bulb attached and see it light up. There is one ring which is nearly completely slitted -- at the remaining small "bridge" of conductor, very large currents flow, and it gets hot very quickly.
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