Electricity and Magnetism Demonstrations

Moving a magnet into a coil of wire will generate a voltage in that coil according to Faraday's Law. The magnitude of the voltage is proportional to the rate of change of magnetic flux in the coil, so that moving the magnet in faster will produce a greater voltage. The polarity of the voltage is such that it opposes the change that produces it, so pulling the magnet out will produce a voltage of the opposite polarity.

A copper rod is placed in the magnetic field of a permanent magnet. If a battery is used to drive an electric current through the rod, it will experience a force perpendicular to the magnetic field in the direction given by the right-hand rule. A double-pole switch is used to switch the current to the opposite direction to show that the force on the wire reverses.

When ordinary AC voltage is applied to a large coil of wire with an iron core, a strong alternating magnetic field is produced. If an aluminum ring is placed over the core and 120 volts AC is connected, a large current is induced in the ring according to Faraday's Law and it is tossed into the air. Note that if the magnetic field on the aluminum ring were perfectly vertical, the magnetic force by the right-hand rule would be radially inward and outward on the ring - and if you hold the ring down, you can hear it hum at 60Hz from that alternating force. But at the location where the magnetic field exits the coil, it diverges and will have radially outward and inward components.The force resulting from those components are upward and downward on the ring. The first upward force pulse will launch the ring into the air. At the end of the video is the contrast with a ring with a slot cut into it so that no circulating induced current is produced.

A large coil of wire with an iron core can produce a large alternating magnetic field in the air, particularly around the iron core because of its ferromagnetic nature. This alternating magnetic field is sufficient to light a lightbulb attached to another coil of wire when that alternating magnetic field is allowed to pass through the coil. This is another example of Faraday's Law and includes the principles used in transformers. Moving the coil and bulb around the end of the iron core allows you to visualize the geometry of the alternating magnetic field from the coil.


Voltage concepts
HyperPhysics***** Electricity and Magnetism J Nave, R Nave
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