Demo videos: electromagnetism
Demonstration videos illustrating key concepts of magnetism, Faraday’s Law, the Biot-Savart Law, and Lentz’s Law, as well as motors and generators.
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|Magnaprobe (3D compass): How it works. A 3D compass always aligns itself to the magnetic field it is in, such as the Earth’s magnetic field. Watch how the needle maintains its position even though the handle is turned.
|Magnaprobe (3D compass): Magnetic Field Lines. A 3D compass always aligns itself to the magnetic field it is in, such as the Earth’s magnetic field. Watch how the needle maintains its position even though the handle is turned.
|Oersted’s Effect, Biot-Savart Law. A current carrying wire will have a magnetic field directed by the right hand rule.
Magnetic forces on moving charges and currents
|Cathode Ray Tube. A stream of electrons will be bent by a magnetic field.
|Magnetic Force on Current Carrying Wire. A suspended horizontal wire passes between the poles of a permanent magnet that provides a vertical magnetic field. When a current is supplied, the wire experiences a force due to a magnetic field.
|Light Bulb in Mag Field. Because an incandescent light bulb works on AC, the filament will vibrate in the presence of a magnetic field.
|Attracting Coils. Two coils can be wired so that they field attract each other when the current is turned on. What must be the orientation of the poles of these two electromagnets?
|Repelling Coils. Two coils can be wired so that their magnetic field oppose each other when the current is turned on.
|Electric Motor. The force of the magnetic field on the coils mounted to the shaft causes this motor to turn. Note that this is the same apparatus as the AC and DC Generator illustrated later on this page. Feed it current and it spins; crank it and it generates electrical power.
|Induction Motor. Eddy currents induced in a floating metal ball by an a.c. magnet are acted on by the magnetic field from the apparatus and make the ball spin. The copper plate placed under the left side of the ball is critical. Why?
Faraday’s Law of Induction
|Faraday’s Law: a magnet and a coil. A changing magnetic field will induced a changing current in the coil.
|AC and DC Generator. A coil is spun in a magnetic field. Initially, the brushes contact rings attached to opposite ends of the coil, generating alternating current. When the brushes are moved to a split ring commutator, the generator puts out direct current. Note that this is the same apparatus as in the Induction Motor above. Crank it and it generates electrical power; feed it electrical power and it spins.
|Transformers. An AC current can not only be transmitted from one coil to another without wires, but due to the ratio of the coils, the voltage can be stepped up or down. An iron core is used to boost the coils’ efficiency in the demo.
|Florescent Bulb in Magnetic Field. The high, rapidly changing magnetic field sets up a corresponding electric field in the bulb, exciting the gas therein and lighting the bulb (light saber).
|Jumping Ring; cold ring, regular core. The current induced in the ring is in such a direction as to oppose the change in the magnetic field that caused it. This makes the ring jump off the apparatus. In this case, the changing field comes from the core of the electromagnet. Cooling the ring before hand in 77K liquid nitrogen lowers its resistance and allows it to jump higher.
|Hovering coke can. The magnetic field induces a current in the can that opposes the changing field, causing the can to hover.
|Pepsi can does not hover. Why? Watch for what is different about the back side of this can, vs. the above Coke can. Although an emf is still induced, the split in the can prevents current from flowing.