Hewitt-Drew-it! 102. Electromagnetic Induction

History, explanations, and implications of E&M inductions.

 

Levitating Barbecue! Electromagnetic Induction

At the Palais de la Decouverte in Paris, they showed me this experiment where a 1kg aluminium plate is levitated above a large coil of wire that is being supplied with 800A of alternating current at 900Hz. This is by far the best demonstration of electromagnetic induction I have ever seen. Back in London, I visited the magnetic lab of Michael Faraday in the basement of the Royal Institution. It was here that he did his groundbreaking work on induction. People had previously observed that current in a wire causes a compass needle to deflect, but more exciting was the prospect of using a magnetic field to generate current. Faraday created his famous induction ring by winding two coils of insulated wire onto an iron ring. When he connected a battery to one coil, a small pulse of current was induced in the other. When the battery was disconnected, current was induced in the other direction. This led Faraday to the conclusion that current was induced in the second coil only when the magnetic field through it was changing. And if they hadn't been wrapped on the same ring, Faraday may have noticed that the two coils repel each other when the current is induced due to the interaction of their magnetic fields. This is the same thing that is happening with the aluminium plate, except we're using alternating current to create a continually changing magnetic field. This induces an alternating current in the plate, producing an opposing magnetic field which levitates the disk.

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Lenz's Law

Two bar magnets of the same size are dropped through an aluminum tube and a glass tube. The magnet dropped in the glass tube falls at the normal rate of acceleration due to gravity, but the magnet falling through the metal tube is slowed. This slowed acceleration occurs because the falling magnet induces currents inside the metal tube. The induced currents then produce a small magnetic field that opposes the direction of the original magnetic field. This effect is known as Lenz's Law, a result of Faraday's law of induction.

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MIT 8.02 Electricity and Magnetism Lecture 17

MIT 8.02 Electricity and Magnetism, Spring 2002

Professor Walter Lewin

Motional EMF, Dynamos, Eddy Currents, Magnetic Braking.

Other lectures from the same course

MIT 8.02 Electricity and Magnetism Lecture 16

MIT 8.02 Electricity and Magnetism, Spring 2002

Professor Walter Lewin

Electromagnetic Induction, Faraday's Law, Lenz Law, Complete Breakdown of Intuition, Non-Conservative Fields.

Other lectures from the same course

Lenz's Law with Copper Pipe

A magnet is dropped down a conducting copper pipe and feels a resistive force. The falling magnet induces a current in the copper pipe and, by Lenz's Law, the current creates a magnetic field that opposes the changing field of the falling magnet. Thus, the magnet is "repelled" and falls more slowly.

Other demonstrations from MIT

 

An Application of Faraday's Law of Induction

This is an illustration of an application of Faraday's Law to a single loop moving through a magnetic field.

Other animations by Penn State Schuylkill

Ring Falling in a Magnetic Field (MIT Tech TV)

Three aluminium objects are dropped through a magnetic field. The objects move slower entering and leaving the field due to changes in the flux through the object. The change in the flux through a conductor results in a force which opposes additional flux change. As a result the force opposes the motion and slows the object's fall. This demonstration illustrates Faraday's and Lenz's law.

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Electromagnetic induction

These animations show a magnet approaching a conducting coil, then getting farther. The magnetic field lines caused by the magnet change color when they get through the coil. The inducted current in the coil is represented by red spheres in motion (they move in the direction of conventional current).

Other animations by Yves Pelletier