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Electromagnetic Induction

Asked by hascott | Jan 19, 2002 | GCSE Level > Physics > Revision
hascott asks:

Could you please explain to me how this works, so that I can explain it clearly to my student. I am a Home Tutor, and Science is not my specialism.

Thank you.

etutor answers:

Electromagnetic Induction

Michael Faraday, an expert chemist-physicist of immense talent, in 1831, discovered the basic principle of electromagnetic induction. Faraday made his discovery by experimenting with conductors in the vicinity of magnetic fields. His investigations involved three basic situations:

  1. moving a wire through the jaws of a horseshoe magnet
  2. plunging a bar magnet into and out of the core of a coil
  3. touching the iron core of a coil with a bar magnet and then removing the magnet

In the first case, he found that electrons only flowed while the conductor was moving through the magnetic field. In the second case, electrons only began to flow when the bar magnet was moving into or out of the coil and in the third example, electrons only moved through the coil when the iron cylinder was being magnetised
or demagnetised.

Hence there are three distinct phenomena that are involved in the process of electromagnetic induction:

  1. the action of the "inducing field"
  2. the resulting "induced current and potential difference"
  3. the magnetic field created by the induced current

A current produces a magnetic field and conversely a magnetic field can produce a current. Actually, the magnetic field induces an EMF which, in turn, can produce a current.

This EMF can be generated in 2 ways:
By relative movement between a magnet and a conductor.
Changing the magnetic field.

Faraday's Law of Electromagnetic Induction is therefore:

Whenever the magnetic field in the region of a conductor is moving, or changing in magnitude, electrons are induced to flow through the conductor.

Mutual Induction

Mutual Induction is the effect that occurs whenever a changing current in one coil induces a current in another coil near by. In fact, the two coils do not have to be coupled with an iron ring, which merely acts to strengthen an effect that would be present in any case.

The Magnitude of the Induced Electric Potential

The three factors affecting the magnitude of the induced current are:

  1. the number of turns on the induction coil
  2. the rate of change, or rate of motion, of the inducing magnetic field
  3. the strength of the inducing magnetic field

An example of how magnetic induction is used is in transformers.
Two coils of wire are wrapped around an iron core.
The primary coils are connected to the power supply.
The secondary coils are connected to the device.
An electromagnetic field is created from the primary coils.
This induces a magnetic field in the core.
This in turn induces a current in the secondary coil.
The number of coils is proportion to the amount of voltage so a transformer is used to increase or decrease the voltage.

Secondary voltage/primary voltage = Secondary turns/primary turns.

Lenz's law
The direction of the induced EMF is such that it tends to oppose the flux change, and it does oppose it if current flows.
Lenz's law is a direct consequence of the principle of conservation of energy. Let's say relative motion produces an induced EMF which causes a current to flow. The magnetic field generated by this current can only help or hinder the motion, it cannot have a neutral effect. Helping the motion would result in the creation of a perpetual motion machine which violates the conservation of energy. Fleming's left hand rule can be modified to decide the direction of the induced current and therefore the direction of the induced EMF. Just reverse the direction of the `current' finger.
Electromagnetic induction is an incredibly useful phenomenon with a wide variety of applications. Induction is used in power generation and power transmission, and it's worth taking a look at how that's done. There are other effects with some interesting applications to consider, too, such as eddy currents.

Eddy currents
An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. By Lenz¹s law, the current swirls in such a way as to create a magnetic field opposing the change; to do this in
a conductor, electrons swirl in a plane perpendicular to the magnetic field. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. In many applications the loss of useful energy is not particularly desirable, but there are some practical applications. One is in the brakes of some trains. During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.

An electric generator
A electric motor is a device for transforming electrical energy into mechanical energy; an electric generator does the reverse, using mechanical energy to generate electricity. At the heart of both motors and generators is a wire coil in a magnetic field. In fact, the same device can be used as a motor or a generator. When the device is used as a motor, a current is passed through the coil. The interaction of the magnetic field with the current causes the coil to spin. To use the device as a generator, the coil can be spun, inducing a current in the coil. An AC (alternating current) generator utilizes Faraday's law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating EMF.

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