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.
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:
- moving a wire through the jaws of a horseshoe magnet
- plunging a bar magnet into and out of the core of a coil
- touching the iron core of a coil with a bar magnet and then removing the
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
Hence there are three distinct phenomena that are involved in the process of
- the action of the "inducing field"
- the resulting "induced current and potential difference"
- 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
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 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:
- the number of turns on the induction coil
- the rate of change, or rate of motion, of the inducing magnetic field
- 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.
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.
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