Electromagnetic
Induction
We have seen previously that when a
DC current pass through a long straight conductor a magnetising force, H and a static magnetic field, B is developed around the wire. If the wire is then wound
into a coil, the magnetic field is greatly intensified producing a static
magnetic field around itself forming the shape of a bar magnet giving a
distinct North and South pole.
The magnetic flux developed around
the coil being proportional to the amount of current flowing in the coils windings
as shown. If additional layers of wire are wound upon the same coil with the
same current flowing, the static magnetic field strength will be increased and
therefore, the magnetic field strength of a coil is determined by the ampere
turns of the coil with the more turns of wire within the coil the
greater will be the strength of the static magnetic field around it.
But what if we reversed this idea by
disconnecting the electrical current from the coil and instead of a hollow core
we placed a bar magnet inside the core of the coil of wire. By moving this bar
magnet "in" and "out" of the coil a current would be
induced into the coil by the physical movement of the magnetic flux inside it.
Likewise, if we kept the bar magnet
stationary and moved the coil back and forth within the magnetic field an
electric current would be induced in the coil. Then by either moving the wire
or changing the magnetic field we can induce a voltage and current within the
coil and this process is known asElectromagnetic
Induction and is the basic principal of operation of transformers,
motors and generators.
Electromagnetic
Induction was first discovered way back
in the 1830's by Michael Faraday.
Faraday noticed that when he moved a permanent magnet in and out of a coil or a
single loop of wire it induced an ElectroMotive Force or emf, in other words a Voltage, and
therefore a current was produced. So what Michael Faraday discovered was a way
of producing an electrical current in a circuit by using only the force of a
magnetic field and not batteries. This then lead to a very important law
linking electricity with magnetism, Faraday's Law of Electromagnetic Induction. So how does this
work?.
When the magnet shown below is moved
"towards" the coil, the pointer or needle of the Galvanometer, which
is basically a very sensitive centre zero'ed moving-coil ammeter, will deflect
away from its centre position in one direction only. When the magnet stops
moving and is held stationary with regards to the coil the needle of the
galvanometer returns back to zero as there is no physical movement of the
magnetic field.
Likwwise, when the magnet is moved
"away" from the coil in the other direction, the needle of the
galvanometer deflects in the opposite direction with regards to the first indicating
a change in polarity. Then by moving the magnet back and forth towards the coil
the needle of the galvanometer will deflect left or right, positive or
negative, relative to the directional motion of the magnet.
Electromagnetic Induction by a
Moving Magnet
Likewise, if the magnet is now held
stationary and ONLY the coil is moved towards or away from the magnet the
needle of the galvanometer will also deflect in either direction. Then the
action of moving a coil or loop of wire through a magnetic field induces a
voltage in the coil with the magnitude of this induced voltage being
proportional to the speed or velocity of the movement.
Then we can see that the faster the
movement of the magnetic field the greater will be the induced emf or voltage
in the coil, so for Faraday's law to hold true there must be "relative
motion" or movement between the coil and the magnetic field and either the
magnetic field, the coil or both can move.
Faraday's Law of Induction
From the above description we can say
that a relationship exists between an electrical voltage and a changing
magnetic field to which Michael Faraday's famous law of electromagnetic
induction states:
"that a voltage is induced in a circuit whenever relative
motion exists between a conductor and a magnetic field and that the magnitude
of this voltage is proportional to the rate of change of the flux".
In other words, Electromagnetic Induction is the
process of using magnetic fields to produce voltage, and in a closed circuit, a
current.
So how much voltage (emf) can be
induced into the coil using just magnetism. Well this is determined by the
following 3 different factors.
1). Increasing the number of turns of
wire in the coil. - By increasing the amount of
individual conductors cutting through the magnetic field, the amount of induced
emf produced will be the sum of all the individual loops of the coil, so if
there are 20 turns in the coil there will be 20 times more induced emf than in
one piece of wire.
2). Increasing the speed of the
relative motion between the coil and the magnet. - If the same coil of wire passed through the same
magnetic field but its speed or velocity is increased, the wire will cut the
lines of flux at a faster rate so more induced emf would be produced.
3). Increasing the strength of the
magnetic field. - If the same coil of wire is
moved at the same speed through a stronger magnetic field, there will be more
emf produced because there are more lines of force to cut.
If we were able to move the magnet in
the diagram above in and out of the coil at a constant speed and distance
without stopping we would generate a continuously induced voltage that would
alternate between one positive polarity and a negative polarity producing an
alternating or AC output voltage and this is the basic principal of how a Generator works similar to those used in dynamos and car
alternators.
In small generators such as a bicycle
dynamo, a small permanent magnet is rotated by the action of the bicycle wheel
inside a fixed coil. Alternatively, an electromagnet powered by a fixed DC
voltage can be made to rotate inside a fixed coil, such as in large power
generators producing in both cases an alternating current.
Lenz's Law of Electromagnetic
Induction
Faraday's Law tells us that inducing
a voltage into a conductor can be done by either passing it through a magnetic
field, or by moving the magnetic field past the conductor and that if this
conductor is part of a closed circuit, an electric current will flow. This
voltage is called an induced emf as it has been induced into the
conductor by a changing magnetic field due to electromagnetic induction with
the negative sign in Faraday's law telling us the direction of the induced
current (or polarity of the induced emf.
But a changing magnetic flux produces
a varying current through the coil which itself will produce its own magnetic
field as we saw in the Electromagnets tutorial. This self-induced emf opposes the change that is
causing it and the faster the rate of change of current the greater is the
opposing emf. This self-induced emf will, by Lenz’s law oppose the change in
current in the coil and because of its direction this self-induced emf is
generally called a back-emf.
Lenz's Law states that: the direction of an induced emf is such that it will always
opposes the change that is causing it". In other words, an induced current will always OPPOSE the motion
or change which started the induced current in the first place and this idea is
found in the analysis of Inductance. Likewise, if the magnetic flux is decreased then the induced
emf will oppose this decrease by generating and induced magnetic flux that adds
to the original flux.
Lenz's law is one of the basic laws
in electromagnetic induction for determining the direction of flow of induced
currents and is related to the law of conservation of energy. According to the
law of conservation of energy which states that the total amount of energy in
the universe will always remain constant as energy can not be created nor destroyed.
Lenz's law is derived from Michael Faraday's law of induction.
One final comment about Lenz's Law
regarding electromagnetic induction. We now know that when a relative motion
exists between a conductor and a magnetic field, an emf is induced within the
conductor. But the conductor may not actually be part of the coils electrical
circuit, but may be the coils iron core or some other metallic part of the
system, for example, a transformer. The induced emf within this metallic part
of the system causes a circulating current to flow around it and this type of
core current is known as an Eddy
Current.
Eddy currents generated by
electromagnetic induction circulate around the coils core or any connecting
metallic components inside the magnetic field because for the magnetic flux
they are acting like a single loop of wire. Eddy currents do not contribute
anything towards the usefulness of the system but instead they oppose the flow
of the induced current by acting like a negative force generating resistive heating
and power loss within the core. However, there are electromagnetic induction
furnace applications in which only eddy currents are used to heat and melt
ferromagnetic metals.
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