Download HSC Physics – Core Module 1 – Space

HSC Physics – Core Module 2 – Motors and Generators
2. The relative motion between a conductor and magnetic field is used to generate an electric
voltage.

Outline Michael Faraday’s discovery of the generation of an electric current by a
moving magnet.
In September 1821, following the 1820 discovery by Hans Christian Oersted that an electric current produces
a magnetic field, Michael Faraday discovered that a current-carrying conductor in a magnetic field experiences
a force. This became known as the motor effect.
Almost ten years later, Faraday discovered electromagnetic induction. This is the generation of an EMF
and/or electric current through the use of a magnetic field. Faraday’s discovery was not accidental. He and
other scientists spent many years searching for ways to produce an electric current using a magnetic field.
Faraday’s break-through eventually led to the development of the means of generating electrical energy in the
vast quantities that we use in society today.
In his first successful experiment, Faraday set out to produce and detect a current in a coil of wire by the
presence of a magnetic field set up by another coil. He appears to have coiled about 70m of copper wire
around a block of wood in the spaces between the first coil. The coils were separated with twine. One coil was
connected to a galvanometer and the other to a battery. (A galvanometer is an instrument for detecting
small electric currents. Faraday’s early efforts to detect a induced current failed because of the lack of
sensitivity of his galvanometers).
When the battery circuit (or primary circuit) was closed, Faraday observed that ‘there was a sudden and very
slight deflection at the galvanometer.” This means that Faraday had observed a small brief current was
created in the secondary circuit. A similar effect was also observed when the current in the battery circuit was
stopped, but the momentary deflection of the galvanometer needle was in the opposite direction.
Faraday was careful to emphasise that the current in the
galvanometer circuit was a temporary one and that no
current existed when the current in the battery circuit
was at a constant value.
Faraday modified this experiment by winding the
secondary coil around a glass tube. He placed the steel
needle in the tube and closed the primary circuit. He
then removed the needle and found that it had been
magnetised. This also showed that a current had been
produced in the secondary circuit. It was the magnetic
field of the induced current in the secondary circuit that
had magnetised the needle.
The next experiment was to place a steel needle in the secondary coil when a current was flowing in the
primary coil. The primary current was stopped and the needle was again found to be magnetised, but with the
poles reversed to the direction of the first experiment.
The Iron Ring Experiment
In a further experiment, Faraday made a ring of soft iron. He wound a primary coil on one side and connected
it to a battery and switch. He wound a secondary coil on the other side and connected it a galvanometer.
When the current was set up in the primary coil, the
galvanometer needle immediately responded, as Faraday
stated: “to a degree far beyond what has been described
when the helices [coils] without an iron core were used, but
although the primary current was continued, the effect was
not permanent, for the needle soon came to rest in its
natural position, as if quite indifferent to the attached
electromagnet”. When the current in the primary coil was
stopped, the galvanometer needle moved in the opposite
direction. He concluded that when the magnetic field of the
primary coil was changing, a current was induced in the
secondary coil.
Using a Moving Magnet
Faraday was also able to show that a moving magnet near a coil could generate an electric current in the coil.
Similar results occur when the S pole is moved near the same end of the coil, except that the galvanometer
needle deflects in the opposite direction.
Another observation from Faraday’s experiments with a coil and a moving magnet is that the magnitude of the
induced current depends on the speed at which the current is moving towards or away from the coil. If the
magnet moves slowly, a small current is induced. If the magnet moves quickly, the induced current has a
greater magnitude


Define magnetic field strength B as the magnetic flux density
Describe the concept of the magnetic flux in terms of the magnetic flux density and
the surface area
Electromagnetic induction is the creation of an EMF in a conductor when it is in relative motion to a magnetic
field, or it is situated in a changing magnetic field. Such an EMF is known as an induced EMF. In a closed
conducting circuit, the EMF gives rise to a current known as an induced current.
Faraday demonstrated that it was possible to produce or induce a current in a coil by using a changing
magnetic field. For there to a be a current in the coil, there must have been an EMF induced in the coil.
Magnetic Flux
The magnetic field in a region can be represented diagrammatically using field or flux lines. The filed lines on
a diagram show the direction of magnetic force experienced by the N pole of a test compass if it were placed
in that point. The closeness (or density) of the lines represents the strength of the magnetic field. The closer
together the lines, the stronger the field.
Magnetic Flux is the name given to the amount of magnetic field passing through a given area. It is given the
symbol
. In the SI system
is measured in weber (Wb). If the particular area, A, is perpendicular
to a uniform magnetic field of strength B then the magnetic flux
is the product of B and A.
The strength of a magnetic field, B, is also known as the magnetic flux density. It is the amount of magnetic
flux passing through a unit area. In the SI system , B is measured in tesla (T) or weber per square meter
(Wb m-2).
The magnetic flux,
, passing through an area is reduced if the magnetic field is not perpendicular to the
area, and
, is zero if the magnetic field is parallel to the area. The above relationship between magnetic
flux, magnetic flux density and area is often written as:
Where,
, is the component of the magnetic flux density that is perpendicular to the area, A.

Describe the generated potential difference as the rate of change of the magnetic
flux through a circuit.
For a current to flow through the galvanometer in Faraday’s experiments there must be an electromotive force
(EMF
). The magnitude of the current through the galvanometer depends on the resistance of the
circuit and the magnitude of the EMF generated in the circuit.
Faraday noted that there had to be a change occurring the apparatus for an EMF to be created. The quantity
that was changing in each case was the amount of magnetic flux threading (or passing through) the coil in the
galvanometer circuit. The rate at which the magnetic flux changes determines the magnitude of the generated
EMF.
This gives Faraday’s Law of Induction:
“The induced EMF in a circuit is equal in magnitude to the rate at which the magnetic flux through
the circuit is changing with time.”
Faraday’s law can be written in the eqn:
The negative sign in the above equation indicates the direction of the EMF (explained through Lenz Law).
Rotating Coils in uniform magnetic fields
When a coil rotates in a magnetic field, as occurs in generators and motors, the flux threading the coil is a
maximum when the plane of the coil is perpendicular to the direction of the magnetic field. If the plane of the
coil is parallel to the direction of the magnetic field, the flux threading the coil is 0.

Account for Lenz’s Law in terms of conservation of energy and relate it to the
production of back EMF in motors
Lenz was a German scientist, who without knowledge of both Faraday and Henry, duplicated many of their
experiments. Lenz discovered a way to predict the direction of an induced current. This method is given the
name Lenz Law:
“An induced EMF always gives rise to a current that creates a magnetic field that opposes the
original change in the flux through the circuit.”
This is a consequence of the Principle of Conservation of Energy. The minus sign in Faraday’s law of induction
is placed there to remind us of the direction of the induced EMF.
Using Lenz’s Law:
When determining the direction of the induced EMF, it is useful to use the field line method for representing
magnetic fields. The figure below shows the effect of a magnet moving closer to a coil connected to a
galvanometer. The coil is wound on a cardboard tube. As the magnet approaches the coil, the magnetic flux
density within the coil increases. The induced current sets up a magnetic field (dotted lines) that opposes this
change. The approaching magnet increases the number of field lines pointing to the left that pass through the
coil. The induced current in the coil produces field lines that point to the right to counter this increase.
The direction of the induced current in the coil can be deduced using the right-hand rule for coils. The thumb
points in the direction of the induced magnetic field within the coil, the curl of the fingers holding the coil show
the direction of the induced current in the coil.
Lenz’s Law and the Principle of Conservation of Energy
What would happen if the opposite of Lenz’s Law were true? That is, if a changing flux in the coil would
produce a magnetic flux in the same direction as the original change of flux. This would lead to a greater
change in flux threading the coil, which in turn would lead to a even greater change in flux. The induced
current would continue to increase in magnitude, fed by its own changing flux. In fact, we would be creating
energy without doing any work.
To create electrical energy in a coil, work must be done. Energy is required to move a magnet towards or
away from a coil. Some of this energy is transformed into electrical energy in the coil.
Lenz’s Law and the production of back EMF in motors
Electric motors use an input voltage to produce a current in a coil to make the coil rotate in an external
magnetic field. It has been shown that an EMF is induced in a coil that is rotating in an external magnetic field
The EMF is produced because the amount of the magnetic flux in that is threading the coil is constantly
changing as the coil rotates. The EMF induced in the motor’s coil, as it rotates in the external magnetic field, is
in the opposite direction to the input voltage or supply EMF. If this were not the case, the current would
increase and the coil would go faster and faster forever. The induced EMF produced by the rotation of a motor
coil is known as the back EMF because it is in opposite direction to the supply EMF.
The net voltage across the coils equals the input voltage (or supply EMF) minus the back EMF. If there is
nothing attached to the electric motor to slow it down, the speed of the armature increases until the back EMF
is equal to the external EMF. When this occurs, there is no voltage across the coil and therefore no current
flowing through the coil. With no current through the coil, there is no net force acting on it and the armature
rotates at a constant rate.
When there is a load on the motor, the coil rotates at a slower rate and the back EMF is reduced. There will be
a voltage across the armature coil and a current flows through it, resulting in a force that is used to do the
work. Since the armature coil of a motor has a fixed resistance, the net voltage determines the magnitude of
the current that flows.
The smaller the back EMF, the greater the current flowing through the coil. If the motor is overloaded, it
rotates too slowly. The back EMF is reduced and the voltage across the coil remains high, resulting in a high
current through the coil that could burn out the motor. Motors are usually protected from the initially high
current produced when they are switched on.

Explain the production of eddy currents in terms of Lenz’s Law
Induced currents do not occur in only coils and wires. They can also occur:
When there is a magnetic field acting on part of a metal object and there is relative movement
between the magnetic field and the object.
When a conductor is moving in an external magnetic field
When a metal object is subjected to a changing magnetic field
Such currents are known as Eddy currents. An eddy current is a circular or whirling current inducted in a
conductor that is stationary in a changing magnetic field, or that is moving through a magnetic field. They
resemble the eddies or swirls left in water after a boat has gone by. They are an application of Lenz’s Law.
The magnetic field set up by the eddy currents oppose the changes in the magnetic field acting in the regions
of the metal objects.
The figure below shows one method of production of an eddy current. A rectangular sheet of metal is being
removed from an external magnetic field that is directed into the page. On the left side of the magnetic field
charged particles in the sheet experience a force because they arte moving relative to the magnetic field. It
can be seen that positive charged will experience a force up the page. To the right of the magnetic field,
charged particles experience no force. Therefore the charged particles that are free to move at the edge of the
field contribute to an upward current that is able to flow downward in the metal that is outside the field. This
forms a current loop that is known as an eddy current.
The side of the eddy current loop that is inside the magnetic field experiences a force. The direction of the
force is always opposite to the direction of motion of the sheet. (i.e. to the left in this case).

Present information to explain how induction is used in cook tops in electric ranges
An effect of eddy currents is that they cause an increase in the temperature of the metal. This is due to the
collisions between the moving charges and the atoms of the metal, as well as the direct agitation of atoms by
a magnetic field changing direction at a high frequency.
Induction heating is the heating of an electrically conducting material by the production of eddy currents
within the material. This is caused by a changing magnetic field that passes through the material. Induction
heating is undesirable in electrical equipment such as motors, generators and transformers, but it has been
put to good use with induction cookers and induction furnaces.
Applying the principle of induction to cook tops
Some electric cook tops contain induction cookers instead of heating coils. An induction cooker sets up a
rapidly changing magnetic field that induces eddy currents in the metal of the saucepan placed on the cook
top. The eddy currents cause the metal to heat up directly without loss of thermal energy that occurs with gas
cooking. The heat produced in the metal saucepan is used to cook the good. The induction coils of the cooker
are separated from the saucepan by a ceramic top plate. Induction cookers have an efficiency of about 80%
while gas cookers have an efficiency rating of 43%.
Extra: Induction furnaces
An induction furnace makes use of the heating effect of eddy currents to melt metals. This type of furnace
consists of a container made from a non-metal material that has a high melting point and that is surrounded
by a coil. The metal is placed in the container. The coil is supplied with an alternating current that can have a
range of frequencies and this produces a changing magnetic field through the metal. Eddy currents in the
metal raise its temperature until it melts. The eddy currents also produce a stirring effect in the molten metal,
making the production of alloys easier. Induction furnaces take less time to melt the metal. They are also
cleaner and more efficient than flame furnaces.

Identify how eddy current have been used in electromagnetic braking
Electromagnetic Braking
Consider a metal disc that has a part of it influenced by an external
magnetic field. As the disk is made of metal, the movement of the
metal through the region of magnetic field causes eddy currents to
flow. Using the right-hand push rule, it can be shown that the
magnetic field will be upwards. The current follows a downward
return path through the metal outside the region of magnetic field
influence.
The magnetic field exerts a force on the induced eddy current. This
can be shown to oppose the motion of the disk. In this way eddy
currents can be utilised in smooth braking devices in trams and
trains. An electromagnet is switched on so that an external magnetic
field affects part of a metal wheel or the steel rail below the vehicle.
Eddy currents are established in the part of the metal that is
influenced by the magnetic field. These currents inside the magnetic
field experience a force that acts in the opposite direction to the
motion of the train. In the case of the wheel, the wheel is slowed
down. In the case of the rail, the force acts in the forward direction
thus resulting in an equal and opposite force that acts in the train.