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Chapter Thirty Seven Notes:
Electromagnetic Induction
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While Oersted's surprising discovery of electromagnetism
paved the way for more practical applications of electricity,
it was Michael Faraday in England and Joseph Henry in the
United States who gave us the key to the practical
generation of electricity: electromagnetic induction. They
discovered that a voltage would be generated across a
length of wire if that wire was exposed to a perpendicular
Magnetic field flux of changing intensity. Until their
discovery, the only current-producing devices were voltaic
cells, which produced small currents by dissolving
expensive metals in acid. These were the forerunners of our
present-day batteries.
An easy way to create a magnetic field of changing
intensity is to move a permanent magnet next to a wire or
coil of wire. Remember: the magnetic field must increase or
decrease in intensity perpendicular to the wire (so that the
lines of flux "cut across" the conductor), or else no voltage
will be induced:
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Faraday and Henry found that moving a
conductor in a magnetic field (or by moving the
magnet field near a stationary conductor)
created a voltage. The wire must be part of an
electrical circuit. Otherwise the electrons have
no place to go. In other words, there is no
electrical current produced with a wire with
open ends. But if the ends are attached to a
light bulb, to an electrical meter or even to each
other, the circuit is complete and electrical
current is created.
Faraday's Law
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• "The induced voltage in a coil is proportional to the product of the
number of loops, the cross sectional area of each loop, and the rate
at which the magnetic field changes within those loops.“
Relates induced voltage to # of turns of coil and rate of change of
magnetic field.
Moving a magnet in a coil induces a voltage in the coil.
Induced voltage creates a magnetic field that opposes the motion of
the magnet
Energy is conserved
◦ If coil has no load no work is done to move the magnet
◦ If coil has a load, work to move magnet = work done in load.
Generators & Alternating Current
◦ Converts mechanical energy to electrical energy
◦ Coil rotates in magnetic field
◦ Turbine
 Mech. energy from falling water or steam
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Simple Generator:
Whereas a motor converts electrical energy into mechanical energy,
a generator coverts mechanical energy into electrical energy.
The generators shown above, and on the previous slide are
considered simple generators. They consist of one or just a few
coils of wire which rotate in a magnetic field.
Complex Generators:
The generators found in power plants are much more complex than
the models shown above. Huge coils made up of many loops of wire
are wrapped on an iron core, to make an armature much like the
armature of a motor. This armature is connect externally to a device
that obtains its energy from a paddle wheel, turned by water, steam,
wind or some other source of energy.
The fact that an electric current is deflected in a magnetic field, which
underlies the operation of a motor, and Faraday and Henry’s
discovery of electromagnetic induction, which underlies the
operation of a generator occurred 10 years apart.
Both of these are based upon the same fact: Moving charges
experience a force that is perpendicular to both their motion and the
magnetic field they traverse.
Alternating Current Motor
ELECTRIC MOTORS
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You may be surprised to know that you already have an electric
motor in your own car. Your ignition system uses a small electric
motor to get your car started. In very early cars, this had to be done
manually. A removable crank fit into the engine, and the driver
would have to turn it to get the car started, acting as a human
generator.
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construction of a transformer includes a
ferromagnetic core around which multiple coils,
or windings, of wire are wrapped. The input line
connects to the 'primary' coil, while the output
lines connect to 'secondary' coils. The alternating
current in the primary coil induces an alternating
magnetic
flux
that
'flows'
around
the
ferromagnetic core, changing direction during
each electrical cycle. The alternating flux in the
core in turn induces an alternating current in
each of the secondary coils. The voltage at each
of the secondary coils is directly related to the
primary voltage by the turns ratio, or the number
of turns in the primary coil divided by the
number turns in the secondary coil. For instance,
if the primary coil consists of 100 turns and
carries 480 volts and a secondary coil consists of
25 turns, the secondary voltage is then:
secondary voltage = (480 volts) * (25/100) = 120
volts
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What makes transformers so useful is that if you change the number
of turns from one side to the other, you change the voltage in the
wire on the right! Transformers can change a high voltage to a lower
one, or a low voltage to a higher one.
Explaining how a transformer works
When an electric current passes
through a long, hollow coil of wire
there will be a strong magnetic field
inside the coil and a weaker field
outside it. The lines of the magnetic
field pattern run through the coil,
spread out from the end, and go
The primary voltage (on the
round the outside and in at the other
left) induces a magnetic field
end.
in the core, which creates the
These are not real lines like the
secondary voltage (on the
ones you draw with a pencil. They
right).
are lines that we imagine, as in the
sketch, to show the pattern of the
magnetic field: the direction in which
a sample of iron would be
magnetized by the field. Where the
field is strongest, the lines are most
closely crowded.
With a hollow
coil the lines form
complete rings. If
there is an iron core
in
the
coil
it
becomes
magnetized,
and
seems to make the
field become much
stronger while the
current is on.
The iron core of a transformer is normally a complete ring with two
coils wound on it. One is connected to a source of electrical power
and is called the 'primary coil'; the other supplies the power to a
load and is called the 'secondary coil'. The magnetization due to the
current in the primary coil runs all the way round the ring. The
primary and secondary coils can be wound anywhere on the ring,
because the iron carries the changes in magnetization from one coil
to the other. There is no electrical connection between the two coils.
However they are connected by the magnetic field in the iron core.
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When there is a steady current in the primary there is no effect in the
secondary, but there is an effect in the secondary if the current in the
primary is changing. A changing current in the primary induces an
e.m.f. in the secondary. If the secondary is connected to a circuit then
there is a current flow.
A step-down transformer of 1,200 turns on the primary coil
connected to 240 V a.c. will produce 2 V a.c. across a 10-turn
secondary (provided the energy losses are minimal) and so light a 2 V
lamp.
A step-up transformer with 1,000 turns on the primary fed by 200 V
a.c. and a 10,000-turn secondary will give a voltage of 2,000 V a.c.
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The iron core is itself a crude secondary
(like a coil of one turn) and changes of
primary current induce little circular
voltages in the core. Iron is a conductor
and if the iron core were solid, the induced
voltages would drive wasteful secondary
currents in it (called 'eddy currents'). So
the core is made of very thin sheets
clamped together, with the face of each
sheet coated to make it a poor conductor.
The edges of the sheets can be seen by
looking at the edges of a transformer core.
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First let's look at a transformer that converts a low voltage to a high
one. This is called a step-up transformer.
If you increase the number of
turns on the right, the voltage
coming off the transformer will
increase in proportion.
Using the numbers in the
example above, you can see that
the right side has four times
more turns. As a result, the voltage on the right has increased four
times (from 100 V to 400 V). The voltage has been stepped up by a
factor of four.
Because current is inversely proportional to voltage, you can see
that stepping up the voltage pays a price ... the current on the right
is only a quarter of what it was on the left. Step-up transformers
increase the voltage, but decrease the current. In our example
above, the current went from 10 A to 2.5 A, a reduction of by a
factor of four.
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Now let's look at a transformer which reduces the voltage. This is
called a step-down transformer.
If you decrease the number of turns on the right, the voltage coming
off the transformer will decrease in proportion.
Using the numbers in the
example above, you can see
that the right side has one
fifth the number of turns. As
a result, the voltage on the
right is only one-fifth as large.
The voltage has been stepped
down by a factor of five (1000 V
down to 200 V).
Because current is inversely proportional to voltage, you can see that
stepping down the voltage gives a bonus ... the current on the right
is five times what it was on the left. Step-down transformers
decrease the voltage, but increase the current. In our example above,
the current went from 2 A to 10 A, an increase by a factor of 5.
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Almost all electrical energy sold today is in the form of
alternating current because of the ease with which it can be
transformed from one voltage to another.
All modern countries are crisscrossed with high-voltage
transmission lines, which transport electrical power from
generators at power plants to substations and ultimately
consumers. Why are high voltages used? What are the
advantages of alternating current (AC) versus direct current
(DC)? How much energy is lost in transmitting electrical
power over long distances? The main physics principle this
topic addresses is electrical resistance.
Power is transmitted great distances at high voltages and
correspondingly low currents, a process that otherwise would
result in large energy losses owing to the heating of the
wires. Power may be carried from power plants to cities at
about 120,000 volts or more, stepped down to 2400 volts in
the city, and finally stepped down again by a transformer to
provide the 120 (240) volts used in household circuits.
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The most fundamental electric and magnetic fields underlie both
voltages and currents.
James Maxwell said that there is a second effect that is a
counterpart to Faraday’s law where only the roles of the electric and
magnetic fields are interchanged.
Faraday’s law states that an electric field is induced in any region of
space in which a magnetic field is changing with time. The
magnitude of the induced electric field is proportional to the rate at
which the magnetic field changes. The direction of the induced
electric field is at right angles to the changing magnetic field.
According to Maxwell: a magnetic field is induced in any region of
space in which an electric field is changing with time. The
magnitude of the induced magnetic field is proportional to the rate
at which the electric field changes. The direction of the induced
magnetic field is at right angles to the changing electric field.
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Just as an electrical current induces a magnetic field so does a
changing magnetic field cause current to flow in a conductor.
Scientists call this property electromagnetic induction. It allows us
to convert mechanical energy, the energy of motion, into electricity.
The impact that this simple physical property has had on the
modern world is incalculable.
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A changing magnetic field passing through a conductor causes a
current to flow in the conductor. This current in turn induces a
magnetic field. The induced magnetic field points in the opposite
direction of the changing magnetic field, opposing the changing
field. (This is called Lenz's Law.) The external field must be
changing! The induced field opposes the change in the external
field, not the field itself. If the field is not changing, there is nothing
to oppose and there is no current.
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IBM Magnetic Tape
TECHNOLOGY
Elvis Presley, Buddy Holly, and magnetic tape all rose to prominence
in the 1950s, and it was the latter that helped shape the recording
industry. Magnetic tape also changed the computing landscape by
making long-term storage of vast amounts of data possible. A
single reel of the oxide coated half-inch tape could store as much
information as 10,000 punch cards, and most commonly came in
lengths measuring anywhere from 2400 to 4800 feet. The long
length presented plenty of opportunities for tears and breaks, so in
1952, IBM devised bulky floor standing drives that made use of
vacuum columns to buffer the nickel-plated bronze tape. This
helped prevent the media from ripping as it sped and up and slowed
down.
Approximate Years in Use: 1951 - Present
Maximum Capacity: About 1TB
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If you shake a charged rod you will produce electromagnetic waves.
This is because the shaking charge can be considered an electric
current.
A magnetic field surrounds an electric current and a changing
magnetic field surrounds a changing electric current.
A changing magnetic field will induce a changing electric field and
a changing electric field will induce a changing magnetic field
An electromagnetic wave is composed of vibrating electric and
magnetic fields that regenerate each other.
The magnitude of each induced field depends not only on the
vibrational rate but on the motion of the other field.
◦ The higher the speed the greater the magnitude of the field that
is induced.
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Physics: Electromagnetic Waves Field Theory
Michael Faraday, James Clerk Maxwell
The greatest change in the axiomatic basis of physics - in other
words, of our conception of the structure of reality - since Newton
laid the foundation of theoretical physics was brought about by
Faraday's and Maxwell's work on electromagnetic field phenomena.
(Albert Einstein, 1931)
Mutual inductance and self-inductance
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Mutual inductance occurs when two circuits are arranged so that the
change in current in one causes an emf to be induced in the other.
Imagine a simple circuit of a switch, a coil, and a battery. When the
switch is closed, the current through the coil sets up a magnetic field.
As the current is increasing, the magnetic flux through the coil is also
changing. This changing magnetic flux generates an emf opposing
that of the battery. This effect occurs only while the current is either
increasing to its steady state value immediately after the switch is
closed or decreasing to zero when the switch is opened. This effect is
called self-inductance. The proportional constant between the selfinduced emf and the time rate of change of the current is called
inductance (L)
Maxwell's equations and electromagnetic waves
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Maxwell's equations summarize electromagnetic effects in four
equations. The equations are too complex for this text, but the
concepts embodied in them are important to consider. Maxwell
explained that electric and magnetic waves can be generated by
oscillating electric charges. These electromagnetic waves may be
depicted as crossed electric and magnetic fields propagating
through space perpendicular to the direction of motion and to each
other, as illustrated in Figure 3 .
Figure 3
An electromagnetic wave consists of perpendicular oscillating magnetic and electric fields.
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When Maxwell (1876) used this field theory to assume that light was
an Electromagnetic Wave, and then correctly deduced the finite
velocity of light, it was a powerful logical argument for the existence
of the electromagnetic force field, and that light was a wave like
change in the field (electromagnetic radiation) that propagated with
the velocity of light c through the ether.
The Electromagnetic Spectrum:
Although all of these waves are electromagnetic
waves, at different wavelengths and frequencies,
they all move at the same speed. The speed of
light. This was another discovery of Maxwell.
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Remember that the electric field lines and the
magnetic field lines are always perpendicular
to each other!