Mr. Smith Physical Science Blizzard Bag Number Three.
Accompanying notes and lesson plan included.
Name:__________________________
Points:___/10
Physical Science Blizzard Bag Number Three
Due:
Chapter 10 Magnetism
Lesson 5 – Electromagnetic Induction, Faraday‘s Law
Lesson 6 – Generators, Transformers, Motors
Lesson Objectives
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State the original current carrying devices
List the name of the two scientists who discovered electromagnetic induction
State the process of electromagnetic induction
State Faraday’s Law
State the two factors that influence the amount of voltage induced
describe the difference between a generator and a motor
list the inputs and outputs of both a motor and a generator
list the steps in transforming energy into electrical energy
describe the parts of a transformer
state the relationship between input coils and output coils and voltage/current conversions
state the conservation of energy that applies to transformers
Vocabulary:
voltaic cell electromagnetic induction –
Guided Notes:
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Faraday’s Law
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The induced voltage in a coil is proportional to the
number of loops, multiplied by the rate at which the
magnetic field changes within those loops.
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Generators:
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10.8 Power Production
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The Transformer – Boosting or Lowering Voltage
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Text Covered:
10.6 Electromagnetic Induction
In the early 1800s, the only current-carrying devices were voltaic cells, which produced small currents by
dissolving metals in acids. These were the forerunners of our present-day batteries. The question arose
as to whether electricity could be produced from magnetism. The answer was provided in 1831 by two
physicists, Michael Faraday in England and Joseph Henry in the United States – each working without
knowledge of the other. Their discovery changed the world by making electricity commonplace –
powering industries by day and lighting up cities at night.
Faraday and Henry both discovered that electric current could be produced in a wire simple by
moving a magnet into or out of a coil of wire (Figure 10.22). No battery or other voltage source is
needed – only the motion of a magnet in a wire loop. They discovered that voltage is caused, or
induced, by the relative motion between a wire and a magnetic field. Whether the magnetic field moves
near a stationary conductor or vice versa, voltage is induced either way (Figure 10.23).
The greater the number of loops of wire moving in a magnetic field, the greater the induced voltage
(Figure 10.24). Pushing a magnet into a coil with twice as many loops induces twice as much voltage;
pushing into a coil with ten times as many loops induces ten times the voltage; and so on. It may seem
that we get something (energy) for nothing simply by increasing the number of loops in a coil of wire,
but we don’t: We find that it is more difficult to push the magnet into a coil made up of more loops. This
is because the induced voltage produces a current, which makes an electromagnet, which repels the
magnet in our hand. So we must do more work against this “back force” to induce more voltage (Figure
10.25).
The amount of voltage induced depends on how fast the magnetic field lines are entering or leaving
the coil. Very slow motion produces hardly any voltage at all. Rapid motion induces a greater voltage.
This phenomenon of inducing voltage by changing the magnetic field in a coil of wire is called
electromagnetic induction.
Faraday’s Law
Electromagnetic induction is summarized by Faraday’s law:
The induced voltage in a coil is proportional to the number of loops, multiplied by the
rate at which the magnetic field changes within those loops.
The amount of current produced by electromagnetic induction depends on the resistance of the coil
and the circuit that it connects, as well as the induced voltage. For example, we can plunge a magnet
into and out of a closed rubber loop and into and out of a closed loop of copper. The voltage induced in
each is the same, providing the loops are the same size and the magnet moves with the same speed.
But the current is quite different. The electrons in the rubber sense the same voltage as those in the
copper, but their bonding to the fixed atoms prevents the movement of charge that so freely occurs in
the copper.
Checkpoint
If you push a magnet into a coil, as shown in Figure 10.25, you’ll feel a resistance to you push. Why is
this resistance greater in a coil with more loops?
Check Your Answer
Simply put, more work is required to provide more energy. You can look at it this way: When you push a
magnet into a coil, you cause the coil to become an electromagnet. The more loops in the coil, the
stronger the electromagnet that you produce and the stronger it pushes back against you. (If the
electromagnetic coil attracted your magnet instead of repelling it, energy would have been created from
nothing and the law of conservation of energy would have been violated. So the coil must repel the
magnet.)
We have mentioned two ways in which voltage can be induced in a loop of wire: by moving the loop
near a magnet, or by moving a magnet near the loop. There is a third way – by changing a current in a
nearby loop. All three of these cases possess the same essential ingredient – a changing magnetic field
in the loop.
We see electromagnetic induction all around us. On the road, we see it operate when a car drives
over buried coils of wire to activate a nearby traffic light. When iron parts of a car move over the buried
coil, the effect of earth’s magnetic field on the coil is changed, inducing a voltage to trigger the changing
of the traffic lights. Similarly, when you walk through the upright coils in the security system at an
airport, any metal you carry slightly alters the magnetic field in the coils. This change induces voltage,
which sounds an alarm. When the magnetic strip on the back of a credit card is scanned, induced voltage
pulses identify the card. Something similar occurs in the recording head of a tape recorder: magnetic
domains in the tape are sensed as the tape moves past a current-carrying coil. Electromagnet induction
is everywhere . As we shall see in Chapter 14, it underlies the electromagnetic waves that we call light.
10.7 Generators and Alternating Current
When a magnet is repeatedly plunged into and back out of a coil of wire, the direction of the induced
voltage alternates. As the magnetic field strength inside the coil is increased (as the magnet enters), the
induced voltage in the coil is directed one way. When the magnetic field strength diminishes (as the
magnet leaves), the voltage is induced in the opposite direction. The frequency of the alternating
voltage that is induced is equal to the frequency of the changing magnetic field within the loop.
It is more practical to induce voltage by moving a coil rather than by moving a magnet. This can be
done by rotating the coil in a stationary magnetic field (Figure 10.28). Such an arrangement is called a
generator. A generator is a motor in reverse. The device is much the same, with the roles of input and
output reversed. In a motor, electrical energy is the input and mechanical energy is the output. In a
generator, mechanical energy is the input and electrical energy is the output. Both devices simply
transform energy from one form to another.
Because the voltage induced by a generator alternates, the current produced is AC, an alternating
current. The alternating current in our homes is produced by generators standardized so that the
current goes through 60 full cycles of change in magnitude and direction each second – 60 hertz.
10.8 Power Production
Fifty years after Faraday and Henry discovered electromagnetic induction, Nikola Tesla and George
Westinghouse put those findings to practical use and showed the world that electricity could be
generated reliably and in sufficient quantities to light entire cities.
Tesla built generators that were much like those still in use, but quite a bit more complicated that the
simple model we have discussed. Tesla’s generators had armatures consisting of bundles of copper
wires that were made to spin within strong magnetic fields by means of a turbine, which, in turn, was
spun by the energy of steam or falling water. The rotating loops of wire in the armature cut through the
magnetic field of the surrounding electromagnets, thereby inducing alternating current and voltage.
We can look at this process from an atomic point of view. When the wires in the spinning armature
cut through the magnetic field, oppositely directed electromagnetic forces act on the negative and
positive charges. Electrons respond to this force by momentarily swarming relatively freely in one
direction throughout the crystalline copper lattice; the copper atoms, which are actually positive ions,
are forced in the opposite direction. But the ions are anchored in the lattice, so they barely move at all.
Only the electrons move significantly, sloshing back and forth in alternating fashion with each rotation of
the armature. The energy produced by this electron sloshing is tapped at the electrode terminals of the
generator.
It’s important to know that generators don’t produce energy – they simply convert energy from some
other form to electric energy. As we discussed in Chapter 3, energy from a source, whether fossil or
nuclear fuel or wind or water, is converted to mechanical energy to drive the turbine. The attached
generator converts most of the mechanical energy to electrical energy. Some people think that
electricity is a primary source of energy. It is not. It is a carrier of energy that requires a source.
10.9 The Transformer – Boosting or Lowering Voltage
When changes in the magnetic field of a current-carrying coil of wire are intercepted by a second coil of
wire, voltage is induced in the second coil. This is the principle of the transformer – a simple
electromagnetic-induction device consisting of an input coil of wire (the primary) and an output coil of
wire (the secondary). The coils need not physically touch each other, but they are normally wound on a
common iron core so that the magnetic field of the primary passes through the secondary. The primary
is powered by an AC voltage source, and the secondary is connected to some external circuit. Changes
in the primary current produce changes in the magnetic field. These changes extend to the secondary,
and, by electromagnetic induction, voltage is induced in the secondary. If the number of turns of wire in
both is the same, the voltage input and the voltage output are the same. Nothing is gained. But, if the
secondary has more turns than the primary, then greater voltage will be induced in the secondary. This
is a step-up transformer. If the secondary has fewer turns than the primary, the AC voltage induced in
the secondary will be lower than that in the primary. This is a step-down transformer.
The relationship between primary and secondary voltages relative to the number of turns is:
Primary voltage
Number of primary turns
=
Secondary voltage
Number of secondary turns
It might seem that we get something for nothing with a transformer that steps up voltage, but we
don’t. When voltage is stepped up, current in the secondary is less than in the primary. The
transformer actually transfers energy from one coil to the other. The rate of transferring energy is
power. The power in the secondary is supplied by the primary. The primary gives no more than the
secondary uses, in accord with the law of energy conservation . If the slight power losses due to heating
of the core are neglected, then
Power into primary = power out of secondary
Electric power is equal the produce of voltage and current, so we can say that
(voltage x current)primary
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(voltage x current)secondary
The ease with which voltages can be stepped up or down with a transformer is the principle reason that
most electric power is AC rather than DC.
10.10 Field Induction
Electromagnetic induction explains the induction of voltages and currents. Actually, the more basic
concept of fields is at the root of both voltages and currents. The modern view of electromagnetic
induction states that electric and magnetic fields are induced. These, in turn, produce the voltages we
have considered. So induction occurs whether or not a conducting wire or any material medium is
present. In this more general sense, Faraday’s law states:
An electric field is induced in any region of space in which a magnetic field is
changing with time
There is a second effect, an extension of Faraday’s law. It is the same except that the roles of electric
and magnetic fields are interchanged. It is one of nature’s many symmetries. This effect, which was
advanced by the British physicist James Clerk Maxwell in about 1860, is known as Maxwell’s counterpart
to Faraday’s law:
A magnetic field is induced in any region of space in which an electric field is changing with time
In each case, the strength of the induced field is proportional to the rates of change of the inducing field.
The induced electric and magnetic fields are at right angles to each other.
Maxwell saw the link between electromagnetic waves and light. If electric charges are set into
vibration in the range of frequencies that match those of light, waves are produced that are light.
Maxwell discovered that light is simply electromagnetic waves in the range of frequencies to which the
eye is sensitive.
On the eve of his discovery, Maxwell had a date with the young woman he was later to marry. While
they were walking in a garden, she remarked about the beauty and wonder of the stars. Maxwell asked
her how she would feel is she knew that she was walking with the only person in the world who know
what the starlight really was. In fact, at the time, James Clerk Maxwell was the only person in the entire
world to know that light of any kind is energy carried in waves of electric and magnetic fields that
continually regenerate each other.
The laws of electromagnetic induction were discovered at about the time the American Civil War was
being fought. From a long view of human history, there can be little doubt that events such as the
American Civil War will pale into provincial insignificance in comparison with the more significant event
of the ninetieth century: the discovery of the electromagnetic laws.
Guided Reading Questions:
use the chapter text and guided notes found above
10.6 Electromagnetic Induction
1. What was the important discovery made by physicist Michael Faraday and Joseph Henry?
2. State Faraday’s law.
3. What are the three ways in which voltage can be induced in a wire?
10.7 Generators and Alternating Current
4. How does the frequency of induced voltage compare with how frequently a magnet is plunged into
and out of a coil of wire?
5. What is the basic difference between a generator and an electric motor?
6. What is the basic similarity between a generator and an electric motor?
7. Why does the voltage induced in a generator alternate?
10.8 Power Production
8. What commonly supplies the energy input to a turbine?
9. Is it correct to say that a generator produces electric energy?
10.9 The Transformer – Boosting or Lowering Voltage
10. Is it correct to say that a transformer boosts electric energy?
11. Does a step-up transformer step up voltage, the current, or the power?
12. Does a step-down transformer step up the voltage, current or the power?
10.10 Field Induction
13. What is induced by the rapid alternation of a magnetic field?
14. What is induced by the rapid alternation of an electric field?
15. What important connection did Maxwell discover about electric and magnetic fields?
Physical Science
Blizzard Bag Notes DAY THREE
Chapter 10 Magnetism and Electromagnetic Induction
Vocabulary
voltaic cell - any device that can produce a difference in potential via chemical reactions
electromagnetic induction – inducing voltage via the movement of a conductor near a magnetic field or
vice-a-versa.
Lesson Five Notes
10.6 Electromagnetic Induction
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In the early 1800s, the only current-carrying devices were voltaic cells, which produced small currents by
dissolving metals in acids.
These were the forerunners of our present-day batteries.
The question arose as to whether electricity could be produced from magnetism.
The answer was provided in 1831 by two physicists, Michael Faraday in England and Joseph Henry in the
United States – each working without knowledge of the other.
Their discovery changed the world by making electricity commonplace – powering industries by day and
lighting up cities at night.
Faraday and Henry both discovered that electric current could be produced in a wire simply by moving a
magnet into or out of a coil of wire
No battery or other voltage source is needed – only the motion of a magnet in a wire loop.
Voltage is caused, or induced, by the relative motion between a wire and a magnetic field.
Whether the magnetic field moves near a stationary conductor or vice versa, voltage is induced either
way
The greater the number of loops of wire moving in a magnetic field, the greater the induced voltage
The amount of voltage induced depends on how fast the magnetic field lines are entering or leaving the
coil.
This phenomenon of inducing voltage by changing the magnetic field in a coil of wire is called
electromagnetic induction.
Faraday’s Law
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Electromagnetic induction is summarized by Faraday’s law:
The induced voltage in a coil is proportional to the number of loops,
multiplied by the rate at which the magnetic field changes within those
loops.
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The electrons in an insulator sense the same voltage as those in a conductor, but their bonding to the
fixed atoms in the insulator prevents the movement of charge that so freely occurs in the conductor.
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Induced voltage can also be achieved by changing the current in a nearby loop.
- this is how a transformer works
Lesson Six Notes
Generators:
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A practical way to induce voltage is by moving a coil rather than by moving a magnet.
This can be done by rotating the coil in a stationary magnetic field.
Such an arrangement is called a generator.
A generator is a motor in reverse.
In a motor, electrical energy is the input and mechanical energy is the output.
In a generator, mechanical energy is the input and electrical energy is the output.
Both devices simply transform energy from one form to another
10.8 Power Production
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It’s important to know that generators don’t produce energy – they simply convert energy from some
other form to electric energy.
Energy from a source, whether fossil or nuclear fuel or wind or water, is converted to mechanical energy
to drive the turbine.
The attached generator converts most of the mechanical energy to electrical energy.
Electricity is not a primary source of energy.
It is a carrier of energy that requires a source.
The Transformer – Boosting or Lowering Voltage
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A transformer is a simple electromagnetic-induction device consisting of an input coil of wire (the
primary) and an output coil of wire (the secondary).
The coils are normally wound on a common iron core so that the magnetic field of the primary passes
through the secondary.
The primary coil is powered by an AC voltage source
The secondary coil is connected to an external circuit.
The resulting induced voltage on the secondary coil has three possibilities:
1) same number of turns in both coils: same voltage induced
2) more turns in secondary: higher voltage
3) less turns in secondary: lower voltage
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The relationship between primary and secondary voltages relative to the number of turns is:
Primary voltage
Number of primary turns
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=
Secondary voltage
Number of secondary turns
It is important to note that what the transformer actually does is transfer power.
Since electrical power is volts times amps, we can see that if the volts are increased, the amps must be
decreased by a corresponding amount, and vice-versa.
Academic/Career & Technical Related/Demonstration Lesson Plan
Title: “Blizzard Bag Number Three Physical Science ”
Scope/Sequence: Third of Three covering chapter 10 Electromagnetic Induction
State Indicator/Competency:
• Electricity and Magnetism
• Charging objects (friction, contact, and induction)
• Coulomb’s law
• Electric fields and electric potential energy
• DC circuits
o Ohm’s Law
o Series circuits
o Parallel circuits
o Mixed circuits
o Applying conservation of charge and energy (junction and loop rules)
• Magnetic fields and energy
• Electromagnetic fields and energy
Instructional Objective(s):
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State the original current carrying devices
List the name of the two scientists who discovered electromagnetic induction
State the process of electromagnetic induction
State Faraday’s Law
State the two factors that influence the amount of voltage induced
describe the difference between a generator and a motor
list the inputs and outputs of both a motor and a generator
list the steps in transforming energy into electrical energy
describe the parts of a transformer
state the relationship between input coils and output coils and voltage/current conversions
state the conservation of energy that applies to transformers
Materials:
Blizzard Bag Three Handout
Vocabulary and Notes Handout
Method of Instruction:
Homework
Activities:
Students will complete worksheet at home including vocabulary, guided notes, and guided reading.
Assessment:
This assignment is worth 10 pts.