Download Phys11U_Unit 5_Ch13_transmittal_July12

[CATCH CH 13: 36pp]
[CATCH Ch13 opening: 2 pp]
13
KEY CONCEPTS
After completing this chapter
you will be able to
describe the law of
electromagnetic induction
use Lenz’s law to predict the
direction of induced current
describe alternating current and
its properties
describe the operation of an
alternating current generator
describe how transformers rely
on alternating current to step up or
step down the voltage
describe how generators,
transformers, and the electrical grid
work to provide electricity for the
population
Electromagnetic Induction
Can magnetic fields create electric currents?
The Back Lot Stunt Coaster at Canada’s
Wonderland is a different type of roller
coaster. It is missing that first big hill
that you slowly get towed up and then released
from. This initial build up of gravitational
potential energy on typical roller coasters is
necessary so that it can be converted into
kinetic energy for the rest of the ride. In
the Back Lot Stunt Coaster, you start
horizontally and then accelerate rapidly from
rest to 64 km/h in 3 s. That is faster than
many sports cars. How is it done? The ride is
accelerated using a linear induction motor.
In chapter 12, you learned about DC
motors and how they convert electrical energy
into rotation. Linear induction motors use
electrical energy to create a magnetic field
that can cause an object to move along a
linear track instead of rotating on the spot.
These motors cause huge accelerations. In
fact, the Kingda Ka roller coaster, at Six
Flags Great Adventure in New Jersey, uses a
linear induction motor to accelerate the
coaster from rest to 206 km/h in 3.5 s.
In this chapter, you will learn about
electromagnetic induction and its many
applications, from causing motion in amusement
park rides to one of the most important:
generating your electrical energy.
STARTING POINTS
Answer the following questions using your current knowledge. You will have a
chance to revisit these questions later, applying concepts and skills from the
chapter.
1. How can a magnetic field be used to induce an electric current?
2. Can we predict in which direction the induced current will flow?
3. What properties of an alternating current make it favourable for electricity
generation?
4. What is the most efficient way to transmit electrical energy?
[END PAGE 1 of 2]
Chapter 13
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[START PAGE 2 of 2]
[CATCH: C13-P001-OP11USB, size CO photo of
the Back Lot Stunt Coaster from Canada’s
Wonderland 2/3 page]
MINI INVESTIGATION ELECTRIC CURRENT FROM MOTION?
Skills: Performing, Observing
In this mini-investigation, you will try to produce an electric current without
touching a loop of wire.
Equipment and Materials: loop of wire; galvanometer; bar magnet; alligator leads
(if necessary)
1. Connect the loop of wire directly to the galvanometer.
2. Move the bar magnet in various directions around or through the loop of wire
without touching the wire.
3. Observe any changes on the galvanometer and record your results.
A. What types of motion show that a current is flowing in the wire? Explain. [T/I]
B. Is any one type of motion more effective at producing a current than others?
Chapter 13
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Explain. [T/I]
[END Page 2 of 2]
[END CHAPTER SUMMARY]
[Start Section 13.1: 4pp.]
13.1
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Figure 1 Michael Faraday was born
September 22, 1791 in London,
England. At 14, he apprenticed with a
local bookbinder. There he read many of
the books that were being bound and
developed an interest in electricity and
chemistry. He was never formally
trained as a scientist, yet he still
published several papers in scientific
journals.
electromagnetic induction the
production of electric current in a
conductor moving through a magnetic
field
law of electromagnetic induction a
change in the magnetic field in the
region of a conductor induces a voltage
in the conductor and this causes an
induced electric current in the
conductor.
Electromagnetic Induction
You know from Chapter 12 that an electric current in a
conductor can produce a magnetic field. But is the
opposite true as well? Can a magnetic field produce an
electric current in a conductor? In 1831, Michael
Faraday, an English scientist, proved just that (Figure
1). This discovery led to many of the technologies that
make the electricity we use everyday.
Discovery of Electromagnetic Induction
In Section 12.2 you learned that a constant electric
current will produce a magnetic field, so it would be
logical to assume the opposite—that a constant magnetic
field will produce an electric current in a conductor
sitting in that constant magnetic field. It does not.
Faraday discovered hat in order to produce an electric
current the magnetic field needed to be continuously
changing. He had discovered electromagnetic induction,
the production of electric current in a conductor within
a changing magnetic field.
Induction means that one action causes another
action to happen, often without direct contact. In his
investigations, Faraday brought a permanent magnet near
a conductor but not in direct contact with it, and
induced a current in the conductor. The electric current
flowed only while the magnet was moving in the vicinity
of the conductor. We call this an “induced current”
because it is not an already existing current; it is
formed by the action of the magnetic field moving along
the conductor. These observations led Faraday to develop
what is now known as the Law of Electromagnetic
Induction.
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Law of Electromagnetic Induction
Any change in the magnetic field in a defined region around a conductor induces a voltage that
causes an induced current in the conductor
The implications of this discovery were
extraordinary because for the first time in history,
electricity had been generated using only a magnet. In
Faraday's time, the only way of producing electrical
energy was by using an electric cell or battery. Of
course, batteries are extremely useful but they have
disadvantages: they operate for a limited amount of
time, they can be heavy and bulky, and they only produce
small electric potential differences. With Faraday’s
discovery all that was needed was to find a way to move
the magnet continuously and the world would be able to
produce electrical energy on a large scale without the
Chapter 13
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limitations of batteries.
[CATCH 2-column Mini Investigation – don’t break over page]
.
MINI INVESTIGATION FARADAY’S RING
Skills: Performing, Observing
Faraday investigated electromagnetic induction using a device that he built himself. It contained two
completely independent circuits. The primary circuit was connected to a source of electrical energy.
The secondary circuit was only connected to a galvanometer.
Equipment and Materials: 2 pieces of conducting wire; battery (or power supply); switch; soft-iron
ring; galvanometer
1. Construct a Faraday’s Ring apparatus as shown in Figure 2. Coil the conducting wire tightly
around the ring. Be sure to coil the conducting wire the same number of times on each side of the
ring.
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Figure 2 Faraday's Ring
2.
3.
A.
B.
C.
Close the switch in the primary circuit and observe the galvanometer in the secondary circuit.
Open the switch in the primary circuit and observe the galvanometer in the secondary circuit.
What happened to the galvanometer when the primary circuit switch was closed? [T/I]
What happened to the galvanometer when the primary circuit was switched off? [T/I]
Was there a difference in the direction of the currents in parts 2 and 3? [T/I]
Electromagnetic Induction and Faraday’s Ring
The Faraday’s Ring (Figure 1) you constructed in the
Mini Investigation is a demonstration of electromagnetic
induction. Closing the switch in the primary circuit
causes an induced voltage in the conducting wire which
in turn causes a constant electric current in the
conducting wire. This constant electric current produces
a magnetic field in the primary coil. The soft-iron ring
enhances the strength of the magnetic field and the ring
itself becomes magnetized. This change of the magnetic
field in the soft-iron ring (from zero to some value)
induces a voltage and an electric current in the
secondary circuit. However, once the magnetic field is
stable and no longer changing, the electric current in
the secondary circuit no longer exists. Remember that
you need a changing magnetic field to induce an electric
current.
When the switch is opened, the magnetic field in
the primary coil disappears because there is no longer
an electric current. The magnetic field in the soft-iron
ring collapses from maximum strength to zero. This
change in the magnetic field causes an induced electric
current in the opposite direction in the secondary
circuit. Direct currents only produce electromagnetic
induction for brief instants when the primary circuit is
switched on or off.
Factors Affecting Electromagnetic Induction
There are several factors that determine the amount of
electric current that can be produced through
electromagnetic induction. Each of the following factors
must be considered independently.
Chapter 13
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Coiling the Conductor
In Section 12.4, you learned that by coiling a conductor
you can create a magnetic field similar to that of a bar
magnet. The magnetic fields from both sides of the loop
interact to produce a more pronounced magnetic field in
the centre of the loop. Similarly, with electromagnetic
induction, a coiled conductor will have more induced
electric current in it than will a straight conductor.
The Number of Loops in the Coil
You know from section 12.4 that increasing the number of
loops in a coiled conductor or solenoid produces a
stronger magnetic field for a given electric current.
With electromagnetic induction, the number of loops in
the coil is directly proportional to the magnitude of
the electric current induced in the conductor for a
given change in the magnetic field. So, the greater the
number of loops in a coil, the more electric current can
be induced for a given change in the magnetic field.
The Rate of Change of the Magnetic Field
There are two cases to consider here: a coiled conductor
with a permanent magnet or a Faraday’s ring apparatus.
In the case of a coiled conductor with a permanent
magnet, the quicker you move the magnet into, or out of,
the coil, the higher the rate of change you cause in the
magnetic field within the coil. This will cause a
larger induced electric current in the conductor
compared to moving the magnet slowly.
In the second case of Faraday’s ring, the quicker
you increase the current in the primary circuit the
higher the rate of change you cause in the magnetic
field in the coiled conductor and the soft-iron ring.
The magnitude of the induced electric current in the
secondary circuit is proportional to the rate of change
of the magnetic field in the soft-iron ring.
The Strength of the Inducing Magnetic Field
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The stronger the inducing magnetic field, the greater
the induced electric current. So, a stronger permanent
magnet will induce a greater electric current in a given
coil. Similarly, in Faraday’s ring, a larger electric
current in the primary circuit will increase the
strength of the magnetic field in the coiled conductor
and soft-iron ring. This will increase the induced
electric current in the secondary circuit.
Applications of Electromagnetic Induction
[CAPTION] Figure 3 An induction
cooking surface
To operate any electrical device you own, you must rely
on electrical power produced and supplied through
generators and transformers—both devices that rely on
the law of electromagnetic induction to operate. You
will learn more about the generators and transformers in
Chapter 13
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Sections 13.4 and 13.5. Now we will look two other
interesting applications of electromagnetic induction:
induction cooking and metal detectors.
Induction in Cooking
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photo of metal detector at an airport]
Figure 4 Metal detectors like this one
are used to prevent dangerous objects
from being carried onto an airplane.
Cooking food involves the transfer of thermal energy. In
an electric stove, an electric current is directed into
the element which converts the electrical energy into
thermal energy. That thermal energy is transferred by
conduction into a metal pot. The pot needs to increase
in temperature to then transfer thermal energy into
food. The efficiency of this process is low because the
stove element has to get hot, the pot has to get hot,
and finally the food is heated. In the process, much
thermal energy is lost to the environment.
Cooking using an induction cooker involves a
rapidly changing magnetic field in the stove element
(Figure 3). The rapidly changing magnetic field induces
an electric current in the pot. The electric current
heats the pot because of the electrical resistance of
the pot. Iron pots work better than copper or aluminum
due to their higher electrical resistance. Insulating
materials, like glass, will not work on an induction
cooker. The main benefit of cooking with an induction
cooker is that it is more efficient because it is a more
direct transfer of thermal energy to the food. Another
benefit is that the induction cooking surface does not
get hot and food that spills onto the cooking surface
will not burn. Since the induction cooking surface is
not hot, the heating of the food immediately stops once
the induction cooker is turned off.
Metal Detectors
Electromagnetic induction is also used to detect metals.
Metal detectors use a coil that generates a rapidly
changing magnetic field. This magnetic field will induce
a current in any metal near it. The induced electric
current in the detected metal will also produce an
induced magnetic field of its own. Sensitive
measurements of the magnetic field near a metal detector
are used to detect the induced magnetic field.
Metal detectors have many uses and have become
quite common. They are used for humanitarian purposes to
help locate buried bombs called “land mines”. Land mines
were often buried during times of war but never removed.
Innocent people are injured or killed when walking
through areas of countryside where no warnings of land
mines exist.
Metal detectors are used for security purposes at
airports (Figure 4). If you have been on a flight, you
will have walked through one of these detectors. There
also handheld devices that security guards can use to
detect any metal objects on you.
Metal detectors are also used by hobbyists
searching for metals that might be valuable buried in
the ground.
Chapter 13
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13.1 SUMMARY
• The law of electromagnetic induction states that a
change in the magnetic field in the region of a
conductor induces a voltage in the conductor and this
causes an induced electric current in the conductor
• Faraday’s ring is a device that demonstrates
electromagnetic induction. A current in the primary coil
creates a magnetic field in the ring. The magnetic field
in the ring then induces a current in the secondary
coil.
• The amount of induced electric current can be
increased by: coiling the conductor, increasing the
number of loops, increasing the rate of change of the
magnetic field, and increasing the strength of the
magnetic field
• Electromagnetic induction is used in many technologies
including: generators, transformers, induction cooking,
and metal detectors.
13.1 QUESTIONS
1. A student demonstrates electromagnetic induction using a straight wire and a permanent magnet.
The wire is part of a circuit that is connected to a galvanometer. What would you expect would
happen in each of the scenarios listed below? [K/U]
(a) The magnet is placed on top of a stationary wire.
(b) The magnet is removed from the top of the stationary wire.
(c) The magnet is moved slowly over the top of the stationary wire.
(d) The magnet is moved quickly over the top of the stationary wire.
(e) The magnet is moved back and forth over the stationary wire quickly.
(f) The wire is moved past the stationary magnet quickly.
2. You need to demonstrate electromagnetic induction and wish to maximize the amount of induced
current. Describe a design to accomplish this. [T/I] [C]
3. A glass cooking pot with an iron handle is placed on the cooking surface of an induction cooker.
Describe how the temperature of the glass and the handle after being on the induction cooker for
some time. [K/U]
4. Could you design a non-metal detector that detects things other than metal and uses
electromagnetic induction? Explain. [K/U]
5. Before going into a metal detector at an airport, you must remove your belt, empty your pockets
and remove your shoes. Explain why. [C][A]
[END PAGE 4 of 4]
[END SECTION 12.1]
Chapter 13
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[START Section 13.2: 3pp]
13.2
Lenz’s Law
In 1834, Heinrich Lenz, a Russian physicist, formulated
a law to describe the direction of induced electric
current. He used logic and the law of the conservation
of energy to deduce the direction of an induced current.
You can observe the direction of current by watching a
needle on a galvanometer move as you move a magnet along
a conductor. Lenz determined how to predict that
direction.
[CATCH 2-column Mini Investigation]
MINI INVESTIGATION OBSERVING THE DIRECTION OF INDUCED
CURRENT
Skills: Performing, Observing
In this activity, you will observe the direction of the induced current when moving a magnet into and
out of a coiled conductor.
Equipment and Materials: coiled conductor, permanent bar magnet, 2 alligator leads,
galvanometer
1. Connect the two terminals of the coil to the galvanometer.
2. At a moderate rate, push the north pole of the magnet into the coil (Figure 1). Note the direction
of the current on the galvanometer
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[CAPTION]Figure 1
3. Pull the north pole of the magnet out of the coil and note the direction of the current on the
galvanometer.
4. Repeat steps 2 and 3 with the south pole of the magnet.
A. How did the direction of the current compare in steps 2 and 3? [T/I]
B. How did the direction of the current compare in step 4? [T/I]
C. Did using the south pole or north pole change your results? Explain. [T/I]
D. Predict the magnetic pole induced at the top of the coil using your right hand rule for a coiled
conductor for both steps 2 and 3. [T/I]
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Direction of Induced Current
Figure 2 North pole of a permanent
magnet pushed into the coiled
conductor.
When a permanent magnet is pushed into a coiled
conductor, the electrons in the coil respond to the
magnetic field by starting to move in a particular
direction by forming an electric current. The direction
of the current depends on which direction the magnetic
field points. For example, if the north pole of a magnet
is pushed into a coil, an induced current is produced in
the coil in one direction (Figure 2). If the south pole
of the magnet is pushed into a coil, the induced current
will be in the opposite direction.
How did Lenz deduce the direction of an induced
current? He used the law of conservation of energy as a
starting point. Recall from Chapter 5 that energy cannot
be created or destroyed. All that can be done is to
Chapter 13
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Lenz’s law If a changing magnetic field
induces a current in a coil, the electric
current is in such a direction that is own
magnetic field opposes the change that
produced it.
transform one type of energy into another. When the
north pole of a magnet is moved into the coil, as in
Figure 2, kinetic energy exists in the movement of the
magnet. The kinetic energy is transformed into electric
energy in the electric current in the coil. As a result,
the electric current in the coil also produces a
magnetic field that points in a particular direction.
Which way does the magnetic field point? Is the right
side of the coil nearest the bar magnet in Figure 2 a
north or a south magnetic pole?
Let us first consider the conventional induced
current to go around the coil in an upwards direction at
the front of the coil (the opposite of the direction
shown in Figure 2). In this case, the right hand rule
for a solenoid indicates that the right side of the coil
is a south magnetic pole. But this finding is
inconsistent with the law of conservation of energy. If
the right side of the coil is a south magnetic pole then
it would attract the north pole of the permanent magnet
into the coil without the need for an external force to
push the magnet through the coil. Therefore, the right
side of the coil cannot be a south magnetic pole.
Let us now consider the conventional induced current
to go around the coil in a downward direction at the
front of the coil. Using the right hand rule for a
solenoid, the right side of the coil is a north magnetic
pole. The permanent magnet would then be repelled by the
north magnetic pole of the coil. Therefore, you would
need to apply a force to push the permanent magnet into
the coil, thereby doing work. You are transforming
kinetic energy into electric energy in the coil. This
idea is consistent with the law of conservation of
energy.
With similar deductive reasoning Lenz was able to
summarize his findings into a law for determining the
direction of an induced current, known as Lenz’s law
(Figure 3).
[CATCH FORMATTER: set the following in a blue screen]
Lenz’s Law
If a changing magnetic field induces a current in a coil, the electric current is in such a direction that
its own magnetic field opposes the change that produced it.
LEARNING TIP
Remembering Lenz’s Law
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The coil opposes whatever the magnet
is trying to do. If north is moving in, the
coil repels it with a north. If north is
moving out, the coil attracts it back with
a south.
Chapter 13
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photo of drop tower at Canada’s
Wonderland]
Figure 3 Lenz's Law
(a) The permanent magnet is being forced into the coil. If north is being forced inward, the coil must
oppose that with a north pole at the top of the coil.
(b) If the top of the coil must be north, then the right hand rule for a solenoid would determine the
direction of the electric current to flow towards the right side of the coil in the front]
(c) The permanent magnet is being pulled out of the coil. If north is being forced upward, the coil
must oppose that with a south pole at the top of the coil.
(d) If the top of the coil must be south, then the right hand rule for a solenoid would determine the
direction of the electric current to flow towards the left side of the coil in the front.
Lenz’s Law at Canada’s Wonderland
Figure 4 Drop tower rides use an
electromagnetic braking system that is
reliable, automatic, and has parts that
do not wear out. The yellow arrow is
pointing at the copper strip.
Drop tower rides are popular attractions at amusement
parks (Figure 4). Riders are strapped into a seat and
can be raised to a height of over 70 m. They are then
released to free fall towards the ground. To prevent
disaster there must be an extremely reliable braking
system. If a friction-based braking system were used, it
would need to be triggered at just the right time and
the system would need constant replacement. Drop tower
rides use an ingenious system that relies on
electromagnetic induction. It can be explained using
Lenz’s law.
Each of the carts that are raised to the top of the
tower have permanent magnets underneath the seats. After
approximately 45 m of free fall an electromagnetic
braking system kicks in. Along the bottom third of the
tower there are copper strips mounted to the tower
vertically. When the carts fall and the permanent
magnets move past the copper conductor, an electric
current is induced in the copper. The induced current
then produces a magnetic field. Applying Lenz’s law, the
induced magnetic field must oppose the field that
created it. The opposing repulsion force acts to create
a reliable, no-friction braking system.
13.2 SUMMARY
• Lenz’s law states: If a changing magnetic field
induces a current in a coil, the electric current is in
such a direction that its own magnetic field opposes the
change that produced it.
13.2 QUESTIONS
1. For each of the diagrams below, determine the direction of the induced current in the coil and the
magnetic poles on the coil. [T/I]
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Chapter 13
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2. How would your answers to question 1 change if the coils were moved in the opposite direction to
the arrows shown above instead of the magnets? [K/U]
3. When a magnet is pushed into a coil to induce a current, the magnetic field that is created will
never attract the magnet into the coil. Explain why this is the case.[K/U]
4. In a drop tower ride, would using an electromagnet in the carts, instead of the permanent
magnets under the seats, work equally well? Would it be equally reliable? Explain. [A]
[END PAGE 3 of 3]
[END SECTION 13.2]
Chapter 13
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[START Section 13.3: 4pp]
13.3
alternating current an electric current
that repeatedly and periodically reverses
direction
Alternating Current
You know that Faraday’s law of electromagnetic induction
requires that a changing magnetic field is needed to
produce an electric current. If you push a permanent
magnet into a coil an electric current is produced as
long as the magnet is moving. Lenz’s law predicts the
direction of the current; however, the current goes in
one direction for only as long as you move the magnet in
one direction. It will not be possible for you to move
the magnet into the coil in the same direction
indefinitely. At some point, you will need to pull the
magnet in the opposite direction. As soon as you reverse
the direction of the magnet, the electric current also
reverses direction. A current that periodically reverses
direction is called an alternating current.
Development of Alternating Current
LEARNING TIP
Short forms for current
Alternating current is shortened to AC
while direct current is shortened to DC.
WEBLINK
War of the Currents
To find out more about Tesla and
Edison’s battle for electrical power
distribution, go to
NELSON SCIENCE
Recall from Section 11.5 that direct current is a flow
of electrons in one direction only. To cause an electric
current, a potential difference is applied across the
circuit by a source of electrical energy, like a
battery. This causes the electrons present throughout
the entire circuit to move in one direction. Charges
flow from one terminal of the battery through the
circuit and eventually back to the other terminal. This
situation describes the current in the small circuits
you studied in chapter 11. Do the principles involved in
these small circuits apply to a large circuit like the
circuits in your home or even the electrical power grid?
Today’s electrical power grid does not rely on
direct current. The transfer of electrical energy using
direct current is limited to how far it can be
transferred without significant energy loss in the form
of thermal energy. However, direct current was the
standard used in the first electrical power grids. In
1882, Thomas Edison built the Pearl Street power station
in Manhattan to illuminate homes and stores. He set up
an electrical power grid using direct current, but he
was only effectively able to transfer electrical energy
to 193 buildings.
Nikola Tesla, an inventor and electrical engineer,
developed a competing system that used alternating
current. The alternating current system could transfer
energy from a power plant more efficiently than the
direct current system could. In 1896, the Edward Dean
Adams Station, a U.S. hydroelectric power station,
delivered the first alternating current along a power
grid to the city of Buffalo and its industries.
Edison and Tesla both fought to have their systems
accepted and in the end alternating current became the
favoured choice. In 1922, the Sir Adam Beck generating
station started producing alternating current
electricity in Niagara Falls, Canada. [CATCH weblink]
Chapter 13
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What is Alternating Current?
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[CAPTION] Figure 1 [to come]
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Figure 2 A distribution panel controls
all the circuits in your home. You can
think of this panel as the power supply
for your home. Note the red, black, and
white service line.
Alternating current is a back and forth motion of
charges. To understand alternating current, look at the
graphs of current and voltage versus time shown in
Figure 1.
As the voltage (V ) increases the conventional
current (I ) in the wire begins in the positive
direction. The voltage increases until it reaches a
maximum positive value. At the same instant, the current
also reaches a maximum positive value. The voltage then
decreases through zero until it reaches a maximum
negative value. The current similarly decreases and then
starts reversing and heading in the negative direction
only when the potential difference passes through zero.
In Figure 1, the process repeats at a frequency of
60 Hz, which is the frequency of electricity in the
North American electrical power grid. This means that
the current is going in the positive direction,
reversing, and going in the negative direction 60 times
each second. So the electrons in the wire never travel
the complete length of the circuit; rather they move
back and forth in more or less the same spot. Note that
the graphs both go through zero twice during each cycle
as the current is changing directions. This implies that
the circuit is effectively off for an instant. You may
ask why we do not notice this effect. In fact, lights do
get dimmer and brighter during each cycle, but the
cycles occur so quickly that our eyes cannot detect it.
Since the electrons move back and forth in more or
less the same spot, you may think the current to be
ineffective at transferring energy. To understand how an
alternating current can cause an energy transfer,
consider a clothes washing machine. In a clothes washing
machine, there is an agitator that swishes the wet
clothes back and forth. The agitator still causes
changes in energy and thus work is done to clean the
clothes. In an alternating current circuit, electrons
move back and forth to cause energy changes in the
electrical device and thus work is done.
All the relationships that you have learned about
circuits and their components still apply with
alternating current, for example, Ohm’s law. In the
graphs in Figure 1, as the voltage increases, so does
the electric current. This is in accordance with Ohm’s
law, and the current depends on the resistance of the
wire that is used to transmit the alternating current.
Household Circuits
Homes have limits to how much alternating current they
can draw. Smaller homes and apartments may only require
50 to 100 A of electric current. Larger homes require
150 to 200 A of electric current. The wire that enters
your home from the power grid is called the service line
and the amount of current it carries will be labelled at
Chapter 13
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CAREER LINK
Electrician
To find out more
GO TO NELSON SCIENCE
[CATCH: stack C13-P007a-OP11USB,
size D and
C13-P007b-OP11USB, size D]
Figure 3 This plug and receptacle is
used for your electric stove or clothes
dryer. It is designed for a maximum
alternating current of 30 A at a voltage
of 240 V.
the point of entry. For example, you may have a 100 A
service going to your home. The service enters your
distribution panel which controls all the circuits in
your home (Figure 2.)
The AC voltage oscillates between a maximum
positive voltage and a maximum negative voltage. Since
the maximum voltage is reached for a brief instant twice
during a cycle, we do not state that voltage. Instead,
we use a value that is called the “root mean squared” or
RMS value. This is an effective voltage that is
available most of the time during a cycle. It also
allows for the use of one voltage in calculations
instead of a constantly changing voltage.
Your home is designed so that an effective voltage
of 240 V is applied to the electrons in the wires in
your home. Only a few of your household appliances
require 240 V, such as an electric stove or clothes
dryer. Most appliances only require an effective voltage
of 120 V. To accommodate both requirements, a three wire
system is used. Two of the wires are considered “hot”
and are coloured red and black. The third wire is white
and is called the neutral wire. The voltage between the
black and the white, and the red and the white, is 120
V. The voltage between the red and the black is 240 V.
This way, an electrician can wire the home’s circuit
with just two of the wires. Typically, the electrician
uses only the black and the white wire when 120 V is
required. If an appliance requires 240 V, then the black
and red wires are used. There is a fourth wire that is
also an essential part of the wiring called the ground
wire. Ground wires can be bare (not-insulated) or green
and are electrically connected to the ground via your
electrical distribution panel. The purpose of the ground
wire is prevent stray currents from reaching you when
you touch a circuit and directs them safely into the
ground. As a safety feature, all electrical receptacles
and switches must be grounded. [CAREER LINK]
The receptacles for 240 V appliances and 120 V
appliances are different. A 240 V plug may have 4
prongs, one for each of the wires: red, black, white,
and bare (or green)(Figure 3). Most 120 V plug require 3
prongs, one for black, white, and ground. There are also
many devices, like lamps, that use 2 prong plugs and do
not have a ground pin.
Some electronic devices (such as laptop computers)
require direct current because they cannot operate with
alternating current. These electronic devices need an
adaptor that converts the alternating current into
direct current.
The electrical requirements of some common
household appliances are listed in Table 1.
[CATCH Table] Table 1 Electrical requirements of some common household appliances
Device
Type of current
Current
Voltage requirements
required
requirements (A)
(V)
Electric stove
AC
30
240
Chapter 13
14
Clothes dryer
AC
30
240
Air conditioner
AC
20
240
Iron
AC
5
120
Hair dryer
AC
12.5
120
Refrigerator
AC
6
120
40 “ LCD television
DC
18
12
Desktop computer
(excluding monitor)
DC
Telephone charger
DC
0.5 to 20
(depends on the
component)
1
3.3 to 12
(depends on the
component)
5
Safety Systems
Many safety systems exist in your home to prevent harm
to you, damage to your appliances, or electrical fires.
These include fuses, circuit breakers, ground fault
circuit interrupters, and arc fault circuit
interrupters.
Fuses
Fuses are devices that are placed in series with one of
the parallel branches in your home. It the current
exceeds a maximum value that the fuse is rated, the wire
inside of the fuse melts which opens the circuit and
prevents any more current. Fuses are one use only. Fuses
may also be found inside of appliances as a further
measure of protection.
Circuit Breakers
Circuit breakers are devices that prevent too much
current in a wire for an extended period of time. If too
much current is in a wire, the wire will heat up and the
insulation may melt and potentially cause a fire.
Circuit breakers work by using a strip of metal
containing two different metals fused together, called a
bimetallic strip. When too much current is present, the
bimetallic strip heats up and bends. This causes the
breaker to "trip" which turns off the circuit. If a
circuit breaker trips, you should consider why it
tripped. It could be because too many loads are
connected in parallel to the same circuit and are
requiring too much current. Using fewer electrical
devices on that circuit should prevent the tripping.
Circuit breakers are reusable and can be reset.
Ground Fault Circuit Interrupters (GFCI)
GFCIs are installed in places like bathrooms. Their
purpose is to detect any differences in the current
going into a circuit compared to the current going out.
Chapter 13
15
If the GFCI does detect a difference it immediately
shuts off and prevents current. Suppose that you touch
an electrical outlet with a wet hand and then reach for
an appliance with another wet hand. It is possible that
you may set up a circuit pathway for electric current to
travel through your body. This could cause an
electrocution. The GFCI would very quickly detect the
difference in current and immediately turn off the
circuit. GFCIs respond much faster than regular circuit
breakers because they are designed to trip with small
current fault whereas regular circuit breakers will only
trip if the current exceeds the maximum rating. For
example, a GFCI will trip if a current difference of
0.006 A is detected whereas a current of over 15 A would
need to be detected to trip a regular breaker.
Arc Fault Circuit Interrupters (AFCI)
AFCIs prevent sparking or arcing, which could start a
fire. Arcing occurs when electric current travels
through the air and causes a spark. This may happen if
the insulation around the wiring has become frayed. The
bare wire can possibly move near a metal part and cause
the arcing to occur. The temperature of an arc is very
high and could cause a fire. Arcing may not be enough to
cause a circuit breaker to trip, so the AFCI prevents
current flow if an arc is detected. AFCIs also act
faster than regular circuit breakers.
13.3 SUMMARY
• Alternating current is an electric current that
repeatedly and periodically reverses direction.
• Alternating current frequency is 60 Hz in the North
American power grid.
• Some appliances require alternating current whereas
others require direct current.
• Your home requires a certain amount of current at a
voltage of 120 V and 240 V.
• The circuits in your home are protected by fuses,
circuit breakers, ground fault circuit interrupters,
and/or arc fault circuit interrupters.
13.3 QUESTIONS
1. Do electrons travel from the power plant to your home to provide electrical energy? Explain your
answer.[K/U]
2. In alternating current electricity, is the voltage proportional to the current? Explain. [K/U]
3. (a) Would you notice if the frequency of the alternating current electricity was reduced to 2 Hz?
Explain. [K/U] [C]
(b) In the late 1950’s the frequency of alternating current was changed from 25 Hz to 60 Hz.
Suggest a reason for this change.
4. How do 120 V plugs differ from 240 V plugs? [K/U]
5. A laptop computer requires 12 V DC and yet it is plugged into a home’s wall outlet. What must be
involved to satisfy the requirements of the laptop? [K/U]
6. Describe the differences between fuses, circuit breakers, GFCIs, and AFCIs. [K/U]
7. Most household circuits in North America use 120 V, while in Europe 240 V are most commonly
used. Research to find out why this difference exists. Is one system better then the other? [T/I]
[END PAGE 4 of 4]
[END SECTION 13.3]
Chapter 13
16
Chapter 13
17
[Start Section 13.4: 6pp]
13.4
electric generator device that converts
other forms of energy into electrical
energy
Electricity Generation
The large-scale production of electrical energy that we
have today is possible because of electromagnetic
induction. The electric generator, which provides
electricity for most places in the world, relies on the
law of electromagnetic induction to operate. An electric
generator is a device that converts other forms of
energy into electrical energy. These other forms of
energy can include thermal, gravitational, or kinetic
energy. As you know from Chapter 5, the energy sources
used to power generators can come from either renewable
or non-renewable sources, each with their own benefits,
disadvantages and environmental impacts that must be
considered.
In Section 13.3, you learned that alternating
current was chosen for the transmission of electrical
energy. So we will first look at the generation of
alternating current.
The Alternating Current Generator
Electromagnetic induction requires a changing magnetic
field to produce an electric current. In an AC generator
there are two ways of changing the magnetic field.
Either a permanent magnet can be spun inside a coil or a
coiled conductor can be spun inside a magnetic field. We
will examine a coiled conductor spinning inside a
magnetic field in a single-loop generator.
The AC generator shown in Figure 1 shows a single
loop of conducting wire set between the poles of two
permanent magnets. There are two slip rings and two
brushes. Each slip ring is connected to a different side
of the loop. Slip ring 1 is connected to the left side
of the loop while slip ring 2 is connected to the right
side of the loop. The slip rings rotate with the loop.
The brushes are stationary and make contact with the
slip rings to allow current to be directed out to the
external circuit. The loop spins on the axis of
rotation. The spinning force is provided by an external
source of energy. For example, recall from Chapter 5
that at a hydro-electric power plant the falling water
turns the blades of a turbine that is connected to the
electrical generator.
[CATCH – C13-F007-OP11USB, size B
Chapter 13
18
[CAPTION] Figure 1 A single-loop AC generator
In Figure 1, the loop of the generator is being
forced clockwise. Since the loop is moving inside the
magnetic field provided by the external magnets, an
electric current will be induced in the loop as
described by Faraday’s law. The direction of the induced
current is predicted by Lenz’s law. Remember that an
induced current also creates an induced magnetic field
around the loop. The induced magnetic field around the
loop will oppose the field that created it. Let us
examine the left part of the loop, in cross section,
closest to the north pole of the external magnet (Figure
2).
[CATCH C13-F008-OP11USB, size B]
[CAPTION] Figure 2 [TO COME]
The magnetic field around the wire will oppose the
external magnetic field. This happens because the field
lines at the top of the wire are pointed in the same
direction as the field lines from the external magnet.
Recall that magnetic field lines pointed in the same
direction will result in a force of repulsion. The force
on the conductor coming from an external source of
energy will overcome the magnetic repulsion force
between the field from the conductor and the field from
the magnets. Using the right hand rule for a straight
conductor, the conventional current points into the
page. Let us now examine the loop, in cross section,
closest to the south pole of the external magnet (Figure
3).
[CATCH C13-F009-OP11USB, size B]
[CAPTION] Figure 3 [TO COME]
On this side of the loop, the conductor is being forced
downward and the magnetic field from the loop will
oppose the external magnetic field. Again the field
lines from the wire will point in the same direction as
the field lines from the external magnet and cause
repulsion. Using the right hand rule for a straight
conductor, the conventional current points out of the
page.
Therefore, the rotation of the loop in a clockwise
Chapter 13
19
direction in the magnetic field will cause a
conventional current in the loop in the direction shown
in Figure 4. In the following tutorial we will
investigate the direction of the current in the external
circuit attached to the generator shown in Figure 4.
[CATCH C13-F010-OP11USB, size B]
[CAPTION] Figure 4 [TO COME]
[CATCH 2-column Tutorial: but place the first 2 sentences in 1-column format (as intro) and then
remaining text runs in 2-columns]
TUTORIAL 1
EXPLAINING THE AC GENERATOR
As the loop of an AC generator spins, a current will form in the external circuit connected to the
generator. What is the direction of the current in the external circuit?
We have showed that a clockwise current forms in the generator’s loop when the loop
rotates in a clockwise direction. We will now turn to determining the direction of the current in the
external circuit. We will use a galvanometer connected to the external circuit to determine the
direction of the induced current. Let us first consider what will happen as the loop is rotated from it
initial orientation.
In Figure 5, the current in the loop heads towards slip ring 2, which contacts brush 2, and
into the external circuit as shown. Since the conventional current goes into the galvanometer at the
positive terminal, the galvanometer needle points to the positive side of the scale.
[CATCH C13-F011-OP11USB, size C]
[CAPTION] Figure 5
In addition the factors discussed in Section 13.1, the amount of induced current also depends on
the angle of the conductor in relation to the external magnetic field. The induced current is at a
maximum when the plane of the loop is parallel to the external magnetic field, As the loop rotates
towards 90 of rotation, the amount of current will decrease. Once the loop reaches perpendicular to
the magnets (or 90 of rotation), the current reads zero as shown in Figure 6.
[CATCH C13-F012-OP11USB, size C]
[CAPTION] Figure 6
As the loop rotates from 90 of rotation and approaches 180, the conventional current in the loop
now goes into slip ring 1. This reverses the direction of the current in the external circuit as shown in
Figure 7. Note that the conventional current goes into the galvanometer at the negative terminal.
The galvanometer needle moves to the negative side.
[CATCH C13-F013-OP11USB, size C]
[CAPTION] Figure7
Chapter 13
20
As the loop rotates away from 180, the current will once again decrease until it reaches zero at
270 of rotation as shown in Figure 8.
[CATCH C13-F014-OP11USB, size C]
[CAPTION] Figure 8
At this point the current will once again reverse direction and enter the external circuit at slip ring 2.
This will start the whole process over again. If the galvanometer readings are plotted on a graph, it
will look like Figure 9.
[CATCH C13-F015-OP11USB, size C]
[CAPTION] Figure 9
The rotation rate of the loop matches the frequency of the current changing directions. In large scale
generators, the frequency of the alternating current is set to 60 Hz in North America. Any generators
connected to the electrical grid are synchronized to this frequency.
[CATCH 2-column Mini Investigation]
MINI INVESTIGATION WHAT FACTORS AFFECT ELECTRICITY
GENERATION?
Skills: Planning, Performing, Observing, Communicating
Generators use coils and magnets to generate electricity. In this investigation, you will determine
what factors affect the amount of current produced.
Equipment and Materials: 2 different coils, galvanometer, 2 alligator leads, 2 bar magnets
1. Connect the coil with fewer windings to the galvanometer using the alligator leads.
2. Plan a procedure to test the factors that affect the amount of current produced.
3. Record your observations in a series of statements that are framed as: changing the
__________, produced a maximum reading on the galvanometer of ______.
A. How did moving the magnet into the coil faster affect the amount of current? [T/I]
B. How did reversing the pole of the magnet put into the coil, while keeping the speed of the magnet
going into the coil constant, affect the amount of current? [T/I]
C. How did using two magnets affect the amount of current? [T/I]
D. How did changing the number of windings in the coil affect the current? [T/I]
Factors Affecting Generator Output
The single-loop AC generator discussed in Tutorial 1 is
useful for demonstration purposes. To increase the
amount of current of the generator, we could use a
coiled conductor wrapped around a soft-iron armature.
This increases the strength of the induced magnetic
field. We could also rotate the armature faster or use
stronger external magnets. In Tutorial 2 we will look at
using a coil in a generator.
[CATCH 2-column Tutorial]
TUTORIAL 2
USING A COIL IN AN AC GENERATOR
A coil-type generator includes a coil wrapped around a soft-iron armature as shown in Figure 10.
This armature is rotated by an external source of energy. How does this design affect the direction
of current in the external circuit?
[CATCH C13-F016-OP11USB, size C]
Chapter 13
21
[CAPTION] Figure 10
The rotation of the generator shown in Figure 10 is clockwise. As the shaded side of the armature
moves away from the north pole of the external magnet, Lenz’s law determines the left side of the
armature to be a south magnetic pole. The armature is being forced away from north. Thus the
induced magnetic field must oppose being pulled away by attracting with a south pole. Using the
right hand rule for a coil, the direction of the conventional current is as shown in Figure 11.
[CATCH C13-F017-OP11USB, size C]
[CAPTION] Figure 11
As the shaded side of the armature spins away from the north pole of the external field magnet, the
amount of current increases to a maximum until the shaded side of the armature starts approaching
the south pole of the external field magnet. The current increase can be explained by referring back
to Tutorial 1. When the single loop was oriented so that it was perpendicular to the external
magnetic field, the induced current was zero. When the single loop was oriented so that is was
parallel to the external magnetic field, the induced current was at a maximum. The same applies
whether you are using one loop or several loops as in the design in this tutorial. Using Lenz’s law,
the shaded side of the armature will resist going towards south by repelling. This would make the
shaded side of the armature a south pole. Using the right hand rule, the direction of the
conventional current is as shown in Figure 12.
[CATCH C13-F018-OP11USB, size C]
[CATCH C13-F021-OP11USB, size D]
[CAPTION] Figure 12
Now the shaded side of the armature will spin away from the south pole of the external field magnet.
This time Lenz’s law predicts the shaded side of the armature to be north. This is because the
shaded side of the armature is moving away from the south pole of the external field magnet. The
shaded side of the armature will have to resist moving away by attracting to a north pole. The
current now reverses direction as shown in Figure 13.
[CAPTION] Figure 15 A DC generator
[CATCH C13-F019-OP11USB, size C]
[CAPTION] Figure 13
As the shaded side of the armature moves away from the south pole of the external field magnet,
the current increases to a maximum value until the shaded side of the armature starts approaching
the north pole of the external field magnet. The shaded side of the armature remains north on the
side approaching north, causing repulsion. The current will go in the direction as shown in Figure
14.
[CATCH C13-F020-OP11USB, size C]
[CAPTION] Figure 14
DC Generators
Chapter 13
22
A DC generator, as shown in Figure 15, has the same
design as a DC motor. It has a split ring commutator
instead of slip rings. The split ring commutator serves
to prevent the current from changing direction in the
external circuit as it does in the AC generator.
However, the induced current in the coil in the armature
is still the same as has been shown in the tutorials.
In the case of the DC motor, electrical energy is
transferred into the motor to cause rotation or kinetic
energy. In the case of a DC generator, rotation or
kinetic energy (for example, from falling water, wind,
or high pressure steam) is used to turn the coil to
generate electrical energy. Therefore, a generator may
be considered a motor in reverse.
RESEARCH THIS
WIND TURBINES
Skills: Researching, Identifying Alternatives, Communicating
Wind is a renewable resource that many are tapping into to generate electricity using an AC
generator. Many different designs of wind turbine are being engineered. Some are large scale and
able to deliver enough electrical energy to power 5000 homes. Others are small scale and can
power only one home. As you learned in this section, in a generator, the rotation rate of the loop is
directly related to the frequency of the alternating current. Our electrical grid requires 60 Hz
electricity. Different design philosophies are put in place to achieve this.
1. Choose a wind turbine technology. You can consider large-scale land-based turbines, off-shore
turbines, or some of the novel small-scale turbines for residential applications.
2. Research how the technology works. Highlight the type of turbine used, the generator used, the
type of technology used to control the electricity to feed it into the grid, and where the electricity is
being used or developed.
A. At what rotation rate does the wind turbine spin and how is the electricity controlled? [T/I] [A]
B. What are the maintenance considerations? [K/U] [T/I]
C. Prepare a tri-fold pamphlet highlighting your research. [T/I][C][A]
[CATCH WEBLINK ICON]
13.4 SUMMARY
• An electric generator is a device that converts other
forms of energy into electrical energy.
• Alternating current generators are designed with loops
of a conductor that spins in a magnetic field. The end
of the loops are connected to two different slip rings
which allow it to produce alternating current.
• Coiling the conductor around an armature increases the
strength of the induced magnetic field making the
generator produce more current.
• Spinning the armature faster and/or using a stronger
external magnetic field will also increase the current
produced by the generator.
• DC generators are designed like a DC motor except
energy is put into spinning the coil to generate
electricity rather than putting electrical energy into
the motor to cause it to spin.
13.4 QUESTIONS
1.Sketch the generator shown in Figure 1, but change the rotation to counter clockwise. Answer the
following questions based on your diagram. Consider the starting angle to be at 0°. [T/I] [C]
(a) At what angle(s) relative to your starting point would you expect maximum current in the loop?
(b) At what angle(s) relative to your starting point would you expect a zero current?
(c) Sketch a graph of the induced current in the external circuit.
(d) What effect would reversing the polarity of the external magnets have on the current?
2. How does the rotation rate of the loop in the generator in Figure 1 compare to the frequency of
the alternating current? [K/U]
Chapter 13
23
3. How many times per second does a generator armature rotate in North America? [K/U]
4. The generator shown in Figure 16 is rotated counter-clockwise. Determine the polarity of the
magnetic field on the armature, the direction of induced current in the coil, and the direction of
current in the external circuit. [T/I]
[CATCH C13-F022-OP11USB, size C]
[CAPTION] Figure 16
5. What effect would each of the following have on the generator shown in Figure 16? [K/U]
(a) increasing the number of loops in the coil
(b) decreasing the rotation rate of the armature
(c) removing the soft iron from the armature
(d) removing the brushes
(e) using electromagnets for external field magnets
6. Is Figure 17 a DC or AC generator? Explain your answer. [K/U]
[CATCH C13-F023-OP11USB, size C ]
[CAPTION] Figure 17
[END PAGE 6 of 6]
[END SECTION 13.4]
Chapter 13
24
[START Section 13.5: 4pp]
13.5
transformer electromagnetic device
that can raise or lower voltage
[CATCH: C13-P008-OP11USB, size D
Figure 1 Photo of a transformer. For
example, the transformer for a
computer.
step-down transformer a transformer
with fewer secondary windings than
primary windings.
step-up transformer a transformer with
more secondary windings than primary
windings.
Transformers
The electrical devices you use everyday all have
different electrical energy requirements. An electric
stove require lots of electrical energy, while an LED
requires very little. Some devices require different
currents and voltages. For example, a computer may
require only 12 V to operate, so the voltage in your
home needs to be lowered from 120 V to 12 V. Devices
that are capable of raising or lowering AC voltage are
called transformers. Transformers are used in many
electronic devices so that the AC voltage is lowered or
raised to a value that the device is designed for
(Figure 1). Adapters, like cell phone chargers have
transformers as part of their circuitry. Adapters also
contain a circuit that converts AC voltage to DC
voltage.
How transformers work
To understand how a transformer works, recall Faraday’s
ring from Section 13.1. The ring had a primary circuit
and a secondary circuit. These circuits are not in
physical contact, but a current in the primary circuit
induces a current in the secondary circuit. According to
the law of electromagnetic induction, a changing
magnetic field is required to induce a current. A
changing magnetic field can be produced by using
alternating current. An alternating current in the
primary coil is the most critical part to producing an
alternating current in the secondary coil of Faraday’s
ring.
Suppose that we change the number of windings in the
coils on either the primary or secondary circuit of
Faraday’s ring. Would the same AC voltage be measured
across both the primary circuit and secondary circuit?
Transformers have different numbers of windings on
the primary circuit compared to the secondary circuit
and a soft-iron core. If there are fewer windings in the
secondary circuit than the primary circuit, then the
voltage on the secondary side will be less than the
voltage on the primary side. Transformers that have
fewer windings on the secondary circuit than the primary
circuit are called step-down transformers (Figure 2(a)).
They are called step-down transformers because they
lower the AC voltage by a specific amount.
If the situation is reversed and there are more
windings on the secondary circuit, then the voltage will
be higher on the secondary side (Figure
2(b)).Transformers that have more windings on the
secondary circuit than the primary circuit are called
step-up transformers. They are called step-up
transformers because they increase the AC voltage by a
specific amount.
So, we can lower or raise the voltage in the
Chapter 13
25
secondary circuit just by changing the number of
windings.
[CATCH – C13-F024a-OP11USB and C13-F024b-OP11USB, total width B]
Figure 2 (a) A step-down transformer has fewer windings on the secondary coil than the primary
coil.
(b) A step-up transformer has more windings on the secondary coil than the primary coil.
[CATCH 2-column Mini Investigation]
MINI INVESTIGATION OBSERVING TRANSFORMERS AT WORK
Skills: Performing, Observing
In this investigation, you will observe how a transformer works with direct current and alternating
current.
Equipment and Materials: variable AC/DC power supply, 2 AC/DC multimeters with probes, 2
alligator leads, transformer with different number of windings on primary and secondary coils.
1. Set up a circuit with the variable DC power supply connected to the transformer using the leads
as shown in Figure 3. Make sure that the power supply is off.
[Catch C13-P009-OP11USB, size C, setup]
[CAPTION] Figure 3
2. With the power supply off, set to the voltage specified by your teacher.
3. Set your multimeter to measure DC voltage and connect one multimeter to the primary coil and
the other multimeter to the secondary coil.
4. While watching the display on your multimeters, turn on the DC power supply. Record your
observations. Turn off the power supply and record your observations.
5. Disconnect the transformer from the DC power supply and connect the alligator leads to the AC
connection. Set your multimeters to measure AC voltages and repeat Step 4.
A. How effectively did the transformer work with direct current? [T/I] [C]
B. How did the AC voltage on the primary coil compare to the AC voltage on the secondary coil?
[T/I]
C. Is your transformer a step-up or step-down transformer? [T/I]
Conservation of Energy in Transformers
Transformers must obey the law of conservation of
energy. Therefore, the energy going into the primary
coil must equal the energy coming out of the secondary
coil if there are no energy losses. Recall, from Section
5.5, that the change in energy is expressed as E  Pt .
Power in an electrical circuit is expressed as the
product of voltage and current, or P VI . Using the
energy and power equations, we can express the law of
the conservation of energy as shown below. (The
subscript p represents primary and the subscript s
represents secondary.)
Chapter 13
26
Ep  Es
Pp t  Ps t
Pp  Ps
substitute P VI
VpIp  VsIs
In the above expression, you can see that both sides of
the equation have the same terms. Whatever amount of
energy goes into the primary coil must come out of the
secondary coil. If the number of windings is the same on
both sides, then the voltages and currents will be the
same.
In a step-down transformer, the voltage on the
secondary coil, Vs, is lower than the voltage on the
primary coil, Vp. So, from the equation above and the
law of the conservation of energy, we can deduce that
the current on the secondary side Is must be greater
than the current on the primary side, Ip.
In a step-up transformer, the voltage on the
secondary coil, Vs, is higher than the voltage on the
primary coil, Vp. To comply with the law of the
conservation of energy, the current in the secondary
coil Is must be less than the current on the primary
side Ip.
Therefore, the voltage and current are inversely
proportional. For example, if voltage is doubled, then
current is halved, and vice versa.
Transformer Equations
From the law of conservation of energy we derived the
following equation:
LEARNING TIP
Transformer Equations
You can just remember one equation:
Vp
Vs

Is Np

I p Ns
VpIp  VsIs
Grouping I and V together gives
Vp
Vs

Is
Ip
(equation 1)
The voltage in the coil is directly proportional to the
number of windings and therefore
Vp
Vs

Np
Ns
(equation 2)
where NP is the number of windings on the primary coil
and NS is the number of windings on the secondary coil.
We can also express current in a transformer with
respect to the number of windings by combining equations
1 and 2.
Chapter 13
27
Vp
Vs

Is
Ip
Vp
and
Vs

Np
Ns
therefore
Is Np

I p Ns
So the current is inversely proportional to the number
of windings. You will use these equations in the
following tutorials.
LEARNING TIP
Significant digits and windings
[CATCH 2-column Tutorial]
The number of winding is an exact
number and does not limit the number of
significant digits in a calculation.
TUTORIAL 1
VOLTAGE IN A TRANSFORMER
We will use the equation
Vp
Vs

Np
Ns
to solve a problem involving voltage in a step-down transformer.
Sample Problem 1
A step-down transformer used in an adaptor for a laptop has a primary voltage of 120 V. There are
250 windings in the primary coil and 25 windings in the secondary coil. Calculate the voltage in the
secondary coil.
Given: primary voltage, Vp = 120 V
number of windings in the primary coil, Np = 250
number of windings in the secondary coil, Ns = 25
Required:
voltage in the secondary coil, Vs
Analysis:
Vp
Vs

Np
Ns
rearrange the equation so that all the variables are as shown because it allows for easier
subsequent rearranging
VpNs  VsNp
solve for Vs
Vs 
Solution:
VpNs
Np
(120V)(25)
250
Vs  12V
Vs 
Statement:
The voltage in the secondary coil is 12 V.
Practice
1. A step-down transformer has a primary voltage of 240 V. The number of windings in the primary
coil is 550 and the number of windings in the secondary coil is 110. Determine the voltage of the
secondary coil. [T/I]
2. A step-up transformer has a primary voltage of 31.0 V. The number of windings in the primary coil
is 211 and the number of windings in the secondary coil is 844. Determine the voltage of the
secondary coil. [T/I]
[CATCH 2-column Tutorial]
TUTORIAL 2
CURRENT IN A TRANSFORMER
Vp
I
We will use the equation s 
to solve a problem involving current in a step-down transformer.
Ip
Vs
Sample Problem 1
A step-down transformer used in the adaptor for a cell phone charger has a primary voltage of 120
V and a secondary voltage of 5.0 V. The current in the primary coil is 1.0 A. Calculate the current in
the secondary coil.
Given: primary voltage, Vp = 120 V
secondary voltage, Vs = 5.0 V
Chapter 13
28
primary current, Ip = 1.0 A
Required:
secondary current, Is
Vp
I
Analysis: s 
Ip
Vs
Is 
VpIp
solve for Is
Solution:
Vs
(120V)(1.0A)
5.0V
Is  24A
Is 
Statement:
The current in the secondary coil is 24 A.
Practice
1. A step-down transformer has a primary voltage of 240 V and a secondary voltage of 12 V. The
primary current is 0.15 A. Determine the current in the secondary coil. [T/I]
2. A step-up transformer has a primary voltage of 620 V and a secondary voltage of 12000 V. The
current in the secondary coil is 1.3 A. Determine the current in the primary coil. [T/I]
Transformer Efficiency
The law of conservation of energy states that no energy
is lost, but in practice some energy will be converted
into unusable energy. In a transformer, some of the
energy is transformed into unusable thermal energy in
the coils, as well as sound energy. Some transformers
make a noticeable hum because the transformer core is
vibrating. Typically, transformers are better than 90%
efficient. To maximize efficiency, the coils are made
from conductors with low resistance like copper and the
core is rectangular in shape to ensure that the magnetic
field lines go through both coils effectively.
13.5 SUMMARY
• A transformer raises or lowers AC voltage. It consists
of a primary coil, secondary coil, and soft-iron core.
• Step-up transformers have more secondary windings than
primary windings and increase the voltage in the
secondary coil
• Step-down transformers have fewer secondary windings
than primary windings and decrease the voltage in the
secondary coil
• The voltage is directly proportional to the number of
windings
• The current is inversely proportional to the number of
windings
Vp
• The equations related to transformers are: Vs
Is Np

I p Ns
.

Np
Ns
,
Is Vp

I p Vs
13.5 QUESTIONS
1. Why do transformers need an alternating current to operate continuously? [K/U]
2. How can you tell the difference between a step-up or step-down transformer? [K/U]
3. A student is discussing transformers and states that "the voltage and current both increase in a
Chapter 13
29
step-up transformer." Explain why this is not possible. [K/U]
4. Suppose that you increase the number of windings on the secondary coil compared to the
primary coil. What would you expect the effect on voltage and current would be? [K/U]
5. Would a device that has the same number of windings on both the primary coil and secondary
coil be classified as a transformer? Explain. [K/U]
6. Are transformers 100% efficient? Explain. [K/U]
7. Copy Table 1 into your notebook and find the missing values. [T/I]
Table 1
Vp
12 V
30 V
Vs
Np
120 V
100
110 V
600
120
Ns
Ip
Is
step up
or step
down
1.2 A
100
150 mA
0.28 A
1.68A
8. The number of windings on the secondary coil of a transformer is 1.5 times less than on the
primary coil. If the primary coil has a current of 3.0 A and a voltage of 12.0 V, what will be the
voltage and current on the secondary coil? [T/I]
[END PAGE 4 of 4]
[END SECTION 13.5]
Chapter 13
30
[START Section 13.6: 4 pp]
13.6
Power Plants and the Electrical Grid
In Section 13.3, the historical competition between the
use of AC and DC electricity was discussed. Efficiency
was one of the main reasons why AC electricity won over
DC. But why is AC more efficient? To answer that
question, we must first examine the transmission of
electrical energy along a conductor.
Transmission Efficiency and Current
A single generator at one of today’s large-scale power
plants can produce over 300 MW of power. If the voltage
is 10 kV and P = 300 MW, then using the power equation
P VI we can calculate for current:
[FORMATTER: please stack the following equation; only 1 equal sign per line]
I  P / V  300 MW/ 10 kV 30 kA
A current of 30 kA will generate a significant amount of
thermal energy in a wire.
To determine how much power is lost in a
transmission wire, again use the power equation:
P VI
Now we substitute for V using Ohm’s law V  IR into the
previous equation to derive a new power equation
P  (IR)(I )
P  I 2R
This equation can be used to calculate the amount of
power lost in the wire due to thermal energy losses.
Assume that a generator produces 300 MW (3 x 108 W) of
power at a current of 30 kA, which travels through a
transmission wire with a resistance of 0.1 . Using the
new power equation
PI R
2
 (30 kA) (0.1 )
2
 (30000 A) (0.1 )
2
 9  107 W
P  90 MW
So 90 MW of power is converted to unusable thermal
energy. This represents a loss of 30%.
Note that the lost power is proportional to the
current squared, so if we could lower the current going
through the wire, there would be much less power lost.
Fortunately, we have a technology that can lower the
current, increase the voltage, and keep the same power:
a transformer. By using a transformer at a power plant
we can step up the voltage. Suppose that we step up the
voltage to 100 kV. This would lower the current to 3 kA.
Repeating the calculation, we find
Chapter 13
31
PI R
2
 (3 kA) (0.1 )
2
 (3000 A) (0.1 )
2
 9  105 W
P  0.9 MW
This represents a loss of 0.3%, which is a significant
improvement. This is the main reason why we generate AC
electricity at power plants. Transformers will only work
with AC electricity. Without the transformer, the amount
of power lost in transmission would be impractical.
The Electrical Power Grid
The electrical power grid is a giant circuit composed of
many parallel circuits. There are many sources of energy
in a variety of different power plants feeding
electrical energy into the grid. The grid transmits AC
power using transformers which step up and step down the
voltage where necessary. Figure 1 shows a representation
of how transformers are used.
[CATCH C13-F025-OP11USB, size B]
[CAPTION] Figure 1 [TO COME]
In Figure 1, the generator produces 20 kV AC which is
immediately stepped up to 230 kV or higher to minimize
energy loss. The electricity is then sent along power
transmission lines suspended high above the ground
supported by towers (Figure 2). If the voltages were
higher, then the electricity could discharge through the
air and into the ground—there is a limit to how much the
voltage can be stepped up. Very specialized training and
equipment is needed to maintain or repair the equipment
used in these high-voltage transmission lines. The
electricity is gradually stepped down in voltage at a
district transformer station, a local transformer
station, a substation (see Figure 3), and then a pole or
ground transformer in your neighbourhood (see Figure 4).
[CATCH run Figures 2, 3, and 4 across the page, size A total]
[CATCH C13-P010-OP11USB]
Figure 2 High-voltage electricity from power plants is transmitted across the province as needed
using these towers to support the wires.
[CATCH C13-P011-OP11USB]
Figure 3 Transformers at this substation step down the voltage so that it is low enough to be
transmitted to neighbourhoods.
Chapter 13
32
CATCH C13-P012-OP11USB]
Figure 4 This residential transformer steps down the voltage to 240 V for use in your home.
CAREER LINK
Power technicians construct and
maintain generation, transmission and
distribution stations. To learn more
about becoming a power technician,
GO TO NELSON SCIENCE
The electrical power grid is monitored, and energy
is fed into the grid on demand. If more energy is
needed, and there is the capacity, then more is fed in.
Power plants only generate the amount of electricity
that is needed because electrical energy cannot easily
be stored. If more is generated than needed, it is sold
to other electrical grids, farther away, that need it.
If we need more electrical energy and we do not have the
capacity, then we purchase it from other grids at a
significantly elevated cost. On some summer days in
Ontario, when many air conditioners are running, we may
use more electrical energy than the power plants are
capable of generating. So we purchase electricity from
the United States in order to make up the shortfall.
The electrical grid needs maintenance from time to
time. This is a very time consuming job due to the large
number of wires needed. As the grid ages, continued
repair and replacement is needed. The costs for this are
passed along to the consumer. If you look at your
electricity bill, you will find that there are costs
listed for the amount of electricity used and its
delivery. The delivery fee is collected to maintain the
grid. [Career Link]
Commercial AC Generators
The generators used in power plants contain multiple
coils and armatures. The field magnets are not permanent
magnets because it is difficult to make a strong enough
magnet. Also, permanent magnets would lose their
magnetism over time because of the strong magnetic
fields in the coils. So, instead, electromagnets are
used. To increase the strength of an electromagnet, you
increase the current. Where does the electrical energy
come from to power the electromagnets? In some cases, it
comes from the AC generator itself. In other cases, the
AC generator has a DC generator that uses permanent
magnets. The DC generator uses a source of energy (such
as falling water) to generate electrical energy which is
then used to power the electromagnets of the AC
generator. The AC generator will use a source of energy
(such as falling water) along with the DC generator to
generate AC electricity. Figure 5 shows a cross section
of a large scale generator and a hydro-electric power
plant.
[CATCH C13-F026-OP11USB and C13-F027-OP11USB,place side-by-side for a total width of B]
Chapter 13
33
Figure 5 (a) Cross section of a large generator, and (b) cross section of a hydro-electric power
plant.
The rotation speed of a generator must be managed to
maintain the desired frequency of 60 Hz. As electricity
demand increases, more current is drawn into the grid
and it becomes more difficult to turn the generator. You
can increase the turning force of the turbine rotating
the generator shaft. If this cannot be done, you can
decrease the strength of the electromagnets inside the
generator. This lowers the voltage. It is for this
reason that the voltages continually fluctuate a small
amount throughout the day. The voltages are required to
be relatively constant however and can only fluctuate
within a regulated amount.
In Section 13.3, you learned about alternating
current. The type of alternating current that was
discussed in section 13.3 was single phase. Generators
in power plants produce three phases of AC. You can
think of the three phases as three independent
alternating currents. To transmit three phases, only
three wires are needed. The three alternating currents
all have the same frequency but are out of phase from
one another. The peak currents are offset from one
another. The net result of using three-phase AC is that
you can transmit more power with only a little extra
conductor.
13.6 SUMMARY
• Transmission of AC electricity requires the use of
transformers to minimize losses.
• Step-up transformers at the power plant are used to
increase the voltage and decrease the current for
transmission.
• Step-down transformers are used throughout the grid to
bring voltages down to levels that can be used in homes.
• Commercial generators have multiple armatures and
coils using electromagnets to generate AC electricity.
13.6 QUESTIONS
1. Describe the main reason why AC power generation was chosen over DC power generation.
[K/U] [C]
2. Determine the power loss in each of the following. Express your answer as a percent. [T/I]
(a) A 200 MW power plant delivers a current of 2 kA in a 10  wire.
(b) A 200 MW power plant delivers a current of 200 A in a 10  wire.
(c) A 10 MW wind turbine delivers a current of 3000 A in a 0.50  wire.
3. Why is electrical energy generated on demand? [K/U] [ [C]
4. What is the difference between the electrical generators you learned about in Section 13.5 and
the commercial generators discussed in this section? [C] [A]
5. What is the benefit of generating three-phase alternating current? [C] [A]
[END PAGE 4 of 4]
[END SECTION 13.6]
Chapter 13
34
Chapter 13
35
[START INVESTIGATION: 1p]
INVESTIGATION 13.2.1 OBSERVATIONAL STUDY
Investigating Electromagnetic
Induction
The generation of electrical energy uses the
principle of electromagnetic induction. What
factors affect the strength of the induced
current and make electricity generation more
effective?
Purpose
To change the magnetic field and
observe the direction of induced
current
Equipment and Materials
• 2 alligator leads
• 2 different coils of wire (one with
more windings than the other)
• galvanometer (or ammeter)
• 2 bar magnets
Procedure
1. Connect the 2 alligator leads to the
terminals of one coil of wire and the
galvanometer terminals. Make note of
which direction the coil is wound.
2. Gently move the north pole of one
magnet into the coil and observe the
direction of the deflection of the
galvanometer needle. Using the
conventional current convention,
describe the direction of current in
the coil.
3. Move the magnet into the coil
quickly, and again observe the
galvanometer. Using the conventional
current convention, describe the
direction of current in the coil.
4. Repeat steps 2 and 3 but this time
pull the magnet out of the coil.
5. Repeat steps 2 and 3 but this time
move the south pole of the magnet into
the coil.
6. Repeat steps 2 and 3 using two
magnets (with poles aligned to increase
the strength of the magnetic field)
7. Repeat steps 2 and 3 by moving the
coil in the direction of the magnet.
SKILLS MENU
Questioning
Hypothesizing
Predicting
Planning
Controlling Variables
Performing
Observing
Analyzing
Evaluating
Communicating
The magnet should be aligned with the
core of the coil.
8. Repeat steps 2 and 3 using the other
coil (with a different amount of
windings).
Analyze and Evaluate
(a) Identify the manipulated and
responding variables when investigating
speed of the moving magnet versus
current. [T/I]
(b) Make a statement about the
direction of conventional current when
moving the magnet into the coil
compared with moving it out of the
coil. [T/I] [C]
(c) Make a statement about the
magnitude of the conventional current
when the speed of the magnet changes.
[T/I] [C]
(d) Increasing the speed of the coil
increases its kinetic energy. How does
the current in the coil ensure that the
law of the conservation of energy is
obeyed? [T/I]
(e) How does changing the magnetic pole
affect the direction of conventional
current? [T/I]
(f) Does moving the coil versus moving
the magnet change the results? Explain.
[T/I]
(g) How did using the second coil
affect your results? Explain. [T/I]
(h) From your results in step 2, use
your right hand rule for a coil to
determine the magnetic pole at the
entrance point of the magnet. Are the
results consistent with Lenz’s law?
Describe how the results are or are not
consistent. [T/I][C]
Chapter 13
36
Apply and Extend
(a) What would happen if you rotate a
magnet inside the coil? [A]
(b) Does the orientation of the magnet
change the amount of current? Explain.
[T/I]
(c) The coil uses an insulated
conductor for the windings. What effect
would using bare conductor for the
windings have? [A]
[END PAGE 1 of 1]
[END INVESTIGATIONS]
Chapter 13
37
[START Chapter Summary: 1 page]
Ch13 Summary
Summary Questions
[FORMATTER: set the following questions in two columns]
1. Create a study guide based on the points listed in the margin on page XXX. For
each point, create three or four sub-points that provide further information,
relevant examples, explanatory diagrams, or general equations.
2. Look back at the Starting Points questions on page XXX. Answer these questions
using what you have learned in this chapter. Compare your latest answers with those
that you wrote at the beginning of the chapter. How has your understanding changed
during the study of this Chapter? Note how your answers have changed.
Vocabulary
law of electromagnetic induction (p. XXX)
Lenz's Law (p. XXX)
alternating current (p. XXX)
electric generator (p. XXX)
transformer (p. XXX)
step-down transformer (p.XXX)
step-up transformer (p.XXX)
[CATCH Career Pathways feature]
Grade 11 Physics can lead to a wide range of careers. Some require a college diploma
or a B.Sc. degree. Others require specialized or post-graduate degrees. This graphic
organizer shows a few pathways to careers mentioned in this chapter.
1. Select an interesting career that relates to Electromagnetic Induction. Research
the educational pathway you would need to follow to pursue this career.
2. What is involved in becoming a [XXXX]? Research at least two pathways that could
lead to this career, and prepare a brief report of your findings.
[CATCH C13-F028-OP11USB Size B.
Career Pathways Graphic organizer]
[END Career Pathways]
[END Chapter Summary]
Chapter 13 Electromagnetic Induction
38
[START Chapter 13 Self-Quiz: 1 page]
To come
[END Chapter 13 Self-Quiz]
[START Chapter 13 review: 6 pages]
To come
[END Chapter 13 review]
Chapter 13 Electromagnetic Induction
39