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SPH3U1
Lesson 10
Magnetism
GALVANOMETERS
If the wire through a magnetic field is bent into a loop, the loop can be made to turn up to 900.
Notice how the current runs in the opposite directions on opposite sides of the loop leading to
opposite forces. This is what causes the turn.
Check out the directions of the forces with the right-hand rule for the motor principle.
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SPH3U1
Lesson 10
Magnetism
A loop like above can be turned into a simple device called a galvanometer. The single loop is
strengthened with several more loops. A needle is attached to act as a pointer and a return
spring is used to make the loop turn back to its start point when the current is. Galvanometers
are used to make ammeters and voltmeters. The addition of a high resistance resistor in series
makes it into voltmeter with a scale that is determined by the resistance of the resistor. Adding
a very small resistor in parallel makes this an ammeter – the size of the resistor determines the
range.
DC MOTORS:HOW A MOTOR WORKS
The purpose of a motor is to turn electrical energy (current) into mechanical energy (spinning
motion). Answer the questions below to see how a motor works.
FIRST WAY TO LOOK AT IT
1.
Use the diagram below to label the stator, armature, brushes and commutator on
diagram #1 on the next page.
In the diagram to the left, the armature spins.
The commutator is attached to the armature, so
it spins too. The brushes don’t move.
2. On the diagram to the left, use the + and
– on the battery to label the direction of the
current.
3. Use your right-hand rule #3 (for the
motor principle) on each side of the wire to verify
the direction of the force.
4. On diagram #1 on the next page, label
the direction of the current. Then use right-hand rule #3 to determine the direction of
the force on each side of the wire. Label the forces.
Does the armature spin clockwise, or counter-clockwise? _______________
5. Repeat question #4 for diagram #2. (Hint: it won’t be much different)
6. Look at diagram #3. IF the current in the darker half of the armature was still going into
the page (like in the first two diagrams), why would the armature get stuck here?
7. Look at diagram #4. Label the direction of the current in the armature. Is the direction
of the current in the dark half of the armature the same or opposite to diagram #2?
_________________
Why did the current in the armature switch direction?
8. Use right-hand rule #3 to determine the direction of the magnetic force on both sides of
the armature in diagram #4.
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SPH3U1
Lesson 10
Magnetism
1.
N
–
+
2.
N
–
+
3.
N
–
+
4.
N
–
+
N
5.
+
–
5
SPH3U1
Lesson 10
Magnetism
9. Label the direction of current flow and use right-hand rule #3 to determine the direction
of the magnetic force on both sides of the armature in diagram #5.
10. Does the direction the motor rotates ever change? If so, where?
11. Does the direction of the current in the armature ever change? If so, where?
12. Does the direction of the current from the source (battery) every change? If so, where?
13. Why is this motor called a DC motor?
ANOTHER WAY TO LOOK AT IT
In the diagram below, the wire coiled around the iron armature causes the armature to have a
south and a north pole. The poles of the armature are attracted and/or repelled by the poles of
the permanent magnets which makes the motor spin.
1. For each diagram, label the direction of the current, the north and south poles of the
armature and show which way the motor turns.
2. Write a brief explanation of how this motor works.
HOMEWORK
1. page 566 #1-3, page 571 #1-3
2. Go the course moodle/shared folder/website. Read the DC motor assignment.
6
Blackline Master 12.5-1
NAME: ___________________________DATE:___________________________
12.5-1 The Right-Hand Rule for the Motor Principle
Examine the diagrams below and use the right-hand rule for the motor principle to answer the questions.
1. Draw an arrow showing the direction of the force on the wire in the diagram below.
2. Label the north and south poles of the magnets in the diagram below. The force on the wire is directed out
of the page.
3. Label the direction of the conventional current in the diagram below. The arrow shows the force on the
wire, directed out of the page.
BLM 12.5-1
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SPH3U
Lesson 11
Electromagnetism
USING MAGNETISM TO INDUCE AN ELECTRIC CURRENT
LEARNING GOALS
Students will:
•
State and apply both Faraday’s Law of Induced current and Lenz’s Law
THE DISCOVERY OF ELECTROMAGNETIC INDUCTION
The production of electricity by magnetism is called ______________________ ____________.
Michael Faraday first demonstrated that magnetism can produce electricity. Faraday showed
that when a magnet approaches a coil, a current is ______________ in the coil. The
____________ of induced current depends on the pole of the magnet that _________________
the coil. A stationary magnet will not induce current. There must be motion of the coil or
magnet to induce current.
(Word Bank: direction, induced, electromagnetic induction, approaches)
CREATING CURRENT FROM A MAGNETIC FIELD
FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION
When a conductor is located near a moving or changing magnetic field, an
electric potential difference is induced over the conductor causing an induced
current to flow.
The induced potential difference (voltage) is called an electromotive force or emf. Although
there is a slight difference between potential difference and emf, this difference is not important
to us at this point.
A galvanometer is a sensitive current detector. This diagram
illustrates electromagnetic induction. As the magnet is
moved into the wire coil, current is generated in the coil.
•
•
Faraday discovered that three factors influence the
magnitude of emf and induced current in the wire:
• The _____________ of the wire – the higher the
velocity, the greater the emf and current.
The _____________ of the magnetic field – the stronger the magnetic field, the greater
the emf and current.
The _____________ of the wire in the magnetic field – the longer the wire, the greater
the emf and current.
A segment of a closed loop of wire moves through a magnetic
field. Note that the wire must be perpendicular to the magnetic
field in order for current and emf to be induced.
SPH3U
Lesson 11
Electromagnetism
THE DIRECTION OF INDUCED CURRENT
LENZ’S LAW
Lenz’s law states that induced current is in a direction that opposes the change
or motion that produced it.
Lenz’s law means that induced current creates a magnetic force that acts on the wire. This is
expected as
•
•
•
•
the wire moves in a magnetic field
a current is induced in the wire
the induced current in the wire creates a magnetic field around the wire
the two magnetic fields interact creating a force
This force always opposes the motion of the wire. Lenz’s law thus obeys the law of conservation
of energy – it takes work to produce energy in a different form.
USING LENZ’S LAW – FINDING DIRECTION OF CURRENT
Lenz’s Law can be looked at in three ways.
I.
A bar magnet approaching a loop of wire
This is like the simulation you just did. Use this idea to draw on the direction of the induced magnetic
field and the induced current and fill in the blanks
The magnet is approaching the loop so the induced
current must create a magnetic field that is to the
_____________ inside the loop. Therefore the current
must be ________________
The magnet is moving away from the loop so the
induced current must create a magnetic field that is to
the _____________ inside the loop. Therefore the
current must be ________________
SPH3U
II.
Lesson 11
Electromagnetism
A bar magnet approaching a solenoid
In this case, the induced current will create a north and a south pole in the solenoid
(RHR #2). The induced poles will oppose the motion of the bar magnet.
A north pole of a permanent magnet is moving away from the solenoid as shown below:
a) To oppose the moving away, a _________ pole must be induced on the left side of the
solenoid to attract the magnet back
b) This south pole is created by an induced current that flows ______________ (label it)
c) Label the induced current and poles on the diagram below
S
III.
A wire moving in a magnetic field
In this diagram, a segment of a closed loop of wire moves
upwards through a magnetic field. Note that the wire must be
perpendicular to the magnetic field in order for current and emf
to be induced.
In this case, the induced current will flow a way that creates a force (RHR #3) opposite to
the direction of motion.
a) In the diagram above, the wire is moving up. The opposite of up is ______________.
b) The induced force will be directed _________________.
c) Use RHR #3 to figure out the direction of the current. Label it. Check your answer.
WHY DOES THIS ALL THIS MAKE SENSE?
It’s conservation of energy – read the steps below...
•
•
•
•
•
the wire moves in a magnetic field
a current is induced in the wire
the induced current in the wire creates a magnetic field around the wire
the two magnetic fields interact creating a force
This force always opposes the motion of the wire. Lenz’s law thus obeys the law of
conservation of energy – it takes work to produce energy in a different form.
SPH3U
Lesson 11
Electromagnetism
PRACTICE
In each situation, use the following steps to determine the direction of the current in the helix.
1. Determine which pole and in what the direction the magnet is moving (e.g. N pole moving
towards coil)
2. Determine what pole (N or S) would oppose that motion. Write this on the
electromagnet.
3. Determine what direction the current must be flowing in order to create the necessary
pole.
a.
N
S
S
b.
c.
d.
S
N
N
S
v
e. Text page 594 #1-3 AND summarize “Lenz’s Law and Drop-Tower Rides”
SPH3U
Lesson 13
Electromagnetism
ELECTRICAL GENERATORS
LEARNING GOALS
Students will:
•
•
•
Compare and contrast a DC motor and a DC generator
Describe the modification needed to turn a DC generator into an AC generator
Use Lenz’s law and Faraday’s law to describe how a generator produces electricity
MOTORS VS. GENERATORS
Motors and generators are essentially the same device.
In an electric motor, _______________ (kinetic or electrical) energy is put into the motor
and _________________(kinetic or electrical) energy is produced.
In a generator, __________________(kinetic or electrical) energy is put into the generator
and ________________(kinetic or electrical) energy is produced.
Describe some methods we use to turn generators:
AC VS. DC GENERATORS
DC = direct current: current flows continuously in one direction.
AC = alternating current: current that periodically changes direction. In North America, AC
current is generated with a frequency of 60 Hz so that it changes direction 120 times per
second.
DC GENERATORS
If a DC motor is connected to some source that causes it to spin, it will act as a DC
generator. The ___________________ commutator that caused the current to flip every
half turn in the motor does the opposite when the motor is used as a generator.
1. As the coil rotates, the black half moves downwards through
the magnetic field (see diagram to the right).
2. Lenz`s law says a force must act upwards on the wire to
oppose the motion.
3. Apply the right hand rule for the motor principle to
determine the current direction in the black wire. Mark this
current on the diagram.
4. Repeat 1 – 3 for the white half of the loop.
SPH3U
Lesson 13
Electromagnetism
As the loop rotates, the current induced in the loop
reverses every half turn (see diagram to the left).
a) Indicate the direction of the INDUCED force
on the loop in each diagram.
b) Why is there no induced current in diagrams A
and C?
c) The current in the loop switched direction
between diagrams _______ and __________
d) Why does the signal to the meter always have
+ on the left even though the current in the loop
switches?
AC GENERATORS
To make an AC generator is simple. The current in the
armature is already AC. It is the split-ring commutator
that “undoes” the alternating. We need to ensure the
same half of the loop remains in contact with the same
brush; this is accomplished with the setup in the
diagram to the right. These two separate rings make up
what is called a slip-ring commutator.
Now we can see that the current coming out of the
generator is alternating (see the galvanometer in the
diagram to the left).
SPH3U
Lesson 13
Electromagnetism
ANOTHER PERSPECTIVE
In this AC generator, the armature is being FORCED to turn clockwise by an external force
(like a windmill). The top brush is always in contact with the inner ring of the commutator
and the lower brush is always in contact with the outer ring.
a) Use Lenz’s Law to determine what pole the dark end of the armature will be and
what direction the induced current will be.
b) When does the current in the solenoid change direction?
2.
1.
CLASS WORK
P604 Q1-4
SPH3U
Lesson 13
Electromagnetism
TRANSFORMERS
LEARNING GOALS
Students will:
•
•
Understand why AC electricity generation was chosen over DC
Calculate currents, voltages and powers from both the input and output sides of a
transformer.
WHY TRANSFORMERS ARE IMPORTANT FOR POWER TRANSMISSION
Generators at power plants produce large amounts of electrical energy at low voltage.
The voltage is increased to allow efficient transmission of the electrical energy over
long distances to end users
High voltages in the lines are reduced by power substations and transformers to an
appropriate voltage (240 V or 120 V)
WHY IS A HIGHER VOLTAGE MORE EFFICIENT FOR TRANSMISSION?
Say you have 10 kW of power to transmit through power lines that have a resistance of 1.0
Ω. Electrical power can be calculated in two different ways:
P = IV
or
P = I2R
a) Say the power is transmitted at a voltage of 200 V.
a. Determine the amount of current in the power lines (Hint: P = IV)
b. With this current, calculated the amount of power that is dissipated from the
resistance of the wire (Hint: P = I2R)
b) Repeat the procedure in (a) but with a voltage of 2000 V
Based on your calculations above, explain why power is actually transmitted at an
extremely high voltage (around 10 000 V)
a. 50 A
b. 2500 W
b) 5 A; 25 W
SPH3U
Lesson 13
Electromagnetism
HOW DO TRANSFORMERS WORK?
The power supply sends current into the primary coil
This current creates a magnetic field around the wires of the
primary coil which moves through the iron core
The magnetic field induces a current in the secondary coil
1. Why does the core of a transformer have to be made of
iron?
2. Why does the primary signal have to be alternating current
(Hint: think of Faraday’s experiments)
CALCULATING THE OUTPUT VOLTAGE AND CURRENT
Because energy cannot be created or destroyed, the electrical energy of the primary signal
(input), must be equal to the electrical energy of the secondary signal (output) which also
means that the primary power must be equal to the secondary power:
Pp = Ps
Use the equation P = IV to determine which of the following ratios is correct:
ܸ௣ ‫ܫ‬௣
=
ܸ௦ ‫ܫ‬௦
ܸ௣ ‫ܫ‬௦
=
ܸ௦ ‫ܫ‬௣
Hence, if the voltage increases, the current ____________________________ and vice
versa.
THE NUMBER OF TURNS IN A TRANSFORMER
It is the number of turns on both sides of the transformer that determine the secondary
voltage and current. The primary or secondary voltage can be determined by the following
equation:
ܸ௣ ܰ௣
=
ܸ௦ ܰ௦
where Np is the number of turns on the primary (input) coil and Ns is the number of turns on
the secondary (output) coil.
1. A transformer that increases the voltage is called a step-up transformer and one that
decreases the voltage is called a step-down transformer. Look at the two diagrams
at the top of the page. Which is a step-up transformer and which is a step-down
transformer? Explain how you can tell. (Answer on last page)
2. Use the two equations above to finish the following equation in terms of Np and Ns:
‫ܫ‬௣
=
‫ܫ‬௦
SPH3U
Lesson 13
Electromagnetism
3. Answer the following:
a. A transformer has more turns in the primary coil than the secondary coil.
How does it change the voltage of the input signal? How will it change the
current?
b. A transformer that doubles the voltage of the primary signal will
______________ the current.
4. A transformer is designed to operate a 12 V door chime from a 120 V supply. If the
transformer has 800 turns on the primary coil and the door chime draws 1.5 A of
current, find
a. the number of turns on the secondary coil
b. the current supplied to the primary coil
HOMEWORK: P 609 #1-5,7-9
Answers:
1. the top diagram is the step-up transformer because there are more turns on the
secondary coil
2.
ூ೛
ூೞ
=
ேೞ
ே೛
3. (a) It will decrease the voltage and increase the current (b) half
4. (a) 80 turns (b) 0.15
Blackline Master 13.5-1
NAME: ___________________________DATE:___________________________
13.5-1 Transformer Equations
The transformer equations relate the number of windings, N,
the voltage V, and the current, I, in a transformer. The
subscript “p” refers to the primary coil, and the subscript “s”
refers to the secondary coil.
Transformer Equations
Vp
Vs
?
Np
Ns
Is Np
?
I p Ns
Vp
Vs
?
Is
Ip
Use the equations to solve each of the following problems.
1. A step-down transformer has a primary voltage of 65 V. The number of windings in the primary coil is 80,
and the number of windings in the secondary coil is 16. Determine the voltage of the secondary coil.
Vs =
2. A step-up transformer has a secondary voltage of 240 V. The number of windings in the primary coil is 15,
and the number of windings in the secondary coil is 60. Determine the voltage of the primary coil.
Vp =
3. A step-down transformer has a primary voltage of 120 V and a secondary voltage of 30 V. The primary
current is 0.12 A. Determine the current in the secondary coil.
Is =
4. A step-up transformer has a primary current of 0.96 A and a secondary current of 0.12 A. The number of
windings in the primary coil is 592. Determine the number of windings in the secondary coil.
Ns =
5. A step-down transformer has a primary current of 0.18 A and a secondary current of 0.72 A. The primary
voltage is 240 V. Determine the voltage of the secondary coil.
Vs =
6. A step-up transformer has 38 windings in the primary coil and 152 windings in the secondary coil. The
primary current is 0.72 A. Determine the current in the secondary coil.
Is =
7. A step-down transformer has a secondary current of 0.91 A and a primary current of 0.13 A. The secondary
voltage is 38 V. Determine the voltage of the primary coil.
Vp =
BLM 13.5-1
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Blackline Master 13.5-A
Name: ___________________________Date: _______________________
13.5-A Transformers in the Distribution of Electrical Energy
1. Which transformers are step-up transformers and which ones are step-down? How is this
indicated in the diagram even if the electric potential differences were not labelled?
2. Which two transformers are usually farthest apart? Describe the voltage and electric current
between these two transformers and explain why they have these properties.
3. Why is the pole transformer located close to homes instead of out in the country?
4. How many different (separate) circuits are shown in the diagram? Discuss how the electric
current changes in the circuits, starting from the generating station and ending at a home.
BLM 13.5-A
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