Download Comparing DC Motors, Generators and Alternators

Unit 4 Physics Electricity and magnetism: Activity booklet
Work Program
Outcome 1: …to investigate and explain the operation of electric motors, generators and alternators, and the generation, transmission, distribution and use of electric
power.
Ls
1
2
3
4
5
6
7
8
13
14
15
Topic
Magnetic fields of bar magnets, coils and
solenoids
Magnetic forces
DC motors
Magnetic flux
Generation of AC electricity (Faraday’s and
Lenz’s Law)
Use of commutators and slip rings
Relationship between AC voltage and
rotating coil
RMS values
DC motors, generators, alternators
Transformers
Electricity transmission and distribution
Power use
Revision
Reading
complete
()
9.1
9.2
9.5
9.3
9.4
9.6
10.1
10.2
10.3
10.5
Topic
summarised
()
Questions
completed
()
Demonstrations, activities and experiments
A Animations (LP and Fendt)
10.4
10.6
10.7
Revision
* SAC task
Revision
Reference: Heinemann Year 12 Physics 3rd edition
Please bring your Library Card to each lesson
Refer to blog www.unit4physics.wordpress.com and Learning Point for course information, answers and dates of tests etc.
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Unit 4 Physics Electricity and magnetism: Activity booklet
Exploring Magnetism
Aim: To investigate magnetism
Instructions
1. What are ferromagnetic materials?
2. Describe the difference between hard and soft ferromagnetic material
Obtain a magnet and some paperclips.
3. Is the paperclip a soft or hard ferromagnet?
4. What is a magnetic domain?
5. Sketch the magnetic domains of a unmagnetised and magnetised block
6. How do we identify the two ends of a magnet?
7. What does dipolar mean?
8. Can we have a monopole?
9. Magnets have a field of influence around them, a force field if you like. Name 2 other fields.
Using two bar magnets circle the correct answer:
10. Like poles repel/attract and unlike poles repel/attract
Using the plastic bag of iron filings, map the field around a magnet.
11. Draw a magnet, labelling the poles, show the field lines.
12. Using a directional compass, show the direction of the field lines.
13. Field lines go from __________ to _________
14. Assuming the Earth behaves as a giant magnet, what magnetic pole do you think is at the North geographic
pole?
Connect a long length of wire to a power supply (4v DC)
15. Using the compass and filings, map the magnetic field around the wire.
Magnetic field strength B is measured in Tesla (T)
16. What is the typical value of the magnetic field strength for bar magnet you have been using?
17. What is the typical field strength of the Earth?
18. Is B a scalar or vector?
19. What is the convention for a vector coming directed out of the page?
20. What is the convention for a vector coming directed into the page?
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Unit 4 Physics Electricity and magnetism: Activity booklet
Summary Questions
1. Which of the following elements cannot be magnetised?
A
gold
B
nickel
C
iron
D
cobalt
2. Explain what happens when a small magnet is placed in an external magnetic field.
3. Consider a conductor of length L (m) carrying a current I (A) placed in an external magnetic field B (T).
a
Write an expression for the magnitude of the magnetic force in F
(N), experienced by the conductor.
b
How would you determine the direction of the force?
4. For the set up shown in the diagram, determine the direction the brass rod
would move.
5. The 3 diagrams below show arrangements of current-carrying
conductors.
Mark in the direction of the magnetic field on the diagrams.
6. What is the magnitude of the force acting on the wire if the magnetic field strength at the wire is 0.083 Tesla,
the length of the wire is 2.5  10 2 m and the magnitude of the current through the wire is 3.0 A?
7. A current-carrying conductor is at right angles to a magnetic field of 0.45 Tesla. The conductor is 15 cm long
and experiences a force of 0.27 N.
What is the magnitude of the current in the conductor?
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Unit 4 Physics Electricity and magnetism: Activity booklet
8. An east–west power line of length L = 400 m is suspended between two towers. Assume that the magnetic field
strength in this region B = 5.0 mT. Calculate the magnitude and direction (out of page or into page) of the
magnetic force on the power line if it carries a current of:
a
I = 0.90 kA from west to east
b
I = 1.0 kA from east to west.
9. A rectangular silver loop is located with its plane perpendicular to a uniform magnetic field of strength B = 0.50
T and directed into the page, as shown in the diagram. The loop is free to rotate about a horizontal axis X–Y. The
loop has a resistance R = 0.50 .
a
How much magnetic flux is threading the loop in this
position?
b
The loop is rotated about X–Y, through an angle of
90, so that its plane is parallel to the magnetic field.
How much flux is threading the loop in the new
position?
c
Assume that the rotation mentioned in part b took 0.50 ms. Calculate the magnitude of the average induced
current in the loop.
d
Assume that the loop is rotated through another 90 so that it returns to its original position, perpendicular to
the field and the loop remains stationary for some time.
The direction of the original field is then reversed. Determine the change in flux through the loop produced by
this reversal of the field direction.
e
Determine the magnitude and direction of the average induced current produced by the change described in
part d, assuming the time interval for the field reversal is 1.0 ms.
10. A gold rod XY of length 20 cm is free to move along a set of parallel conducting rails, forming a square circuit
with total resistance R = 0.50 . A uniform magnetic field B = 8.0 mT directed out of the page is established, and
the rod is moved toward the right with a speed of 5.0 m s –1.
a
What is the magnitude the average EMF induced in the rod?
b
Calculate the magnitude and direction of the average current induced in
the rod.
c
What is the magnitude and direction of the magnetic force on the rod?
d
At what rate is electrical energy being converted into heat energy in the resistance?
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Unit 4 Physics Electricity and magnetism: Activity booklet
Alternators and Generators Application
From Learning Point, run the following applications to answer the questions. You will learn from exploring!
Alternator
This application is from the University of NSW.
Play, stop and step and observe the voltage plot.
1. Describe the direction of the magnetic field.
2. Rewind to start at the beginning. After one step, is the flux in the coil increasing or decreasing?
3. Is the voltage increasing or decreasing?
4. State the formula to determine the value of the generated voltage. What part of this formula relates your
answer to question 2. and 3.?
5. Is the voltage generated AC or DC?
6. The mechanism for electrically connecting to the moving coil is called a slip ring. Can you think of how you
could obtain DC?
AC generator
This application is from the Heinemann CD.
1. From the Introduction, observe the voltage generated from a changing flux. Why don’t these graph match up
with each other?
2. The application has a few minor errors. Select “current” and observe the current arrows. Describe the error with
the arrows.
3. No select “slow turning rate”. Why does the generated voltage decrease?
4. Now, work through the questions on the bottom of the screen. Write down your answer before checking with
the “a” button. Grade yourself for these 9 questions.
/9
Fendt Physics generator
Run the Walter Fendt application showing how generators work.
1. What is the effect of the commutator?
2. What effect does changing the direction of movement have?
3. What effect does changing the speed of rotation have? (hint: it changes 2 variables)
4. For your answer in 3. describe why each variable changes.
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Unit 4 Physics Electricity and magnetism: Activity booklet
Electromagnetism questions
1. A solenoid is wound on an iron rod of radius 50 mm. A current is passed through the solenoid, producing a
magnetic field of strength 1.00 T. Determine the magnetic flux through the solenoid.
2. A rectangular nickel loop is located with its plane
perpendicular to a uniform magnetic field of strength
B = 8.0 mT and directed into the page, as shown in the
diagram. The loop is free to rotate about a horizontal
axis X–Y. The loop has a resistance R = 2.0 .
a. How much magnetic flux is threading the loop in this
position?
b. The loop is rotated about X–Y, through an angle of 90, so that its plane is parallel to the magnetic field. How
much flux is threading the loop in the new position?
c. Calculate the average EMF induced in the loop if the rotation took 20.0 ms.
d. Assume that the loop is rotated through another 90 so that it returns to its original position, perpendicular to
the field. The loop remains in this position for some time. The magnetic field is then switched off and reaches a
zero value in 1.0 ms. Determine the magnitude of the average current induced in the loop.
3. A circular coil of radius 3.0 cm has 50 turns. It is oriented so that the field lines of a magnetic field are normal to
the area of the coil. The magnetic field is varied so that B increases from 0.10 T to 0.35 T in a time interval of 2.0
ms.
a. Determine the change in flux through the coil during the 2.0 ms interval.
b. Calculate the average EMF induced in the coil.
4. During a physics experiment, a student pulls out a horizontal coil from between the poles of two magnets in
0.20 s. Initially the coil is located entirely in the field of the magnets with its plane perpendicular to the field. The
final position is field free. The coil has 50 turns, each of area 1.0 cm 2, and a total resistance R = 0.50 . The field
strength is B = 2.0 T.
a. What is the magnitude of the average EMF induced in the coil?
b. Determine the average current induced in the coil.
c. The student makes some modifications to the equipment and then repeats the experiment. Which one or more
of the following would result in a greater induced EMF in the coil?
A
The number of turns in the coil is increased.
B
The area of each turn is reduced.
C
The coil is pulled out from between the field faster than before.
D
The direction of the magnetic field is reversed before the coil is pulled out.
1. 7.9 mWb 2a 8.0 Wb b. 0 c 4.0×10–4 V d. 4.0mA 3a 7x10-4Wb b -17.7V 4. 0.05V b. 0.10A c. A and C
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Unit 4 Physics Electricity and magnetism: Activity booklet
Motor, generator & Lenz’s Law questions
1. Thomas is trying to build a simple motor that will operate on
DC current. He first decides to study the magnetic forces on a
current-carrying wire. He places a single loop of wire in a
uniform magnetic field, and connects a battery, as shown.
Use this answer key for the following
a. Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point x?
b. Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point y?
c. Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point z?
In order to convert the arrangement above into a motor, Thomas provides an
axis for rotation. He realises that there must be current flowing through the
coil when it is rotating, so he attaches a set of slip rings that rotate with the
coil as illustrated.
The coil is initially set with its plane parallel to the magnetic field as shown,
and the switch is open so that no current flows.
d. The switch is closed. Which one of the statements (A-D) best describes
the situation after current begins to flow?
A. The coil begins to rotate, but stops after turning through 90°.
B. The coil begins to rotate and, after rotating one half turn, rotates back to
its original position.
C. The coil does not move.
D. The coil rotates continuously.
e. If the slip rings used by Thomas in the circuit were replaced with a commutator, which one of the statements
(A-D) best describes the situation after the switch is closed, and the current begins to flow?
A. The coil begins to rotate, but stops after turning through 90°.
B. The coil begins to rotate and, after rotating one half turn, rotates back to its original position. It then continues
to oscillate in this way.
C. The coil does not move.
D. The coil rotates continuously.
2. In the following arrangements, show the direction of the induced current in the part of the coil closest to you (ie
the thicker line)
S
N
S
N
N
S
3. A
rect
ang
ular
loo
p of
are
a
0.2 m2 is pulled out of a magnetic field of strength 1.2T in 0.5s in the direction shown.
a. What is the induced EMF in the loop and b. in which direction will it flow?
1a H b I c G d A e D 2 up, down, down 3 -0.48V clockwise
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Unit 4 Physics Electricity and magnetism: Activity booklet
AC voltage and current, transmission losses
1. An electric heater has an element with resistance 30  and is connected to the 240 V AC RMS supply.
Determine the peak current that flows through the element.
2. The primary and secondary coils of a power line transformer have 50 and 25000 turns respectively. Neglecting
energy losses, if 240 V AC is applied to the primary, what would be the voltage in the secondary?
3. A 240 V to 12 V transformer has a 100 W light globe connected to it. If Np =200, find
a) Ns
b) Ip
4. A 5000 W generator supplies electricity to a remote building. The wires connecting the generator to the building
have a total resistance of 1.8 . The output voltage is 350 V AC RMS and the current is 15 A AC RMS.
a) What is the peak voltage output of the generator?
b) Find the peak current flowing in the wires.
c) What electric power is dissipated in the wires?
d) What is the total voltage drop in the wires?
e) What is the voltage at the remote building?
f) How could this situation be improved with the use of transformers?
5. A coal-fired power station in the Latrobe Valley generates electricity with an RMS voltage of 20 kV. It is stepped
up to 500 kV for transmission to Melbourne.
a) Determine the ratio Np/Ns for the step up transformer.
b) Is a current of 7000 A flows in the primary coil, how much current flows in the secondary coil?
c) the transmission lines are 150 km and have a total resistance of 10 . Find the power loss in the transmission
lines.
d) the step-down transformer has 50 000 windings in one coil and 1000 windings in the other coil. Does the
primary or secondary have the greatest number of windings?
1. 11.3A 2. 120 kV AC 3. 10 0.415 4. 495V 21.2A 405W 27V 323V 5. 0.04 280A 784kW primary
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Unit 4 Physics Electricity and magnetism: Activity booklet
Power Generation Questions
1. A power station generates 500 MW of electrical power, which is fed to a transmission line. Determine the
current that would flow in the transmission line if the line voltage is:
a
200 kV?
b
400 kV?
2. A 100 km transmission line is made from aluminium cable of total resistance of 10 . The line carries the
electrical power from the 500 MW power station to a substation. Calculate the percentage power loss in the line
when the power station is operating at:
a
250 kV
b
500 kV.
3. A solar-powered generator produces 5.0 kW of electrical power at 500 V. This power is transmitted to a distant
house via twin cables of total resistance 4.0 .
a
What is the current in the cables?
b
What is the total power loss in the cables?
c
What is the percentage power loss in the cables?
d
What is the voltage available at the house?
4. The cost of electricity in Canada is 20c per kilowatt hour.
a
Determine the cost of running a 1.0 kW heater for 2.0 h.
b
Determine the cost of running an 80 W light globe for 30 min.
c
Determine the cost of running a 250 W television for a day.
d
Determine the cost of running a 6.0 W clock radio for a week.
e
Determine the cost of running a 1.0 mW calculator for a year.
5. An RMS voltage of 500 kV provides exactly the same amount of power as a 500 kV DC voltage. However, a
transmission line that is safely insulated for a DC voltage of 500 kV is not necessarily safe for the 500 kV RMS.
Why not?
6. The voltage at the secondary coil of a transformer can be increased by (one or more answers):
A increasing the number of turns of wire wound around the secondary
B decreasing the number of turns of wire wound around the secondary
C increasing the number of turns of wire wound around the primary
D decreasing the number of turns of wire wound around the primary
7. You are going to the USA for a tour and you need to take a transformer so that your 120W hair drier which runs
on 240V here can be used there where the mains is 110V.
a. What is the resistance of your hair drier’s heating element?
b. Is the transformer you need a step up or a step down transformer?
c. For this transformer, what will be the Ns/Np turns ratio?
d. You buy a transformer at a travel goods store and the label on the box says that the primary has 55 turns. How
many turns on the secondary?
e. What current will the drier draw from the transformer when you are using it in the USA?
f. How much current will the transformer be drawing from the mains?
Ans 1a 2.5x103 A b 1.25x103 A 2 a 8.0% b 2.0 % 3 a 10A b 400 W c 8.0 % d 460V 4 a 40c b 0.8c c 120c d 20c e 0.18c 6 A
& D 7 a 480 b up c 2.2 d 120 e 0.5 A f 1.1 A
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Unit 4 Physics Electricity and magnetism: Activity booklet
VCE Physics Unit 4 Electricity and Magnetism Practice Test
Q1 At a particular moment, the coil of an electric motor is flat between the two poles of its magnet, as shown in
the diagram. Using words like up, down, left, right, in and out, determine the direction of:
a) the magnetic field
b) the current in ab
c) the force on ab
d) the force on bc
e) the force on cd
f)
the net force on the coil
After a one quarter rotation (90o from the position shown in the diagram), determine the direction of:
g) the torque due to the force on ab
h) the torque due to the force on bc
i)
the torque due to the force on cd
After a one half rotation (180o from the position shown in the diagram), determine the direction of:
j)
the force on ab
k) the force on bc
l)
the force on cd
Q2 5MW of power is transmitted down power lines (R = 1500
a) the current through the power lines
) at 250 000V. Determine:
b) the voltage loss
c) the percentage power loss
d) would the percentage power loss increase or decrease if the voltage was 500 000V
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Unit 4 Physics Electricity and magnetism: Activity booklet
Q3 A 120 turn rectangular coil, 1.5 x 1.5cm, and a horse-shoe magnet of field strength 0.4T are used to construct
a dc motor. The motor is connected to a 12V battery and draws a current of 2.5A. Determine:
a) the maximum force exerted on each side of the coil
b) the total force exerted by both sides of the coil
c) the maximum torque of the motor
d) the average torque of the motor
e) how much current is needed to produce a maximum torque of 0.135Nm
Q4 A loudspeaker has 125 turns around a magnetic core, 1.4cm in diameter. The diagram below shows a side
view and a view from above the speaker. The radial magnetic field has a strength of 0.12T. Determine:
a) the length of wire perpendicular to the field
b) the current through the coil needed to produce a force of
0.2N on the coil
c) the force on the coil if it draws a current of 0.45A
d) the direction of the current if the speaker coil is pushed out
Q5 12kV rms voltage is produced by a generator when its 0.6T electromagnet is rotated at 40Hz. Determine:
a) the time taken for the flux to change from zero to the maximum.
b) the rms voltage generated if the rotation period is 0.01s
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Unit 4 Physics Electricity and magnetism: Activity booklet
Comparing DC Motors, Generators and Alternators
DC Motors
Generators
Alternators
Energy Input
Energy Input
Connection
Magnetic Field
Wire Coils
Energy Output
Energy Output
Connection
Sketch the rotation of a coil in a generator in 4 different positions
Show the flux
No show the change in flux
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Unit 4 Physics Electricity and magnetism: Activity booklet
Transformers
Questions
1. To correctly explain the operation of a transformer, select the correct statements in the correct order from the
following:
a) A changing magnetic field induces a changing voltage
b) A direct current produces a large magnetic field
c) A changing current produces a changing magnetic field
d) An unchanging magnetic field induces a voltage
2. What do transformers transform?
3. A transformer with 5 primary turns and 25 secondary turns is connected to a 240 V AC supply. The input
current is measured to be 20 A.
a) What is the output voltage?
b) What is the output current?
c) What is the input power?
d) What is the output power?
e) What reasonable assumption did you make to answer part d)?
4. In a car, a coil is used to transform 12 V to a large enough voltage to make a spark jump across the spark plug
gap. A car battery only produces DC, so how does the coil produce a large voltage?
5. A 20 km power line has a resistance of 1.5 Ω/km and carries a current of 80 A. It has a supply voltage of 25 kV.
a) What power is lost?
b) What is the voltage drop from one end of the line to the other?
c) What is the voltage at the far end of the line?
6. A very long extension lead is used to supply power to a shed. The lead is connected to 240 V AC and the far
end connects to a light globe. The globe delivers 100 W of power, but the voltage is measured to be only 200 V.
a) Calculate the current in the extension lead.
b) What is the total power delivered from the power point the lead is plugged into?
c) Find the power lost in the power lead.
d) Calculate the efficiency of this system.
e) Suggest a more efficient may of delivering power to the shed.
1. c then a 2. voltage and current 3. 1200VAC 4A 4.8kW 4.8kW transformers are 100% efficient 4. The DC is switched on and off with the
points – producing a changing voltage 5. 192kW 2400V 22600V 6. 0.5A 120W 20W 83% Use a transform to increase the voltage, hence
decrease the current and reduce power lost.
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Unit 4 Physics Electricity and magnetism: Activity booklet
Electricity Distribution Systems Demonstration
Complete the following table
1. To
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