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QUESTIONS & PROBLEMS
MOTORS AND GENERATORS
 How to answer a question: problem solving (t0_372.pdf)
 View periodic table (cited Aug 2012)
CONSTANTS
electron charge
e = |qe| = 1.60210-19 C
1 eV = 1.60210-19 J
electron mass
me = 9.10910-31 kg
proton mass
mp = 1.67510-27 kg
neutron mass
mn = 1.67510-27 kg
speed of light
c = 3.00108 m.s-1
Planck’s constant
h = 6.62610-34 J.s
magnetic force constant
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k
o
 2.010-7 N.A-2
2
1
QUESTIONS AND PROBLEMS FROM MODULE NOTES
P6021 01/14
Two straight metal rods, P and Q,
have the same length. They are each
pivoted at one end and rotated with
the same angular velocity so that
they sweep out horizontal circular
paths as shown in diagrams X and Y.
A constant current I is flowing along
each rod. In diagram X, a constant
magnetic field is applied at right angles to the plane of the circular path.
In diagram Y, a uniform magnetic field of the same magnitude is applied
in the plane of the circular path. Comment on the magnitude and direction of the
forces acting on P and Q. Explain.
P6060 04/24
In the late nineteenth century Westinghouse and Edison were in competition to
supply
electricity to cities. This competition led to Edison holding public demonstrations to
promote his system of DC generation over Westinghouse’s system of AC generation.
Propose arguments that Westinghouse could have used to convince authorities of
the
advantages of his AC system of generation and distribution of electrical energy over
Edison’s DC supply.
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P6069
An electron travels at 2.0107 m.s-1 in a plane perpendicular to a 0.010 T magnetic
field.
(A)
(B)
(C)
(D)
Describe the path of the electron.
Calculate the radius of the circular orbit.
Calculate the period of motion.
Calculate the frequency of the electron.
Hint: Centripetal force FC = m v2 / R
q = -1.60210-19 C
m = 9.1110-31 kg
P6077 02/8
A single-turn coil of wire is placed in
a uniform magnetic field B, so that
the plane of the coil is parallel to the
field, as shown in the diagrams. The
coil can move freely. An electric
current I flows around the coil. In
which direction does the coil begin
to move as a consequence of the
interaction between the external
magnetic field and the current?
Explain.
P6091 02/7
A student performed an experiment to
measure the force on a long current-carrying
conductor placed perpendicular to an external
magnetic field. The graph shows how the force
on a 2.2 m length of the conductor varied as
the current through the conductor was
changed. What was the magnitude of the
external magnetic field in this experiment?
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P6092 06/7
A current-carrying conductor passes
through a square region of magnetic field,
magnitude 0.55 T. The magnetic field is
directed into the page. What is the force
on the conductor (magnitude and
direction)?
P6121 01/10
An electric motor is connected to a power supply of constant voltage. The motor is
allowed to run at different speeds by adjusting a brake. Sketch a graph that best
shows how the current through the motor varies with speed?
P6130 M762
Describe a first-hand investigation to demonstrate the effect on a generated electric
current when the strength of the magnet is varied. In your description, include:
• A labeled sketch of the experimental set-up.
• How you varied the magnetic field strength.
• How other variables were controlled.
P6133 02/23
State Lenz’s law.
When the metal rod is
moved upwards
through the magnetic
field an emf is induced
between the two ends.
Which end of the rod is negative?
Explain how the emf is produced in the rod.
Explain how the principle of induction can be used to heat a conductor.
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P6146
A neon sign requires a 6000 V supply for its operation. A transformer allows the neon
sign to operate from a 240 V supply. What is the ratio of the number of secondary
turns to the number of primary turns for the transformer?
P6148
Determine induced voltage in a coil of 100 turns and coil area of 0.05 m2, when the
magnetic field 5.0 T (passing through coil) is reduced to zero in 0.25 s.
P6162 04/20
The photograph shows a transmission line support tower.
The insert shows details of the top section of the tower.
Describe the role of each of the parts labeled A and B in
the photograph.
(A) What is the main cause of energy losses in
transmission lines?
(B) How are energy losses in transmission lines minimised?
(C) In the 1880s two systems of power generation were in
competition to supply domestic electricity to consumers.
Which was the preferred system at that time?
(D) What was the deciding factor in the WestinghouseEdison debate?
(E) How are transmission towers protected from lightning strikes?
(F) What stops the current in high voltage overhead transmission lines from running
to
earth?
P6173 07/9
A stationary exercise bike has a solid metal wheel that is rotated by a chain
connected to the pedals. An array of strong permanent magnets provides a magnetic
field close to the face of the wheel. The exercise level can be selected from 1(easy) to
6(hard) using a control panel. Explain how the level is changed.
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P6174
An experiment was set up with a 1 turn rectangle coil of wire suspended from a
spring balance so that its lower side of length 65 mm was between the poles of a
large permanent magnet. The force on the spring balance was measured as the
current through the coil was varies
Current I (A) 0 0.5 1.0 1.5 2.0 2.5
Balance F (N) 3.7 3.8 3.9 4.0 4.1 4.2
Sketch a graph of the experiment and show the direction of the current through the
coil.
Graph the results.
Determine: mass of coil; strength of magnetic field; the spring balance reading if a
current of 5.0 A was flowing through the coil; the reading of the balance if 2.0 A was
flowing in the opposite direction through the coil.
The experiment was repeated with the same coil but with a magnetic whose field
strength was twice as great. On your axes, draw the graph to predict the new results.
The experiment was repeated with a coil of 4000 turns (assume coil has same mass
as previous) with a different magnet. Identical results were obtained as in the first
experiment. What is the strength of the new magnetic field?
P6215 M762
In your course you had to gather information to explain how induction is used in
certain applications. With reference to TWO applications, describe how you assessed
the reliability of information you found.
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P6234 07/21
A simple motor consists of a flat rectangular coil with n turns in a magnetic field B.
The coil has an area of 0.01 m2 and carries a current of 1.7 A. The motor drives a
pulley of diameter 222 mm, and weights can be
hung from either side of the pulley at point X or
point Y.
(A) In order to prevent rotation, should a
weight be hung at point X or at point Y? Explain.
(B) What is the magnitude of the torque
provided by a mass of 0.15 kg suspended from
either point X or point Y?
(C) If the motor is just stopped by a mass of 0.15 kg, how many turns n does the coil
have?
P6235 04/7
Why do some electrical appliances in the home need a transformer instead of
operating
directly from mains power?
P6238 03/7
A non-ferromagnetic metal disk is at
initially at rest and balanced on a
support so that it is free to rotate. A
magnet is moved in a circular path above
the surface of the disk. Explain why the
disk starts to rotate and in what
direction will it rotate? What device
operates on this principle?
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N
magnet rotated above
ferromagnetic disk
S
7
P6241
magnetic field in +x direction
n
D

N
E
B
S
C
F
slip rings
y
brush contacts
direction of
rotation
clockwise
around z axis
(out of page)
A
B
to external circuit
x
z
Given:
magnetic field B = 1.25 T
LCD = LEF = 12 mm LDE = LCF = 6.0 mm
Rotation rate = 20 rpm (revolutions per minute)
Number of turns of coil N = 100
Total resistance of coil Rcoil = 10 
Resistance of external load connected between A and B Rload = 125 
Calculate
(A) For the rotation:
angular frequency  = ? rad.s-1
frequency of rotation f = ? Hz
period T = ? s
For angles  = 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 300, 360 deg
(B)
magnetic flux B = ? T.m2
induced polarity of point A with respect to the point B emf = ? V
the induced current I = ?
(C)
(D)
Graph the above quantities.
The rotation speed was doubled.
On your graphs add curves to show the resulting changes that would
occur.
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The generator is now to be used a motor. The coil is no longer rotated by an external
mechanical energy supply and the load is removed from between A and B.
The point A was connected to the positive terminal of a 12 V battery and point B was
connected to the negative terminal.
(E)
Explain why this arrangement does not work as a motor.
(F)
The commutator of the slip rings is replaced by a split ring.
The motor now works. Why?
P6272 06/14
A potential difference of 50 V is applied between two identical, parallel aluminum
plates which are separated by a distance of 10 mm. In order to double this electric
field
strength, which new arrangement should be used?
Explain
P6276
(A) Can you arrange three parallel wires so that they attract each other?
(B) Can you arrange three parallel wires so that they repel each other?
Explain.
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P6275 07/24
(A) A negatively charged
cylinder is fixed in position near
a positively charged plate.
Sketch the electric field lines
between the cylinder and the
plate.
(B) A tiny particle of mass 10-30 kg and charge +6×10-12 C is released at point Y. The
particle initially accelerates at 7.0×1021 m.s-2. Calculate the electric field intensity at Y.
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P6283 02/10
The coil of an AC generator rotates at a constant rate in a magnetic field.
Which of the following diagrams represents the curve of induced emf against
position? Explain.
P6329
The diagram shows (top view) of a simple
DC generator. If the motion of the wires P
and Q of the coil are as shown, what are
the directions of the induced currents in P
and Q?
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P
S
N
Q
11
P6333
A simple AC generator develops a sinusoidal EMF with a maximum value of 90.0 V
and frequency 50 Hz. The coil has an area of 200 cm2 and rotates in a field of 2.39 T.
(A) How many revolutions per second does the coil make?
(B) What is the EMF produced by a single coil?
(C) How many turns does the coil have
P6349
M501
In a particular experiment a long length of copper wire of very low resistance is
rotated by two students. The ends of
the wire are connected to a
galvanometer, G, and a current is
detected. Which of the following is
LEAST likely to affect the amount of
current produced?
(A) The length of the rotating wire
(B) The thickness of the rotating wire
(C) The speed with which the wire is rotated
(D) Whether the wire is oriented north-south or east-west
Explain.
P6354 03/20
Two solenoids (coils) with hollow cores are suspended using string. The solenoids are
free to move in a pendulum motion. In the first investigation shown in Figure 1, a
strong bar magnet is moved towards the solenoid until the north end of the magnet
enters the solenoid and then the motion of the magnet is stopped. In the second
investigation, shown in Figure 2, a thick copper wire is connected between the two
terminals, A and B, at the ends of the solenoid. The motion of the magnet is repeated
exactly in this second investigation. Explain the effect of the motion of the magnet
on the solenoid in the two investigations.
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P6373 01/8
A light rod has a coil of insulated copper wire fixed at
one end and is pivoted at the other end. The result is a
pendulum which is free to swing back and forth. A
magnet is placed underneath this pendulum. The
pendulum is pulled back and then allowed to swing.
What can you do so that the pendulum comes to rest
most quickly?
P6390
A current-carrying coil in a magnetic field experiences a turning effect. How can the
turning effect be increased?
P6402 06/9
Early electric generators were
often very simple. A handoperated version is depicted
below. Brush X touches the
metal axle and Brush Y
touches the rim of the disc.
The metal disc is rotated
uniformly as shown. Describe
the current through the globe.
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P6428 06/6
The diagram shows a magnet standing on the bottom of a dish filled with a
conducting solution. A copper wire is suspended freely from a point above the
magnet with its tip in the conducting
solution. It is held in the position
shown. The switch is closed and the
wire released.
Which of the following will be
observed?
(A ) The wire will rotate about the
magnet.
(B) The wire will be attracted to the magnet.
(C) The magnet will rotate about its vertical axis.
(D) The solution in the dish will rotate about the magnet.
Explain.
P6447 06/22
A student drops a bar magnet onto a large block of copper resting on the floor. The
magnet falls towards the copper, slowing down as it comes close, then landing gently.
(A) Explain the physics responsible for this observation.
(B) Predict what will happen if the experiment is repeated with a copper block
cooled to approximately –50°C. Justify your prediction.
P6457 03/10
A flexible wire loop is lying on a frictionless
table made from an insulating material. The
wire can slide around horizontally on the
table and change shape freely, but it cannot
move vertically. The loop is connected to a
power supply, a switch and two terminals
fixed to the table. When the switch is closed,
a
current I flows around the loop. Sketch a
diagram that would most closely represents the final shape of the loop after the
switch is closed?
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P6461 07/6
An electric motor is setup as shown.
Why does the coil not rotate? What is
required for the coil to rotate?
P6464 M456
Two thin metal tubes 1.00 m long were supported in a vertical wooden rack as
shown in the diagram. The two ends were connected together, then the other two
ends were connected briefly to a car battery as shown in the diagram. It was
observed that one of the tubes jumped upward as the connection was made.
(A) Explain why only one tube jumped upward.
(B) Each tube has a mass of 110-2 kg, and the tubes lie on the rack 100 mm apart.
What minimum current flows when one tube jumps?
(C) What is the implication of this result for power distribution networks?
P6480
The variation in magnetic flux through a
coil is shown. Sketch the induced emf in
the coil.
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P6481
A horizontal wire carries a current of I1 = 80 A. How much current I2 does a second
parallel wire 200 mm below carry so that the wire does not fall due to gravity? The
lower wire has a linear mass density of 0.12 g.m-1.
P6506 M018
A schematic diagram of a system to supply electricity to a house is shown below.
The step-down transformer in the substation has a turns ratio of 30 : 1.
(A) What is the voltage carried by the high voltage transmission line?
(B)
(B) Identify the causes of the two main energy losses in the transmission of
electricity between the power plant and the house.
(C) Explain how the application of superconductivity could minimise energy loss in
the system.
P6546 04/21
An ammeter was used to
measure the current through
a small DC motor. While it was
running freely, a current of
0.89 A was recorded. While
the motor was running, the
axle of the motor was held
firmly, preventing it from rotating, and the current was then recorded as 2.14 A.
Explain this observation.
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P6547 06/19
The diagram shows the structure of a typical
galvanometer. Describe how the
galvanometer operates as an application of
the motor effect.
P6569 A DC generator has a coil with 500 turns, an area of 0.125 m2 and rotates at
50 Hz in a 1.5 T magnetic field.
(A) What is the maximum EMF generated?
(B) Draw a diagram of the EMF against time, clearly showing the times at which
the maximum EMF occurs.
P6576 06/20
A balance was used to investigate the relationship between current and force. The
balance was set up with one copper rod fixed to it and a second rod fixed above it.
Each rod was connected to a source of current. The copper rods were rigid, each was
2.6 m long, and they were parallel. The current in the upper rod was kept constant at
50 A.
(A) What must be the directions of the current through each wire? Explain.
Different currents were passed through the lower rod and the balance reading
recorded for each current.
Current in lower rod
(A)
5.6
Balance reading (kg)
0.5485
15.7
0.5480
24.2
0.5474
33.8
0.5470
39.3
0.5465
(B) Graph the data (X-axis current).
(C) Find the mass of the copper rod resting on the balance.
(D) Calculate the distance between the two copper rods.
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P6582 01/21
An electric pump is run by a very large DC electric motor. This motor is connected in
series with a variable resistor to protect the windings in the coil. When the motor is
starting up, the variable resistor is adjusted to have a large resistance. The resistance
is then lowered slowly as the motor increases to its operating speed. Explain why no
resistance is required when the motor is running at high speed, but a substantial
resistance is needed when the motor is starting up.
P6598
A Christmas tree contains a string of 24, 1.5 V lights in series with each other. They
are connected through a transformer to a 240 V supply. The transformer has 640
turns
in its primary coil. How many turns does it have in its secondary coil?
P6636
You are offered a house plot close to a high-voltage transmission line. The plot has
marvellous views and is cheap. Discuss the pros and cons of living on this site.
Would you buy the land? Why? What enquiries might you make of the electricity
company in arriving at a decision? What measurements would you make to put your
mind at rest?
P6642 02/6
What is the role of a transformer at an electrical power station? Explain.
P6674
Two conducting coils P and Q are placed near each other.
When the switch is closed at time t = 0 and then later
opened. How will the force on Q vary with time t. Give
your answer as a graph with repulsion to be positive.
P
Q
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P6691 07/7
In the graphs, the solid
curve shows how the emf
produced by a simple
generator varies with time.
The dashed curve is the
output from the same
generator after a
modification has been
made to the generator. What modifications were made?
P6727
If an AC generator is rotated faster, how will the voltage output change?
P6730
A coil containing 15 windings is suspended in
a magnetic field B. Calculate the strength of
the magnetic field if the coil experiences a
torque of 0.15 N.m when the current through
the coil is 0.5 A. Which way will the coil start
to turn?
100 mm
B
100 mm
I
I
P6739 02/22
Two types of generator are shown in the diagra
(A) What is the function of the brush in a generator?
(B) Which of these generators is a DC generator? Justify your choice.
(C) Outline why AC generators are used in large-scale electrical power production.
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P6754 M860
Three rings are dropped at the same time
over identical magnets as shown below.
What is the order in which the rings P, Q and
reach the bottom of the magnets? Explain.
R
P6755
A single-turn coil of wire is placed
a uniform magnetic field B at right
angles to the plane of the coil as
shown in the diagrams. The coil is
then rotated in a clockwise
direction as shown.
Which of the following shows the
direction of current flow in the coil
it begins to rotate? Explain your
answer.
in
as
P6794 01/4
Two types of
generator are shown.
What type of current
is produced by each
generator when
connected to an
external resistance?
Explain.
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P6799
06/10
The apparatus is designed to investigate the operation of a transformer. A student
closes the switch for a short time, then opens it. The data logger records values of
voltage for both coils for the duration of the investigation. The data logger software
displays the results as a pair of voltage–time graphs on a computer monitor. Sketch
the two voltage–time graphs for the coils X and Y. Explain
P6801 07/13
An electron is moving near a long straight wire. When a current is applied to the wire
the electron experiences a force in the same direction as the current flow in the wire.
What was the electron’s initial direction of motion? Explain.
P6806 A transformer is to be
designed so that it is efficient, with
heating by eddy currents
minimised. The designer has some
iron and insulating material
available to build the transformer
core. The windings are to be made
with insulated copper wire.
Which of the following designs
minimises the energy losses in the
core? Explain your answer
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P6813 06/12
A charged non-magnetic particle is moving in a magnetic field. What would NOT
affect the magnetic force on the particle?
(A) The strength of the magnetic field.
(B) The magnitude of the charge on the particle.
(C) The velocity component parallel to the magnetic field direction.
(D) The velocity component perpendicular to the magnetic field direction.
Explain.
P6835
Electric power generators are much harder to turn when current is drawn from the
generator. Power stations need to burn much coal to supply power during times of
peak demand. Use Lenz’s Law to explain why this is so.
P6838
What principle was the deciding factor in the AC/DC competition?
P6845
An average of 120 kW of electric power is sent to a small town from a power point
10 km away. The resistance of the transmission lines is 4.010-5 .m-1. Calculate the
power loss and the percentage power loss if the power is transmitted at 240 V and
24 kV.
P6847 04/9
An electric DC motor consists of 500 turns of wire formed into a rectangular coil of
dimensions 0.2 m  0.1 m. The coil is in a magnetic field of 1.010-3 T. A current of
4.0 A flows through the coil. What are the magnitude of the maximum torque, and
the orientation of the plane of the coil relative to the magnetic field when this occurs?
P6849
Westinghouse created opportunities for his AC distribution system at the right time
by underbidding Edison on two extremely important projects. What were the
projects? Why were the projects important? If Edison had one of these projects –
would it have made a difference? How would our electricity supply system be
different if DC transmission was used today instead of AC? What are the limitations
of DC transmission?
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P6859
A rectangular loop of mass 2.456 g is
suspended vertically from a spring balance in a
uniform magnetic field which is directed out of
I
I
120 mm
the page as shown. The width of the conductor




in the magnetic field is 120 mm. The top
portion of the loop is free of the magnetic field.




-3
The spring balance reading is 4.89610 N




when the conducting loop carries a current of
uniform magnetic field out of page B
0.451 A. What is the magnitude of the uniform
magnetic field? (This technique is a highly accurate means of measuring the strength
of magnetic fields).
P6862 04/22
Describe the main features of an ac
induction motor.
P6913
Thomas Edison was a great scientist. He had over 150 inventions and held over 1900
scientific patients. However, his tactics in the competition against Westinghouse
seem to be a bit unethical. What were three of his possible motives? Critically
evaluate each motive. Assess the fairness of Edison’s tactics. Was Edison justified at
his win at all costs attitude?
P6946 04/8
A transformer which has 60 turns in the primary coil is used to convert an input of
3.6 V into an output of 12 V. What type is the transformer? What is the number of
turns in the secondary coil?
P6981
A magnetic field is produced when an electric current is
1. steady
2. increasing.
3. decreasing
4. alternating.
What answers are correct?
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P6985
The secondary coil of a step-down transformer consists of larger diameter wire than
in the primary coil. Why is this?
P6998 06/8
A square loop of wire, in a uniform magnetic
field, is rotating at a constant rate about an
axis as shown. The magnetic field is directed
out of the plane of the page. At time t 0
the plane of the loop is perpendicular to the
magnetic field. Graph the variation of the
magnetic flux through the loop with time?
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MISCELLANEOUS QUESTIONS AND PROBLEMS
M6071
Explain the concept of magnetic flux in terms of magnetic flux density and surface
area.
M6072
A closed circuit is placed in a magnetic field. The magnetic flux through the circuit
depends on
1. the strength of the magnetic field.
2. the area enclosed by the circuit.
3. the relative orientation of the circuit and magnetic field.
What answers are correct?
M6073
What will produce an induced emf in a circuit?
1. changing the relative orientation of the circuit and magnetic field.
2. changing the strength of the magnetic field.
3. changing the area enclosed by the circuit.
What answers are correct?
M6074
An aeroplane is flying horizontally over the north
pole. The diagram shows the plane when viewed
from above. How will the plane be charged?
Specify the charge at each of the points P, Q, R
and S.
R
direction
of flight
Q
P
S
M6075
What will make a simple DC motor more powerful.
1. increasing the strength of the pole pieces.
2. increasing the number of turns in the coil.
3. winding the wire on a soft-iron armature.
4. using high resistance wire for the windings.
What answers are correct?
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M6076
A disc magnet has its poles on its opposing flat surfaces. An insulated copper wire
was placed on the disc magnet as shown in the diagram. The instant the wire was
connected to a DC battery, the wire was observed to move in the direction of the
arrow when the current was from X to Y.
What is the direction of the magnetic field and identify the poles of the magnet?
F
I
M6077
An electromagnet is attached to the bottom of a light train which is travelling from
left
to right. When a large current is passed through the coils of the electromagnet, the
train slows down as a direct result of the law of conservation of energy.
Explain why the train slowed down.
M6078
In which of the following devices is the law of
conservation of energy applied in the same way?
Explain your choice. What does the picture
represent?
(A) DC motor
(B) Loudspeaker
(C) Induction motor
(D) Induction cooktop
M6079
A current of 5.20 A flows in a wire that is placed in a magnetic field of 0.516 T. The
wire is 0.745 m long and is at an angle of 60.6° to the field. Calculate the magnitude
of the force on the wire? What is the direction of the force?
M6080
Assess the impacts of the development of AC generators and tranformers on today’s
society and the environment.
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M6081
A student claims that a DC generator is an ‘electric motor in reverse’. Analyze this
claim with reference to the structure and function of a simple DC generator and an
electric motor. Include diagrams in your answer.
M6082
An electricity substation delivers a current of 10.0 A at a voltage of 11.00 kV to an
office complex. The office complex uses a transformer to provide a current of 185 A
at a voltage of 240Vac.
(A) Explain why AC is preferable to DC as an input current for transformers.
(B) Outline possible causes of energy loss in the transformer.
(C) Calculate the energy lost by the transformer in eight hours.
(D) Assuming there was no energy losses and the primary voltage was 11.00 kV and
had and the primary coil had1.2104 turns, calculate the number of turns in the
secondary windings of the transformer.
M6083
Two parallel wires are separated by a distance of 0.75 m. Wire X is 3.0 m long and
carries a current of 2.0 A. Wire Y can be considered to
be infinitely long and carries a
18 mm
current of 5.0 A. Both currents flow in the same
direction along the wires.
355 mm
(A) What is the direction of the force that exists
between the two wires?
(B) Sketch a graph that shows how the force between
the two wires would vary if the length of Wire X was
conducting
salt solution
decreased.
(C) Sketch a graph that shows how the force between the two wires would vary if
the distance between the wires was decreased.
(D) In your Physics course you have performed a first-hand investigation to
demonstrate the motor effect. Explain how your results demonstrated that effect.
M6084
Two straight copper wires are suspended so that their lower ends dip into a
conducting salt solution in a beaker as shown. The length of the straight section of
each wire above the conducting salt solution is 355 mm and they are placed 18 mm
apart. The ends of the wire do not touch the bottom of the beaker. The two wires
are connected to a DC power supply. A current of 2.23 A flows from the battery.
Calculate the magnitude and direction of the initial force on each wire.
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M6085
A pair of parallel metal plates, placed in a vacuum, are separated by a distance
of 5.75010-3 m and have a potential difference of 1222 V applied to them.
(A) Calculate the magnitude of the electric field strength between the plates.
(B) Calculate the magnitude of the electrostatic force acting on an electron between
the plates.
(C) A beam of electrons is fired with a velocity of 3.30106 m.s-1 between the
plates. A magnetic field is applied between the plates, sufficient to cancel the force
on the electron beam due to the electric field. Calculate the magnitude of the
magnetic field required between the plates to stop the deflection of the electron
beam.
(D) Draw a diagram showing the charged plates (one positive and the other
negative). Show on the diagram the directions on the magnetic force and electric
force acting on the electrons. Also show the electric and magnetic fields.
M6086
The diagram shows a generator connected to a
cathode ray oscilloscope (CRO).
Sketch the output voltage that would be
observed for this generator on the CRO.
Is the generator AC or DC?
M6087
A positively-charged ion travelling at
250 m.s-1 is fired between two parallel
charged plates, M and N. There is also a
magnetic field present in the region
between the two
plates. The direction of the magnetic field is into the page. The ion is travelling
perpendicular to both the electric and the magnetic fields. The electric field between
the plates has a magnitude of 200 V m-1. The magnetic field is adjusted so that the
ion passes through undeflected. Calculate the magnitude of the adjusted magnetic
field, and the polarity of the M terminal relative to the N terminal?
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M6088
Explain the relationship between the current in the primary coil and the current in
the secondary coil of an ideal step-down transformer in relation to the conservation
of energy
Explain why a transformer will work in an AC circuit but not in a DC circuit.
M6089
The diagram shows a two-pole DC motor as
constructed by a student. Identify THREE
mistakes in the construction of this DC
motor.
M6090
A conductor ABCD is situated in a magnetic field directed out of the page. The
conductor has a galvanometer inserted in side BC and a conducting rod XY connects
the sides AB and CD. The rod slides 50 mm to the left in one second and then stops
for a further one second. Then the rod slides at the same speed,100 mm to the right
of its initial position. Sketch the possible current changes observed on the
galvanometer. What are the directions of the induced currents?
M6091
A magnetic is dropped vertically (N pole lower end) from above a coil connected to a
galvanometer. Sketch a graph of the current through the galvanometer as the
magnetic falls through it. Explain your sketch.
M0692
An electron enters a very strong magnetic
field. Sketch the most likely path of the
electron.
How does the speed of the electron change?
Explain.
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M6093
A flat rectangle coil with 900 turns is in a magnetic field B = 0.48 T. The plane of the
coil is parallel to the magnetic field. The coil has sides of 400 mm (across B) and 160
mm (parallel B) and carries a current of 15 A.
(A) Sketch the coil and magnetic field so that the coil could act as a DC motor. Label
all parts of the sketch.
(B) Calculate the magnitude of the force on the coil and show on your sketch the
direction of the force.
(C) Calculate is the maximum torque on the coil?
(D) If the plane of the coil was at an angle of 30o to the direction of the magnetic
field, calculate the force and the torque acting on the coil.
(E) When is the net force and torque on the coil both zero.
M6094
Three wires are laid parallel to each
other. Wires 1 and 3 are in a fixed
positions. Wire 2 can slide across a
frictionless surface. What will happen
to wire 2 when all currents are
simultaneously turned on?
I1 = 2.5 A
1 fixed wire
220 mm
2 moveable wire
I2 = 4.5 A
89 mm
I3 = 1.5 A
3 fixed wire
M6095
A flat horizontal metal plate is perpendicular to a magnetic field. The plate is moved
at a constant speed through the magnetic field. As the metal plate is moved out of
the magnetic field:
(A) Draw a diagram showing the eddy current (include direction) induced in the
metal plate.
(B) Explain why the eddy current is induced and how does this affect the motion of
the plate through the magnetic field?
(C) Give one practical example of this phenomena.
M6096
The manufacturer of a small DC motor made the following claim: “The torque of the
DC motor has a constant value of 1.5 N.m. Analyse this claim with reference to the
structure of a simple DC motor.
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M6097
Explain why transformers are used to transmit electricity to our homes.
A student constructed and tested a model transformer and obtained the results:
Vp = 12.0 V IP = 0.045 A VS = 6.3 V IS = 0.065 mA
Calculate the power lost from this transformer.
Identify a feature of a real transformer that would reduce this power loss and explain
your answer in terms of the key physical principles.
S
M6098
A magnetic moving near a loop induces a current in
the circuit.
What will the direction of the current when the
magnetic enters the loop?
What will the direction of the current when the
magnetic exits the loop/
Explain.
N
C
D
G
A
B
M6099
Identify and describe the function of the
parts shown. How could the device shown be
used? Justify your answers. Identify the
direction of the current and any polarities.
M6100
Sketch the voltage output for a simple DC generator.
Add to your sketch for the voltage output to change the changes if:
(A) A stronger magnet was used.
(B) The armature was rotated faster.
(C) The number of turns in the coil was increased.
M6101
In a “real” generator, why is there a rotating coil with many turns rather than a single
loop of wire. Explain.
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M6102
When a generator is supplying current to an external circuit, it is found that the force
required to keep the coil turning is much greater than the force needed to overcome
friction. What is the origin of the extra force that opposes the rotation of the coil?
M6103
Compare the output from
the two generators.
Are the generators AC or
DC. Explain.
Discuss advantages and
disadvantages of AC and
DC generators and relate
these to their use.
What the function of the commutator?
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M6104
In this device, the powerful, central magnet
spins in a circle between the two coils.
What is the device? Explain.
What would be an advantage that a device
designed like this would have over
conventional devices built for the same
purpose?
What would be a disadvantage that a device designed like this would have over
conventional devices built for the same purpose?
M6105
In an experiment to demonstrate the production of AC current a group of students
duplicated one of Faraday's
experiments by winding two coils
on a soft iron ring as shown in the
diagram. They attached a DC
power supply and a switch to one
coil and
a milliammeter to the other.
(A) The students switched the DC supply on and then observed the ammeter. What
would they observe?
(B) The students then replaced the DC supply with an AC supply. What difference
would
they observe on the meter?
M6106
An electron travels at 2.0107 m.s-1 in a plane perpendicular to a 0.010 T magnetic
field. Calculate the radius of the electron’s orbit and its period of rotation.
M16107
Consider the following diagram of a
transformer. Describe the transformer.
What causes the heating effects that
occur in the core of transformers?
What is the main cause of inefficiencies
in transformers?
M108
Which of the following core designs would minimise heating effects due to eddy
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currents? Explain.
M6109
A battery charger is used to recharge 1.5 V batteries through a 12 V transformer.
The transformer has 2400 turns in the primary coil. Assuming 100% efficiency,
calculate: (A) The number of turns in the secondary coil. (B) The output current if the
input current is 10 mA.
M6110
If a power line has a resistance of 6.2×10-4 .m-1, calculate the power lost over 100
km of line if a current of 50 A is flowing through the line.
M6111
Which statement correctly describes an induction motor? It is:
(A) An AC or DC motor in which torque is produced by the interaction of a rotating
magnetic field in the stator with induced magnetic fields of the induced current in
the rotor.
(B) An AC only motor in which torque is produced by the interaction of a rotating
magnetic field in the stator with induced magnetic fields of the induced current in
the rotor.
(C) An AC or DC motor in which torque is produced by the interaction of a rotating
magnetic field in the stator with induced magnetic fields of the supplied current in
the rotor.
(D) An AC only motor in which torque is produced by the interaction of a rotating
magnetic field in the stator with induced magnetic fields of the supplied current in
the rotor.
M6112
Which statement about an AC motor is correct?
(A) It has slip rings and carbon brushes.
(B) It has slip rings but no carbon brushes.
(C) It has a split ring commutator and carbon brushes.
(D) It has a split ring commutator but no carbon brushes.
M6113
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Which statement about split rings and slip rings is correct?
(A) Neither are commutators.
(B) Only split rings are commutators.
(C) Only slip rings are commutators.
(D) Both are commutators
M6114
Describe, with the aid of a diagram, the main features of an AC motor.
About 90% of motors are AC induction motors. Compare the AC induction motor
with other forms of electric motors.
M6115
Explain in terms of physical principles, the diagrams showing how an AC induction
motor works.
M6116
The diagram shows an alternate structure for an
induction motor.
Explain how this might work. Predict the efficiency
an induction motor designed on this principle.
Justify your answer.
of
M6117
Explain why it is difficult to move a bar magnet rapidly near a sheet of copper.
M6118
Suppose that a square loop of wire is moved through a uniform magnetic field in a
direction at right angles to the field. Will there be an induced current around the
loop?
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