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| = 1.60210-19 C
1 eV = 1.60210-19 J
electron mass me = 9.10910-31 kg
proton mass mp = 1.67510-27 kg
neutron mass mn = 1.67510-27 kg
speed of light c = 3.00108 m.s-1
Planck’s constant h = 6.62610-34 J.s
magnetic force constant 2.010-7 N.A-2
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.
P6069
An electron travels at 2.0107 m.s-1 in a plane perpendicular to a 0.010 T magnetic field.
(A)Describe the path of the electron.
(B)Calculate the radius of the circular orbit.
(C)Calculate the period of motion.
(D)Calculate the frequency of the electron.
Hint: Centripetal force FC = m v2 / R q = -1.60210-19 C m = 9.1110-31 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?
P6092 06/7
A current-carrying conductor passes through a square region of magnetic field, magnitude 0.55T. 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.
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 labeledA 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 Westinghouse-Edison 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.
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.
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 Xor 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?
P6241
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
periodT = ? 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) Graph the above quantities.
(D) The rotation speed was doubled.
On your graphs add curves to show the resulting changes that would occur.
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.
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.
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?
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.
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 hand-operated 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.
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?
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 110-2 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.
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.
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 themaximum 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 50A.
(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) / Balance reading (kg)5.6 / 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.
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.
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?