Year 12 Physics Motors and Generators Practice Quiz 1
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Data
𝐹
𝐼1 𝐼2
=𝑘
𝑙
𝑑
𝜏 = 𝐹𝑑
𝜏 = 𝑛𝐵𝐼𝐴𝑐𝑜𝑠𝜃
𝑉𝑝 𝑛𝑝
=
𝑉𝑠 𝑛𝑠
𝐹 = 𝐵𝐼𝑙𝑠𝑖𝑛𝜃
𝑃 = 𝑉𝐼
k=2.0x10-7N.A-2
1. A stationary wire carrying an unvarying current in a constant magnetic field will Multiple Choice: 1 mark each
a) Experience no force.
b) Experience a force perpendicular to the direction of the magnetic field.
c) Experience a force parallel to the direction of the current.
d) Experience a force only when the current is turned and off.
2. What is the force per unit length on two straight parallel wires separated by a distance of 12cm and each
carrying a current of 3A in opposite directions?
2 marks
3. A straight 2m wire carries a current of 8A. A second wire of the same length 8cm below and parallel to the
first carries a current of 10A in the same direction. The first wire is fixed and cannot move. The second wire
is attached to 15cm long piece of wood in a horizontal position that is free to pivot. Ignoring the effect of
gravity, what is the initial torque on the piece of wood?
3 marks
4. If the left-hand side of a rectangular loop of wire in an electric motor is experiencing a force upwards, which
direction will the force on the right-hand side of the same loop be?
1 mark
5. Name four parts of a DC electric motor.
2 marks
6. What is responsible for the motor effect?
a) Magnetic induction.
b) Eddy currents due to Lenz’s law produced by the movement of conductor in a magnetic field.
c) Motion of a conductor relative to a magnet or a magnet relative to a conductor.
d) The force acting on a current-carrying conductor in a magnetic field.
7. Which type of electricity can be transformed to change the current and voltage?
a) BC.
b) All types.
c) AC.
d) AC/DC.
8. Current must periodically reverse direction in an electric motor for the motor to rotate. Explain how this is
achieved in a DC motor.
2 marks
9. Name the two main types of magnets the can be used in electric motors and generators.
2 marks
10. In 1820, Oersted and Ampere discovered that an electric current produces a magnetic field.
______________ believed that a magnetic field should also be able to produce an electric current so, in
1831, he attached two wires through a sliding contact to touch a rotating copper disc located between the
poles of a horseshoe magnet. This induced a continuous direct current. This device is considered to be the
first electric generator. Prior to this, continuous electricity could only be produced by batteries or galvanic
cells.
Name the person described above.
1 mark
11. Magnetic field strength is:
a) Defined as the rate of change of magnetic flux, is given the symbol T and represented in diagrams by the
closeness of magnetic field lines.
b) Defined as magnetic flux density, is given the symbol B and represented in diagrams by the closeness of
magnetic field lines.
c) Defined as magnetic flux, is given the symbol Z and represented in diagrams by the direction of magnetic
field lines.
d) Defined as electromagnetic density, is given the symbol T and represented in diagrams by the magnetic
field lines.
12. Magnetic flux is defined as:
a) the time averaged magnetic field strength in a certain area. It is represented diagrammatically by the
thickness of the magnetic field lines.
b) the total amount of magnetic field in the universe. It is represented diagrammatically using dots and
crosses.
c) the average density of magnetic field lines in a certain area. It is represented diagrammatically by the
closeness of magnetic flux lines that pass through area.
d) the amount of magnetic field flowing through a certain area. It is represented diagrammatically by the
total number of magnetic flux lines that pass through area.
13. What is defined as the rate of change of magnetic flux through a circuit?
2 marks
14. Lenz’s law is an example of:
a) Eddy currents.
b) The conservation of energy.
c) The motor effect.
d) Magnetic introduction.
15. What does “A” represent in the equation 𝜏 = 𝑛𝐵𝐼𝐴𝑐𝑜𝑠𝜃?
1 mark
16. Why do some DC electric motors employ a shunt resistor in series during start up?
a) There is no resistance at low speed so a shunt resistor is added to increase resistance.
b) The back emf depends on the total resistance so an extra resistor is added at start up to increase the
amount of back emf.
c) To reduce back emf in order to allow the motor to speed up more quickly.
d) The back emf is low at low speed meaning that the current is high and heat generation due to resistance
is high, which can damage the motor.
17. What is the magnitude and direction of the force on the wire?
1 mark
𝑙 = 1.8𝑚 𝐼 = 3.0𝐴 𝐵 = 0.80𝑇 𝜃 = 47°
18. Which of the following IS NOT a difference between AC and DC generators?
a) DC generators have brushes but AC generators do not have brushes.
b) AC generators have slip rings but DC generators have split rings.
c) The output of AC generators can be transformed but the output of DC generators cannot.
d) AC generators are more commonly used than DC generators.
19. Indicate the direction of the current and label the positive and negative terminals on the following
diagram of an AC generator. 2 marks
20. Which equation describes transmission losses in power lines?
a) P=I2R
b) R=V/I
c) 𝑃𝑆 = 𝜂𝑃𝑃
d)
𝑉𝑝
𝑉𝑠
=
𝑛𝑝
𝑛𝑠
21. What is the relationship between the number of turns in the primary coil and the number of turns
in the secondary coil of a step-up transformer?
1 mark
22. What type of transformer is the current flowing out of the generator fed into in a typical electrical
distribution system?
a) Step-down transformer to decrease the voltage.
b) Step-down transformer to reduce resistive losses.
c) Step-up transformer to increase the power.
d) Step-up transformer to decrease the current.
23. VP=240V, nP=30, nS=120, VS=?
2 marks
24. VP=240V, VS=720V What is the ratio of turns in the primary coil to secondary coil?
1 mark
25. IP=10A, IS=34A, nP=400, nS=100, What is the efficiency of the transformer?
26. What type of transformer is electricity from transmission towers fed into?
2 marks
1 mark
27. How can eddy currents in the iron core of a transformer by reduced?
1 mark
28. What is the main cause of inefficiency in transformers?
29. Indicated the direction of the induced current in each of the following diagrams.
1 mark
4 marks
30. Assess the effects of the development of AC generators on society and the environment. 5 marks
31. Name the individuals who were competing to supply electricity to cities in the USA in the late 19th Century.
1 mark
32. The upper most wires on transmission towers are called shield wires. Shield wires do not normally carry a
current. What is their function?
2 marks