AC induction motors:

(A) have a simple design?

(B) have high efficiency?

(C) have relatively low cost?

(D) all of the above?

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A

Q

In keeping with the format of the HSC, sample marks are given for each question.

Answer the following questions in approximately three lines of writing, calculation or diagram.

16 Two parallel wires AB and CD are carrying currents of 2 A and 4 A respectively. They are separated by 1 cm in air.

70 What is the force per unit length acting between the wires? (2 marks)

71 If the current AB is reversed what happens to the force? (1 mark)

72 This question refers to Figure 2.54.

Figure 2. 54

The diagrams are of a wire carrying a current in a magnetic field. Draw on them the direction of the force acting on the wire. (2 marks)

73 A copper rod of length 20 cm and mass 0.20 kg is suspended by insulating threads from a spring balance so it hangs horizontally in a north-south direction. The rod is placed in a horizontal magnetic field of intensity 0.50 T directed due east as shown in Figure 2.55.

Figure 2. 55

What is the reading on the spring balance when:

74 no current flows in the rod? (1 mark) 75 current of 10 A flows from north to south? (1

mark)

76 current of 10 A flows from south to north? (1 mark)

77 A 0.1 m long conductor XY which is free to move rests on horizontal conducting rods A and B. A vertical uniform magnetic field of intensity 0.5 T acts out of the page as shown in Figure 2.56.

Figure 2. 56

When switch S is closed, determine:

78 the direction of the force on XY. (1 mark) 79 the magnitude of the force on XY. (1 mark) 80 This question refers to Figure 2.57.

Figure 2. 57

A student set up the equipment above. When he closed the switch the coiled spring started to jump up and down. Explain why this occurred.

(3 marks)

81 This question refers to Figure 2.58.

Figure 2. 58

A rectangular coil ABCD of 100 turns lies in a plane parallel to the magnetic induction B of 10 T. A current of 2 A flows from a battery along the path ABCD.

82 What is the direction of the force on the sides AB and CD? (1 mark)

83 What is the size of the force acting on sides AB, BC and CD? (2 marks)

84 If the side BC is 2 cm long and AB is 4 cm, what is the maximum torque acting on the coil? (2 marks)

85 A 240 V transformer may burn out when

connected to a 240 V DC supply. Explain. (Hint:

The resistance of transformer windings is low.) 86 Calculate the power wasted by a transmission

line cable of resistance 0.5 ohm when 10 kW is to be transmitted:

87 at 1000 V. (1 mark) 88 at 200 000 V. (1 mark)

89 Explain how electromagnetic braking works.

(3 marks)

90 What is the basic principle behind the operation of AC induction motors? (3 marks)

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A

Q

In keeping with the format of the HSC sample marks are given for each question.

Answer the following questions in approximately five lines of writing, calculation or diagram.

91 A rectangular coil of wire 6 cm long by 2 cm wide consisting of 100 turns and carrying a current of 5 A is placed in a magnetic field of intensity 0.2 T as shown in Figure 2.59.

Figure 2. 59

92 In what direction will the coil begin to rotate when viewed from A? (1 mark)

93 To ensure the coil rotates continuously, what must be done to the current in the coil? (2 marks)

94 What is the flux through the coil when the coil is:

(i) parallel to the field?

(ii) perpendicular to the field?

95 What is the torque when the coil is:

(i) parallel to the field? (1 mark) (ii) perpendicular to the field? (1 mark) 96 In an experiment designed to measure the

magnetic field in the centre of a solenoid, the apparatus in Figure 2.60 was used.

Figure 2. 60

The loop is balanced when no current flows in the solenoid. When current flows in the solenoid and in the loop circuit, the loop experiences a downward force on the end inserted in the solenoid. To rebalance the loop, a weight of 2.0 x 10-4 N is required.

97 List two changes you could make to the circuit(s) that would change the direction of the force acting on the loop. (2 marks)

98 If the loop is 2 cm long, and a current of 4 A flows through the solenoid when a current of 2 A flows in the loop, determine the magnetic field in the solenoid. (2 marks)

99 In measuring the loop, the end piece only was considered. Why can we neglect the long sides of the loop? (1 mark)

100 Figure 2.61 shows a simple DC motor.

Figure 2. 61

101 Label all parts. (2 marks)

89 Core Topic Two: Motors and Generators 102 Explain the function of a ‘split-ring’ device. (2

marks)

103 List three things that could be changed in the design to improve the function of the motor. (3 marks)

104 Figure 2.62 is a schematic diagram of a moving coil meter.

Figure 2. 62 A moving coil meter

105 Briefly describe how it works. (3 marks) 106 What is the significance of the radial magnetic

field? (1 mark)

107 Figure 2.63 shows a simple generator.

Figure 2. 63

108 Label its parts. (2 marks)

109 Explain why an emf is induced when the coil is rotated. (2 marks)

110 What is the relationship between the plane of the coil and the direction of the magnetic field when the induced emf is a maximum? Explain.

(2 marks)

111 The output is not suitable to charge a car battery. How could the generator be modified to charge the battery? Explain. (1 mark)

112 Transformers are used to change AC voltages.

113 The primary and secondary coils of a transformer have 4000 and 200 coils

respectively. If the primary is connected to 240 V AC, what voltage is across the secondary?

(Assume 100% efficiency). (2 marks)

114 How are energy losses in transformers kept to a minimum? (2 marks)

115 A student sep up an experiment shown schematically in Figure 2.64

Figure 2. 64

116 What is observed in loop B when the circuit is as shown, with a steady resistance? Explain. (2 marks)

117 What happens in loop A, as the resistance is steadily decreased in the variable resistor (rheostat)? Explain. (2 marks)

118 A current balance that can be used to

investigate the relationships between the forces on a current-carrying wire, the currents in the wire, the length of the wire and the separation of the wires is shown in Figure 2.65. Current can be varied in the ‘fixed loop’ and the ‘movable loop’; the separation between the coils can be varied; and the length of the movable loop can also be changed.

Figure 2. 65 A current balance

119 Why does a force act on the loop when a current flows through both loops? (2 marks) 120 If current is arranged to flow from A to B in the

fixed loop, in what direction must the current flow in the movable loop if the pointer is to swing upwards? (1 mark)

121 Briefly explain how the force between the two loops can be measured. (2 marks)

122 Sketch a graph that you might expect showing the relationship between the product of the currents (I₁I₂) and the separation of the loops (d). (2 marks)

123 Briefly discuss the impact on society of electric generators and transformers. (6 marks)

124 You are given the opportunity to purchase a home at what appears to be a bargain price.

The home is within 100 m of high voltage power lines. Would you purchase the home?

(Assuming you had sufficient money.) Explain your reasons for or against the purchase by reference to some research you have

conducted on the health implications of living near power lines.