ELECTRODYNAMICS: ELECTRICAL MACHINES QUESTIONS AND ANSWERS

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Activity 1
A simplified sketch of a generator is shown below.
25

  1. Is the output voltage AC or DC? Give a reason for your answer. (2)
  2. What type of energy conversion takes place in the above generator? (2)
  3. State TWO effects on the output voltage if the coil is made to turn faster. (2)
  4. What is the position of the coil relative to the magnetic field when the output voltage is a maximum? (1)
    [7]

Solutions

  1. AC — The generator has slip rings. (2)
  2. Mechanical energy is converted to electrical energy. (2)
  3. Output voltage increases and the number of cycles per second increases. (2)
  4. The coil position is parallel to the magnetic field.(1)
    [7] 


Activity 2
Lights in most households are connected in parallel, as shown in the simplified circuit below. Two light bulbs rated at 100 W; 220 V and 60 W; 220 V respectively are connected to an AC source of rms value 220 V. The fuse in the circuit can allow a maximum current of 10 A.
26

  1. Calculate the peak voltage of the source. (3)
  2. Calculate the resistance of the 100 W light bulb when operating at optimal conditions. (3)
  3. An electric iron, with a power rating of 2 200 W, is now connected across points a and b. Explain, with the aid of a calculation, why this is not advisable. (5)
    [11]

Solutions

  1. Vmax 
       √2
    220 = Vmax ∴ Vmax = 311,1 V (3)
               √2
  2. P = V2rms
            R
    100 = 220 2 ∴ R = 484 Ω (3)
                R
  3. Pave = VrmsIrms    2 200 = (220)Irms
    Irms = 2200 
               220
    Irms = 10A
    The iron draws 10 A of current. Together with the lights the total current will exceed 10 A causing the fuse to blow.(5)
    [11] 


Activity 3
The essential components of a simplified DC motor are shown in the diagram below.
27
When the motor is functioning, the coil rotates in a clockwise direction as shown.

  1. Write down the function of each of the following components:
    1. Split-ring commutator (1)
    2. Brushes (1)
  2. What is the direction of the conventional current in the part of the coil labeled AB? Write down only FROM A TO B or FROM B TO A. (1)
  3. Will the coil experience a maximum or minimum turning effect (torque) if the coil is in the position as shown in the diagram above? (1)
  4. State ONE way in which this turning effect (torque) can be increased. (1)
  5. Alternating current (AC) is used for the long-distance transmission of electricity.
    Give a reason why AC is preferred over DC for long-distance transmission of electricity. (2)
  6. An electrical appliance with a power rating of 2 000 W is connected to a 230 V rms household mains supply.
    Calculate the:
    1. Peak (maximum) voltage (3)
    2. rms current passing through the appliance (3)
      [13]

Solutions

    1. Reverses current direction in the coil every half cycle. (1)
    2. Connects external circuit 3 to split ring commutator. (1)
  2. B to A (1)
  3. Maximum (1)
  4. Increase current strength / Increase number of coils / Use stronger magnets.(any one) (1)
  5. AC can be stepped up to high voltages and low current.
    Less energy loss with low current (W = I2RΔt).(2)
    1. Vrms = Vmax ∴ 230 = Vmax ∴ Vmax = 325,27 V (3)
                    √2                 √2
    2. Pave = VrmsIrms
      2 000 = (230)Irms
      Irms = 2000 / 230
      Irms = 8,695 A  (3)
      [13] 


Activity 4
Electric motors are important components of many modern electrical appliances. AC motors are used in washing machines and vacuum cleaners, and DC motors are used in toys and some tools.

  1. What energy conversion takes place in electric motors? (2)
  2. What is the essential difference in the design between DC motors and AC motors? (4)
  3. List THREE ways in which the efficiency of the motor can be improved. (3)
  4. Consider the diagram. The conventional current direction is indicated by the arrows.
    1. In which direction (clockwise or anti-clockwise) will the coiled armature rotate if the switch is closed? (1)
    2. Why does the armature continue moving in the same direction once it has reached the vertical position? (2)
      [12]
      28

Solutions

  1. Electric energy converted to mechanical energy. (2)
  2. A DC motor reverses current direction with the aid of the commutator whenever the coil is in the vertical position to ensure continuous rotation.
    An AC motor, with alternating current as input, works without commutators since the current alternates/slip rings can be used. (4)
  3. Increase the number of turns on each coil; increased current; stronger magnets. (3)
    1. Anticlockwise (1)
    2. The armature’s own momentum/ the split ring commutator changes direction of current, every time the coil reaches the vertical position.(2)
      [12] 


Activity 5
In the circuit the AC source delivers alternating voltages at audio frequency to the speaker.

  1. What is the peak voltage that the source can deliver? (3)
  2. Calculate the average power delivered to the speaker. (7)
    [10]
    29

Solutions

  1. Vrms = Vmax/√2
    ∴ Vmax =15/√2 = 21,21 V (3)
  2. Rtotal = 8,2 + 10,4 = 18,6 Ω 
    I = V/R = 15 / 18,6 = 0,81 A 
    P = I2R = (0,81)2(10,4) = 6,76 W  (7)
    [10] 
Last modified on Wednesday, 22 September 2021 11:45
Published in Physical Sciences Paper 1 Grade 12 Study Guide and Notes

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