KCSE 2014 Electricity Paper 1 Questions with Marking Scheme

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QUESTIONS

SECTION A (48 marks)
Answer all the questions in this section.

1. List the three classes of materials according to their electrical conductivity and for each class give one example.(3 marks)
2. State the meaning of the term career.(1 mark)
1. List four characteristics of an entrepreneur.(2 marks)
2. For each of the following activities, state one safety precaution to be taken when:(2 marks)
  1. removing a 470 uF/240V capacitor from a television set circuit during repair,
  2. handling rotating parts of electrical machines.
With the aid of graphical symbols, describe the difference between a potentiometer and a Rheostat.(5 marks)
The average daily power consumption of a domestic consumer is as follows:
- Lighting 0.5kw for 6 hours
- Water heating 3.0kw for 2 hours
- Cooking 5.0kw for 2 hours
- Other appliances 3.5kw for 3 hours
  1. Determine the daily consumption in kWh.(3 marks)
  2. If the monthly standing charge is Ksh.100.00, calculate the bill for a month of 30 days.(3 marks)
1. State four factors that determine the strength of an electromagnet.(2 marks)
2. Figure 1, shows the direction of force on a current carrying conductor in a magnetic field. Sketch the magnetic flux lines to create the force.(2 marks)
  
  Figure 1
Figure 2
From the circuit shown in figure 2 calculate the values of:(3 marks)
1. E
2. R₂
1. Name four accessories used in domestic lighting installation.(2 marks)
2. State one difference between MIMS and PVC sheathed cables.(2 marks)
1. Name two types of D.C generators.(1 mark)
2. With the aid of a diagram, describe the operation of a simple A.C. generator.(5 marks)
1. Perform each of the following number conversions.(2 marks)
  1. 101101₂ to decimal
  2. 46₁₀ to binary
2. Figure 3, shows a symbol of a logic gate.
  
  Figure 3
  1. Name the gate.(1 mark)
  2. Draw and complete the truth table for output of the logic gate.(2 marks)
3. Figure 4, shows a basic regulator circuit.
  
  Figure 4
  State the effect on voltage across RL when:(2 marks)
  1. Zener diode is shorted.
  2. Zener diode open.
Figure 5, shows orthographic views of a block, drawn in first angle projection.

Figure 5
On the isometric grid paper provided, draw the block in isometric projection with P as the lowest point.(5 marks)

SECTION B (52 marks)

1. Answer any four questions from this section in the spaces provided. With reference to Sinusoidal waveforms, explain each of the following terms:(3 marks)
  1. Amplitude;
  2. Period;
  3. Frequency.
2. A Sinusoidal voltage trace displayed on an oscilloscope has peak to peak voltage of 24 V and a period of 40 ms.
  Draw the waveform and calculate:(10 marks)
  1. Frequency:
  2. Average value;
  3. RMS value.
1. With the aid of a labelled diagram, explain the operation of a split-phase AC motor.(8 marks)
2. An ideal transformer is used to step down 480 V to 240 V, The 240 V output is connected to a 24 KW resistive load. Determine the:(5 marks)
  1. load current;
  2. input current;
  3. transformer ratio.
1. Draw a wiring diagram of a final circuit of one lamp controlled from three different points.(4 marks)
2. With the aid of a labelled diagram, explain the operation of a switch start fluorescent lamp.(9 marks)
1. Describe each of the following processes of making a permanent magnet:(4 marks)
  1. heating;
  2. Stroking.
2. With the aid of a labelled diagram, explain how a relay operates a high current load.(9 marks)
1. Draw and label a schematic symbol for each of the following two input logic gates.(3 marks)
  1. OR
  2. NAND
2. Construct a truth table for a two input NAND gate.(4 marks)
3. An electric kettle suddenly stops heating water yet there is power in the socket outlet. Outline the procedure for identifying the possible faults using an ohmmeter.(6 marks)

MARKING SCHEME

1. Classes of materials
  1. Conductor - silver, copper, gold, aluminium.
  2. Semi-conductor - carbon, silicon, germinium.
  3. Insulators - PVC, porcelain, rubber, mica.
    Classification (3 x ¹/₂ ) = 1 ¹/₂ marks
    Examples (3 x ¹/₂ ) = 1 ¹/₂marks
    3 marks
2. Career - profession or occupation chosen as one's life's work. (1 x 1) = 1 mark
1. Four characteristics.
  - Self confident.
  - Optimistic.
  - Action oriented.
  - Futuristic.
  - Achievement motivation.
  - Respond positively to challenges.
    (Any 4 x ¹/₂ ) = 2 marks
2. 1. Discharge the capacitor by shorting the terminals 1 guard against contact with any part of the body.
  2. Be dressed so that no 1 part of the clothing may accidentally be caught up in the rotating part of the machine.
    2 x 1 = 2 marks
1. Potentiometer.
2. Rheostat
  
  Terminals (6 x ¹/₂) = 3 marks
  Correct diagram (1 x 2) = 2 marks
  5 marks
1. W = Pt
  = (0.5 × 6) + (3 × 2) + (3.5 × 3) + (5 × 2)
  = 29.5 kwh
2. Cost of electricity
  = (29.5kwh × 7.00) × 30
  = 6195
  = 100 + 6195 = 6295
  (6 x 1) = 6 marks
1. - Number of turns.
  - Amount of current.
  - Type of core material.
  - Ratio of length to width of coil.
    (4 x ¹/₂ ) = 2 marks
2. - lines of flux around the conductor.
  - Lines of flux between the magnets.
    (2 x 1) = 2 marks
1. R₂ = 5V - 1V = 4 = 2KΩ
  2mA 2
2. E = 5 + (2mA x 2 k)
  = 5 + 4 = 9 V
  3 marks
1. - Lamp holders.
  - Bulbs/tubes.
  - Switches.
  - CCU.
    (4 x ¹/₂) = 2 marks
2. - MIMS sheathing is made of copper while PVC sheathing is made of PVC.
  - MIMS has a good heat resistance while PVC cannot withstand high temperatures.
    Any 1 x 2 = 2 marks
1. - Separately excited.
  - Shunt wound.
  - Series wound.
  - Compound wound.
    (Any 2 x ¹/₂ ) = 1 mark
2. - A loop of conductor is rotated between poles of a pair of magnets. ¹/₂ The loop is mounted in a such away that it is free to be rotated between the fields of the two magnets.
  - If the loop is rotated and cuts the magnetic flux, an emf is induced ¹/₂ in the loop.
  - The two ends of the loop are connected to two copper rings called 1 slip rings which provide a path for the induced current to the external circuit.
    2 marks
1. _{5 4 3 2 1 0}
  2 2 2 2 2 2
  1 0 1 1 0 1
  (32 x 1) + (8 x 1) + (4 x 1) + (1x 1)
  = 32 + 8 + 4 + 1
  = 40 + 5 = 45₁₀
  
  = 1 0 1 1 1 0₂
2. 1. NOR gate 1
    
    A B Q
    
    0 0 1
    
    0 1 0
    
    1 0 0
    
    1 1 0
3. 1. Zener diode shorted
    - Zero voltage across RL. 5 marks
  2. Zener diode open
    - Increased voltage across RL. 2 marks
1. 1. Amplitude is the maximum value, positive or negative of an alternating quantity.
  2. Period is the time taken by an alternating quantity to complete one cycle.
  3. Frequency is the number of cycles of an alternating quantity completed in a unit time.
    (1 x 3) = 3 marks
2. Waveform
  
  Axes - 1
  Peak values - 1
  Wave form - 1
  Period - 1
  4 marks
  1. Frequency
    f = 1 = ( 1 = 1 = 25Hz 2 marks
    T 40ms 0.04
  2. Average value,
    Vav = 0.637 x Vmax
    = 0.637 x 12 V
    = 7.644 V
    2 marks
  3. Rms value, VRMs = 0.707 x Vmax
    0.707 x 12 = 8.484 V
    2 marks
    Total = 13 marks
1. Correctness = 1 mark
  Components 5 x ¹/₂ = 2 ¹/₂ marks
  3 ¹/₂marks
  - A rotating field is achieved for starting purpose by having the motor with two stator windings in order to produce a phase difference between the currents in the two windings.
  - The method of switching the starting winding is by a centrifugal switch. This allows the starting winding to be switched off before the motor reaches its normal speed.
  - Once the motor reaches its normal speed, it runs with only one winding energized.
    4 marks
2. 1. Load current I₂ = P
    V
    = 24000 = 100A
    240
  2. For an ideal transformer
    Output power = input power = I₂ = power
    voltage
    Input current I₂ =24000 = 50A
    480
  3. Transformer ratio
    N1 = V1 = 240 = 1:2
    N2 V2 480
    13 marks
1. 4 marks
2. Six items labelled @ ¹/₂ 3
  Correctness = 1
  Total = 4 marks
  Operation
  - When the supply is switched on with starter switch closed, current flows through inductor (L) and through lamp electrodes E.
  - This initial currents heats the lamp electrodes in readiness for striking the lamp.
  - The starting switch is now opened making a sudden interruption in the current flowing through the inductor and causing a high voltage to be induced.
  - The voltage starts a discharge between the two lamp electrodes and current rises rapidly due to the inductor.
  - The starter switch is left open while the lamp is alight, the electrodes maintaining their operating temperature as long as they continue to pass the discharge current.
    5 marks
1. 1. Heating.
    - A ferromagnet metal alloy is heated to bright red hot 1 then allowed to cool in a powerful magnetic field.
  2. Stroking
    - This involves stroking a piece of hard steel with one pole of magnet along the length of metal piece. 1 At the end of each stroke the magnet is lifted up and taken through a reasonable curve. The starting end takes the same polarity as the stroking pole of the magnet.
      4 marks
2. 8 parts @ ¹/₂= 4 marks
  Low / high current circuit = 2 marks
  TOTAL = 6 marks
  - When the remote switch is closed. Current flows through the coil making it an electromagnet.
  - The electromagnet attracts the armature which closes the circuit.
  - The load in the controlled circuit then operates and when the switch is opened, the load stops to operate.
    9 marks
1. 1. OR gate
  2. NAND gate
    
    (6 x ¹/₂ = 3 marks)
2. Truth table for NAND gate
  
  A B Q
  
  0 0 1
  
  0 1 1
  
  1 0 1
  
  1 1 0
  
  Inputs (4 x ¹/₂ )= 2
  Output (4 x ¹/₂ ) = 2
  4 marks
3. - Inspect the top plug, cable and kettle for burn-out signs.
  - Open the top plug and check for loose connections.
  - Check for fuse continuity.
  - Check for continuity of cables/conductors.
  - Remove the element and check continuity of the element.
    6 marks