KCSE 2012 Electricity Paper 1 with Marking Scheme

SECTION A (48 marks)

Answer all the questions in this section.

1.  
   1. State four categories of institutions that train electrical technicians in Kenya,  (2 marks)
   2. List four key components of a business plan.  (2 marks)
2.  
   1. State how each of the following electrical waste materials should be disposed: 
      1. lead acid battery;
      2. fluorescent tube.      (1 mark)
   2. State where each of the following type of fire extinguisher is suitably applied:
      1. foam; 
      2. water;
      3. dry powder.    (3 marks)
3. A one-watt resistor has the colour code; blue, grey and brown. Determine:
   1. the value of the resistor.
   2. the maximum value of the current that can flow through it without exceeding its power rating,
      (5 marks)
4.  
   1. State Lenz's law of electromagnetic induction.  (1 mark)
   2. State two characteristics of magnetic lines of force.  (2 marks)
5.  
   1. Explain the meaning of "sensitivity" as used in meter movement.   (1 mark)
   2. With the aid of a circuit diagram, show how the linearity of a meter is determined. (4 marks)
6.  
   1. With the aid of a diagram, describe "armature reaction in a de generator. (4 marks)
   2. Outline two methods of reducing armature reaction.  (2 marks)
7.  
   1. Distinguish between intrinsic and extrinsic semi-conductors and give one example of Cach.  
      (3 marks)
   2. List four uses of an ohmmeter in trouble shooting electric circuits.   (2 marks)
8.  
   1. Name four conductor materials used in electric circuits.  (2 marks)
   2. State two advantages of MIMS over PVC cables.  (2 marks)
9. Figure 1 shows a series-parallel circuit connected across a 240V supply.
   
   Calculate the: 
   1. total circuit current.   (2 marks)
   2. voltage drop across;
      1. R₃
      2. R₄                 (4 marks)
10.  
    1. Name four marking out tools used in metal fabrication.   (2 marks)
    2. Figure 2 shows the orthographic views of a bracket drawn in first angle projection
       
       Taking N as the lowest point, make a free hand isometric sketch of the bracket.     (4 marks)

SECTION B (52 marks)

Answer any four questions from this section, in the spaces provided after question

11.  
    1. Convert:
       1. 41₁₀ to binary;
       2. 1101101₂ to decimal.     (4 marks)
    2. Sketch the symbol for each of the following logic gates:
       1. AND
       2. OR
       3. NAND    (3 marks)
    3. Draw a truth table for each of the following logic gates:
       1. NAND;
       2. NOR       (6 marks)
12.  
    1. State three advantages of torroidal type transformer over shell type transformer.    (3 marks)
    2.  
       1. Outline three assumptions made in order to consider a transformer as an ideal machine.
          (3 marks)
       2. A 5000/500V, 10KVA ideal single-phase transformer has 40 turns on the secondary. Calculate:
          1. primary turns; 
          2. primary full load current; 
          3. Secondary full load current.     (7 marks)
13.  
    1. State the phase relationship between current and voltage in circuits that are purely:
       1. resistive;
       2. inductive.       (2 marks)
    2. Figure 3 shows an RLC circuit.
       
       Calculate the:
       1. inductive reactance;
       2. capacitive reactance;
       3. circuit impedance;
       4. circuit current;
       5. power dissipated in the circuit.       (11 marks)
14.  
    1. State:
       1. two IEE requirements regarding bell transformers;
       2. two advantages of MCB over cartridge fuses.      (4 marks)
    2. Outline the procedure of carrying out an insulation resistance test on a new domestic insulation.
       (9 marks)
15.  
    1. With the aid of a diagram, explain how the right hand grip rule is used to determine the direction of the magnetic field around a current carrying conductor.     (3 marks)
    2. With the aid of a labelled diagram, explain the principle of operation of a trembler bell.  (10 marks)

MARKING SCHEME

1.  
   1. Institutions that train electrical technicians in Kenya: 
      • University colleges 
      • Institutes of technology 
      • Technical training institutes 
      • Vocational Training centres 
      • National polytechnics
        Any 4 x ½  = 2 marks
   2. Components of a business plan:
      1. Business description
      2. Organisation/ management plan 
      3. Marketing plan 
      4. Production/ operation plan 
      5. Financial plan
         Any 4 x ½ = 2 marks
2.  
   1. Disposal of electrical waste materials 
      • Lead acid battery - return to the manufacturer.
      • Fluorescent tube - break and bury.
        2 x  ½ = 1 mark
   2. Use of extinguishers
      • Water to put out fires on burning solid materials
      • Foam - to put out fires on burning oils and chemicals
      • Dry powder - used to deal with fires on burning flammable liquids and some solids as wood and paper.
3.  
   1. Value of resistor = 680 Ω (1 mark) ± 20% (1 mark)
   2. Maximum current I = √^P/_R            1 mark
                                P = 1 W
                                I  =  √¹/_680.68     ½mark
                                   = √¹/₅₄₄           ½mark
                                   = 42.87 Amps    1 mark
                                                           5 marks
4.  
   1. Lenz's law of electromagnetic induction states that the direction of an induced emf is always such that it tends to set up a current opposing the motion or change of flux responsible for inducing that emf.
   2. Characteristics of magnetic lines of force.
      • They have a direction from north to south poles.
      • They form complete loops.
      • They do not cross cach other.
        Any 2 x 1 = 2 marks
5.  
   1. Sensitivity is the amount of current ½ required to provide full sclae ½ deflection of the pointer.
      (1 mark)
   2.  
      
6.  
   1. Armature reaction:  This refers to the distortion of the main magnetic field of a d.c generator by the magnetic field created by a generated current around the conductor.
      
      OR
      Armature reaction changes the neutral plane of the main field such that it becomes irregular. This causes arcing between the brushes and the commutator and also lowers generator output. 
      (4 marks)
   2. Methods of reducing Reaction:.
      • Adjusting the brushes to the new neutral plane.
      • Use of interpoles between main field poles to cancel its effect. U
      • se of compensating windings in series with the armature to counter its effects.
        Any 2 x 1 = 2 marks
7.  
   1. Intrinsic semiconductor is made of semiconductor material in its pure form. Extrinsic semiconductor is intrinsic semiconductor to which some suitable impurity or duping agent has been added in small amounts.    (3 marks)
      Examples are:
      Intrinsic -
      Extrinsic -
   2. Uses of ohmeter in trouble shooting
      • To check short circuits
      • To test open circuits
      • To ascertain polarity of diodes and transistors
      • To measure values of resistors.
        4 x ½ = 2 marks
8.  
   1. Conductor materials used in electric circuits
      • Copper
      • Aluminium
      • Silver
      • Brass
      • Gold
      • Steel
      • Mercury
        Any 4 x ½ = 2 marks
   2. Advantages of MIMS over PVC cables
      • They require no further protection/ mechanically stronger
      • They are impervious to oil
      • They last longer
      • They have better heat resistance
        Any 2 x 1 = 2 marks
9.  
   1.  
      1. ∴ current through C₁R₃ = I_t −I₄
         = 1.8 - 12 = 0.6 A ✓½
         ∴ p.d across R₃ = I₃ x R₃
         = 0.6 X 50
         = 30 v. ✓1
         AWARD FULL MAKES FOR CORRECT ALTERNATIVE METHOD.
      2. I_t = V
               R_t
         R_t = R₁ + (R₂+R₃)//R₄ ✓½
         R₂ + R₃ = 50+ 50 = 100Ω
         100Ω// R₄ = 100 x 50 = 5000 = 33.3Ω ✓½
                            100 + 50     150
         R_t = R₁ +33.3Ω
            = 100+ 33.3 = 133.3Ω
         I_t = V =  240 
               R_t    133.3
            = 1.8 A
   2.  
      1. I_t = I₃ +I₄
         V₄=240 - I_tR₁ = 240 - 180 = 60 V
         ∴ p.d across R4 = 60 V
      2. P.d across R₄ = P.d across R₂ +R₃
         Current through R₄ = V₄ = 60 V
                                         R₄    50Ω
         I₄ = 1.2 A
10.  
    1. Marking out tools
       • Scriber
       • Try square
       • Centre punch
       • Steel rule
       • Calliper
         Any 4 x ½ = 2 marks
         
11.  
    1.  
       1. 41 ten to binary
          Divide 241
          2 20 - 1
          2 10-0
          2 5-0
          22-1
            1-0
          Correct method = 1
          Correct answer = 1
          = 101001₂                         2 marks
       2. 1101101₂ to decimal
          2⁶ 2⁵ 2⁴ 2³2²2¹2⁰
          1  1   0  1  1  0  1
          64 +32 +0+8+4 +0+1
          = 109₁₀
          Correct method = 1
          Correct answer = 1                 2 marks
    2. A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output.
       
    3. NAND gate 
       

       Input 1	Input 2	Output
       0     0     1     1	0      1      0      1	1     1     1     0

       Input  2 x ½ = 1 mark
       Output  4 x ½ = 2 mark
       NOR gate
       

       Input 1	Input 2	Output
       0     0     1     1	0      1      0      1	1     0     0     0

       Input  2 x ½ = 1 mark
       Output  4 x ½ = 2 mark
12.  
    1. Advantages 
       • Size is smaller for a given kilo-volt - ampere rating
       • The core is more rigid 
       • They are cheaper 
       • Lower in iron losses at higher densities
         Any 3 x 1 = 3 marks
    2.  
       1. Assumptions
          • No core losses 
          • Windings have negligible resistance 
          • All the flux produced links the primary and the secondary 
          • Negligible emf is required to set up the flux as the core permeability is very high
            Any 3 x 1 = 3 marks
       2.  
          1. Ideal transformer has no losses
             E₁ = V₁ = 5000V ✓½
             E₂ = V₂ = 500V ✓½
             Turns Ratio = E₁ = N₁ = 5000 = 10
                                  E₂    N₂     500
             ∴ N₁ =10  ⇒  N₁ =10x N₂ 
                N₂
             N₁ = 10 x 40
             = 400 turns
          2. I₁V₁ = VA (input)
             ∴ Primary full load current (1)
               = kVA = 10 x 10³
                    V₁      5000
                = 2A
          3. I₂V₂ = VA(output)
              ∴ Secondary full-load current (I₂)
             = kVA = 10x10³
                  V₂     500
             = 20 A                                        TOTAL = 7 marks
13.  
    1.  
       1. Current and voltage are in phase.  (1 mark)
       2. Current lags voltage.   (1 mark)
    2.  
       1. X_L = 2πfL
               = 2π x 50 x0.4
          = 125.6Ω 
       2. X_C = 1   
                 2πfc
                      1                
          2 x 3.14 x 50 x 50 x 10⁻⁶
          = 63.7Ω
       3. Z(impedance) = √(R²+(X_L - X_C)²)
                               =√(50²+ (125.7-63.7)²)
                               = 79.6Ω
       4. Current I = ^V/_Z 
                         = 240 
                            79.6
                         = 3A
       5. Power (P) = I²R
                         = 3² x 50
                         = 450 W
                                        TOTAL = 11 marks
14.  
    1.  
       1. IEE requirements
          • Must be double - wound
          • Should be earthed at one point of secondary winding iron-core of transformer and metal casing
          • Should have a separate control switch and connected on its own final circuit
          • Should have a high grade insulation of supply cable to transformer
            Any 2 x 1 = 2 marks
       2. Advantages of MCB
          • Easy to reset therefore replacement not necessary
          • Gives better overall protection against fire and shock
          • Cannot be replaced by an inexperienced person
          • Highly discriminative
          • Sustained overload and reject harmless transient overcurrent.
            Any 2 x 1 = 2 marks
    2. Insulation resistance test
       • Set the ohmeter/megger to a suitable range. Ensure the following:
         • the main supply is disconnected
         • all switches are on ON position
         • all MCB are on ON position
         • all loads e.g. bulbs are in position or join conductors where there's no load
       • Connect the ohmeter/megger between live and neutral terminals with earth terminal
       • Carryout the resistance test.
       • Get the required readings.
       • Repeat the procedure by taking measurements between the L and N conductors
       • The reading should not be below/ mega ohm.
         9x1 = 9 marks
15.  
    1.  
       
       When conductor is gripped with right hand:
       Thumb points in direction of current
       Fingers point in direction of magnetic fields      (3 marks)
    2.  
       
       • When the push button is pressed, current flow through the circuit.
       • The coils become energized and attracts the armature and the striker hits the gong. This movement of the armature away from the contact - screw breaks the circuit.
       • The coils are denergized and the armature falls back to its original position and its circuit is completed once again.
       • The sequence of movements recurs causing a continuous ringing/trembling sound.