Kenya Certificate Of Secondary Education(KCSE 2013) Electricity Paper 1 with Marking Scheme

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

1.     
   1. Outline the procedure of connecting an ammeter to take a measurement in a circuit. (2 marks)
   2. Figure 1 shows a resistor with colour bands.
      
      Determine its:
      1. nominal resistance;
      2. maximum resistance.
2.    
   1. State two reasons for using a circuit diagram when troubleshooting an electrical equipment (2 marks)
   2. State four components of a bill of materials in project fabrication. (2 marks)
3.    
   1. Explain the effect of each of the following in a p-n junction: (2 marks)
      
      1. forward bias;
      2. reverse bias.
   2. State the meaning of each of the following ratings of a light emitting diode: (2 marks)
      
      1. I_(Fmax)
      2. V_(Ftyp)
4. Figure 2 shows a transformer whose primary impedance is 1 kΩ
   
   Determine the:
   
   1. primary current 1
   2. voltage across R.
5. With the aid of labelled diagrams, illustrate the arrangement of magnetic domains in:
   
   1. unmagnetised material;
   2. magnetised material. (3 marks)
6.    
   1. Figure 3 shows a voltage divider circuit.
      
      Determine the value of;
      
      1. E;
      2. R₂;
      3. R₃
         (4 marks)
   2. A consumer has the following loads connected to the supply:
      
      1. five 60 W lights for 4 hours;
      2. one 2 kW kettle for hour.
         Calculate the total:
         1. energy consumed;
         2. cost of energy used if the rate is 80 cents per unit. (3 marks)
7.    
   1. State three safety precautions to be observed by an operator using a portable electric drill. (3 marks)
   2. Name four communication service provider companies currently operating in Kenya. (2 marks)
8.    
   1. Name four types of insulating materials used in electrical circuits. (2 marks)
   2. State three advantages of PVC conduit wiring systems. (3 marks)
9.    
   1. Calculate the inductance required to cause resonance at 150 kHz when the capacitance is 1.0 pF. (2 marks)
   2. In a 240V circuit, the load current is 2.5 A. If the power factor is 0-6, calculate:
      
      1. apparent power;
      2. true power (3 marks)
10. Make a free hand isometric drawing of a conduit saddle. (5 marks)
    

SECTION B (52 marks)
Answer any four questions from this section in the spaces provided.

11. Figure 4, in the next page, shows the front elevation, end elevation and an incomplete plan of an object.
    1. Complete the plan;
    2. On the isometric grid provided, draw the isometric projection of the object making X the lowest point. (13 marks)
       
12. Figure 5 shows waveforms A and B. Their vertical and horizontal scales are given.
    
    
    1. Name each of the two waveforms. (1 mark) (2 marks)
    2. State the number of cycles displayed in each waveform.
    3. Calculate:
       1. frequency of waveform A;
       2. amplitude of each waveform;
       3. RMS voltage of waveform A. (10 marks)
13.    
    1. Draw a labelled diagram of a consumer unit with the following final circuits.
       1. lighting circuit;
       2. water heater circuit;
       3. bell circuit;
       4. cooker circuit;
       5. ring circuit.(11 marks)
    2. State the typical fuse ratings for any four of the final circuits in (a).(2 marks)
14. Figure 6 shows an amplifier circuit whose current gain is 50.
    
    
    1.    
       1. Name the type of transistor;
       2. State the function of the capacitor C.
       3. Name the type of biasing. (3 marks)
          (10 marks)
    2. Calculate the values of: (i) voltage across R,; (ii) base current 15;
       1. collector current I;
       2. voltage V.CE
15. Figure 7 shows an R-L-C circuit.
    
    
    1. Calculate the:
       1. impedence of the circuit; (7 marks)
       2. Current (3 marks)
    2. Draw the phasor diagram. (3 marks)

MARKING SCHEME

SECTION A

1.    
   1. Procedure of connecting an ammeter to take measurements in a circuit
      • Turn - off the power
      • Ammeter should be connected in series with the load current.
      • Observe polarity.
      • Select the range starting from the highest.
   2.    
      1. Nominal resistance
         Orange Black    Brown
            3         0    x   10¹    = 300 Ω
         ` Nominal = 300 Ω (1 mark)
      2. Maximum resistance
         300 + 5% = 315 Ω (2 marks)
2.    
   1. Circuit diagram
      • Shows connection of every component.
      • Shows values of components.
      • Shows the position of the components.
      • Shows functionality of the circuit.
   2. Bills of materials
      (any 2 x 1 = 2 marks)
      • Materials/parts.
      • Quantity.
      • Size.
      • Estimate costs.
3.    
   1.    
      1. Forward bias
         reduces the PN-junction (depletion layer) and hence the diode conducts
      2. Reverse bias
         increases the PN-junction (depletion layer) hence the diode does not conduct  (2 marks)
   2.    
      1. IF(max): is the maximum forward current that the diode can pass without burning out. (1 mark)
      2. VF(typ): is the forward voltage across the diode at the typical operating current. (1 mark)
4.    
   1. I_p =  V  
              R
      = 100 V_rms
           1 kX
      = 0.1 A
   2. N₁I₁ = N₂I₂ (1)
      `1200 x 0.1 = 400 x I₂
      I₂ = 120 = 0.3A
             400
      V₂ = I₂R₂
      = 0.3 x 8000
      = 2,400
      (5 marks)
5.    
   1. 
      
      Drawing 
      Labelling 
      Direction 
   2. 
      
      Drawing 
      Labelling 
      Direction 
6.  
   1.    
      1. E = 5 + ^I x R1h
         = 5 + ^2 # 10^-3 x 2000h
         = 5 + 4
         =9V
      2. R₂ = V₂ = 4V  = 2kX 
                     I     2mA
      3. R₃ = V₃ = 4V  = 0.5k
                 I      2mA
   2.    
      1. Energy consumed
         Lights 5 x 60 x 4 = 1.2 kwh
         Kettle 1 x 2 x 0.5 = 1.0 kwh(4 marks)
         Total energy = 2.2 kwh (1)
      2. Cost of energy
         = 2.2 x 80 = 1.76 sh (1)
7.    
   1. Safety precautions to be observed (3 marks)
      • Ensure that the equipment is properly earthed.
      • Do not use it in damp areas.
      • Always remove the plug from the socket when the equipment is not in use.
      • When using extensions, ensure the joints are firm and insulated using the electricians insulation tape.
      • Hold it firmly.
      • Avoid loose clothing like ties.
        (any 3 x 1 = 3 marks)
   2. Communication service providers in Kenya
      • Telkom Kenya
      • Safaricom
      • Airtel
      • Yu
8.    
   1. Insulating materials used in electrical circuits
      (4 x 21 = 2 marks) or any other existing ones
      • PVC
      • Porcelain
      • Magnesium oxide
      • Paper
      • Rubber
      • Air
      • Formica = 2 marks)
   2. Advantages of PVC
      • Easy of erection.
      • It is cheap.
      • It is resistant to corrosion.
      • It is light.
      • There is no risk to earth leaks.
9.  
   1. Inductance required
      (any 3 x 1 = 3 marks)
      =               1              
       4π²(1.5 x 10⁵)²(10^-12)
      = 1.13 x 10^-3 H
      = 1.13 H
   2.    
      1. Apparent power
         = IV
         = 2.5 x 240
         = 600 VA
      2. True power
         (2 marks)
         = apparent power x power factor
         = 600 x 0.6
         = 360 w
10. 
    
    Faces 
    Holes 
    Projection
    (5 marks)

SECTION B

11.      
    
12.  
    1. Name of waveforms
       1. sine wave
       2. saw tooth
    2. Number of cycles
       1. 2 cycles (1)
       2. 3 cycles (1)
    3.    
       1. Frequency of waveform A
           1 Where T = period
           T
          T = 50n x 4
          = 200 μs (1)
          f = 1  =      1        =  10  ⁶
               T     200 x 10^-6   200
          = 5 kHz (1)
       2. Amplitude
          A = Vpk = 200 mV x 3 (1)
          = 600 mV (1)
          = 0.6 V
          B = V_pk = 0.5 V x 2 (1)
          = 1 V_pk (1)
       3. RMS value of A
          = 0.707 x V_pk (1)
          = 0.707 x 0.6
          = 0.424 V (1)
          (13 marks)
13.    
    1. 
       
       Correct Drawing = 8
       Labelling 6 items = 3
    2.    
       1. Lighting circuit = 5A
       2. Ring circuit = 30 A
       3. Water heater = 20 A
       4. Door bells = 5 A
       5. Cooker unit = 45 A (11 marks)
          (Total = 11 + 2 = 13 marks)
14.    
    1.    
       1. Type of transistor
          NPN (1)
       2. Function of capacitor C
          To block D.C (1)
       3. Type of biasing
          Fixed bias (1)
    2.    
       1. VR₁ = V_CC - V_be (1)
          = 6.0 - 0.6
          = 5.4 V (1)
          (3 marks)
       2. IB = VR₁ =     5.4    
                        100 x 10³
          5.4 x 10⁵
          = 5.4 x 105
          = 54 μA
       3. I_C = βI_B (1)
          = 54 x 10^-6 x 50 (1)
          = 2.7 mA
       4. Voltage VCE
          VR2 = IC x R2 (1)
          = 2.7 mA x 1 x 10³
          = 2.7 V (1)
          V_CE = V_CC - VR₂ (1)
          = 6 - 2.7 V
          = 3.3 V (1)
15.    
    1.  
       1. XL = 2rfL
          = 2r x 50 x 0.05
          = 15.70 Ω (1)
          X_C =    1   
                  2rfc      (1)
          = 2r x 50 x 2 x 10^-6
          = 1592 Ω
          Z = √R₂ + ^X_C - X_Lh₂
          1000² + ^1592 - 15.7h2
          = 1866 Ω (1) (7 marks)
       2. Current = V 
                         Z
          = 240 
            1866
          = 0.12 A
16. 
    
    Axes 
    Labelling
    (3 marks)