KCSE 2011 Electricity Paper 1 Questions with Marking Scheme

ELECTRICITY

Electricity Paper 1 (448/1)

SECTION A (52 marks)

1.      
   1. State three safety precautions to prevent accidents from overhead power lines.(3 marks)
   2. Name four areas of specialization in electrical engineering at diploma level.(2 marks)
2.       
   1. List four layout tools used in metal fabrication.(2 marks)
   2. Describe three characteristics of magnetic lines of force.(3 marks)
3.      
   1. Name two types of inductor cores.(1 mark)
   2. State two reasons why silver is not used to manufacture electrical conductors.(2 marks)
4.       
   1. Determine the resistance of each of the following carbon resistors if their colour code are:
      1. Blue Orange Brown Silver
      2. Grey Red Yellow
      3. Green White Black Gold(3 marks)
   2. State two factors that determine the inductance of a coil.(2 mark)
5.       
   1. State two ways of identifying the polarity of an electrolytic capacitor.(2 marks)
   2. An electric shaver supplied from a 12V source takes a current of 800mA. Calculate:
      1. its power rating;
      2. its equivalent circuit resistance.(3 marks)
6.       
   1. With the aid of circuit diagrams distinguish between a centre-tapped isolation transformer and an autotransformer.(3 marks)
   2. Explain how eddy currents are minimized in a transformer.(2 marks)
7.       
   1. List four equipment that belong to the supply authority at the consumer's intake point.(2 marks)
   2. State three reasons why protective switchgear is installed in a domestic installation.(3 marks)
8.      
   1. State three functions of light emitting diodes.(3 marks)
   2. Distinguish between a rectifier diode and a zener diode.(2 marks)
9.      
   1. Name two types of electrical indicating instruments and for each state one method of damping.(3 marks)
   2. List three visual inspections carried out when trouble shooting a faulty circuit in a printed circuit board.(3 marks)
10.       
    1. Name the materials used to make the parts of each of the cells shown in the following table:
       

       CELL	POSITIVE  ELECTRODE	NEGATIVE ELECTRON	ELECTROLYTE
       Lead Acid
       Leclanche

    2. Figure 1 shows a layout of an electrical installation in which lamps L, and L, are controlled by switches S, and S, Draw the wiring diagram of the circuit.(5 marks)
       

SECTION B (48 marks)
Answer any four questions from this section.

11. Figure 2 shows two views of a block drawn in first angle projection. On the grid paper provided, draw an isometric view of the block taking point X as the lowest point. (12 marks)
    
12. Figure 3 shows an R-L circuit.
    
    Calculate:
    1. circuit impedance:
    2. circuit current;
    3. voltage drop across the inductor,
    4. apparent voltage:
    5. true power:
    6. power factor
       (12 marks)
13.      
    1. Name two types of controlling devices used in measuring instruments.(2 marks)
    2. State three advantages of permanent magnet moving coil instrument over moving iron instrument.(3 marks)
    3. A meter movement has resistance of a 292 full scale deflection current of 200mA.
       1. Calculate the value of the resistor to enable it to measure:
          1. voltage upto 10 volts
          2. current upto 10 amperes
       2. Draw the circuit diagram in each case.(7 marks)
14.            
    1. Draw a labelled circuit diagram of a capacitor-start induction motor
    2. With the aid of a labelled circuit diagram, explain the operation of a buzzer.(9 marks)
15. Figure 4 shows a resistive circuit.
    
    Calculate:
    1. the total resistance of the circuit.
    2. the voltage drop across R.(12 marks)

MARKING SCHEME

1.        
   1. Safety precautions
      • Do not climb electric power posts
      • Avoid touching any broken overhead cable
      • Do not climb trees near overhead cables
      • Avoid felling trees near overhead power lines
      • Never erect building below power lines.
        (Any 3 x 1)
   2. Areas of specialisation
      • Electrical (power option)
      • Electronics
      • Telecommunication
      • Instrumentation
        (4 x ¹/₂)
2.        
   1. Tools
      • Hacksaw
      • Scriber
      • Steel rule
      • Try square
      • Tape measure
      • Chisel   -   Dot punch
        (Any 4 x ¹/₂)
   2. Magnetic lines of force
      • Each line forms a closed loop
      • Lines never intersect
      • Stretched elastic cords always trying to shorten themselves
      • Direction of line is that of north-seeking pole
        (Any 3 x 1)
3.       
   1. Inductor cores
      • Air core
      • Ferrite core
      • Iron core
        (Any 2 x ¹/₂)
   2. Silver
      • is very expensive/ rare
      • not mechanically strong
        (2 x 1)
4.       
   1. Resistance
      • 630 Ω ± 10% 
      • 820 kΩ ± 20%
      • 59Ω ± 5%
   2. Inductance of a coil
      • Number of turns in a coil
      • Length of the coil
      • Cross-section area of the coil
      • Relative permeability of the core
        (Any 2 x 1 = 2 marks)
5.            
   1. Determining polarity
      • The terminal at the crimped end of capacitor is the positive.
      • The negative terminal is identified by a broad strip marked (-) on the body and vice versa.
      • The shorter terminal of an unused capacitor is the negative and vice versa.
      • The positive terminal is identified by a red spot.
        (Any 2 x 1)
   2.      
      1. Power rating
         =I x V
         = 12 x 0.8 A = 9.6 W
         (1 ¹/₂ marks)
         =P=  V²
                 R
         =R =  V²
                  P
         =122 = 15Ω
            9.6
         (1 ¹/₂ marks)
6.           
   1. Transformers
      
   2. Eddy currents
      Minimized by -
      • Laminations
      • Insulating material,(2 x 1)
7.           
   1. Equipment
      • Meter
      • Main fuse
      • Sealing chamber
      • Armoured cable
      • Ripple timer
        (Any 4 x ¹/₂)
   2. Protection gear
      • Protects circuit against excess current
      • Protects circuit against earth leakage
      • Enables isolation of the installation from the supply.
        (3 x 1)
8.           
   1. Uses of LED
      • indicators in instrument panels.
      • numerical displays
      • lighting
      • photocopying
   2. Diodes
      • Rectifier diode operates in forward bias.
      • Zener diode operates in the reverse bias.
9.          
   1. 
      

      Indicating instruments	Method of damping
      Permanent magnet moving soil	Eddy current
      Moving iron	Air
      Thermocouple	Eddy current
      Electrostatic	Air
      (Any 2 x 1)	(2 x 1/2) = 1

   2. Visual inspections
      Check for:
      • Broken conductor tracks
      • Metal lying across conductor tracks
      • Components showing sign of damage - colour
      • Dry joints
10.          
    1. Materials
       

       	Lead Acid	Leclanche
       + electrode	lead dioxide	Carbon
       - electrode	lead	Zinc
       Electrolyte	dilute sulphuric acid	Potassium hydroxide

       
       (6 x ¹/₂ = 3)
            
    2. Wiring diagram
       
        
       • Correct cable routing
         Correct circuit
         Correct symbols
11.       
    
12.            
    1.        
       • Z= √x²_L +R₂ = √250,000 + 1,000,000
         = 1118 Ω
    2. Circuit current I_T = ^V_s/_Z
       ¹²⁵/₁₁₁₈
       = 0.1118 A
    3. Voltage drop across:
       Inductor = I_T X_L = 0.1118 x 500 = 55.9 V
    4. Apparent power = Vs x IT
       = 125 x 0.1118
       = 13,975 VA
    5. True power dissipated by resistor
       PT =, (L_T)²R = (0.1118)2 x 1000 = 0.0125 x 1000
       = 12.5 W
    6. Power factor
       PF= PT/PA =12.5W/13.975VA = 0.89
       (2 x 6 = 12 marks)
13.         
    1. Controlling devices
       • Spring control
       • Gravity control
         (2 x 1)
    2. Advantages
       • High sensitivity
       • Uniform scale
       • Well shielded from stray magnetic field
         (3 x 1)
    3.           
       1. Potential drop = 2 x 200 = 0.4V
          Voltage drop across R = 10 -0.4 = 9.6 V
          hence R = 9.6/0.2 = 48Ω
          (4 x 1)
       2. Low resistance shunt S is connected across the milliameter shunt carries the rest of the current i.e 10 -0.2 = 9.8A
          ∴=0.2 x 2=Sx 9.8 = 0.04 Ω
          
          (3 x 1)
14.       
    1. Capacitor - start induction motor
       
       Sketch = 1
       Labelling (4 x ¹/₂) = 2
       3 marks
    2.      
         
       • When push button is closed, the circuit is completed.
       • Current flows through the coils which become an electromagnet , and attract the armature.
       • The armature pulls the spring and disconnects the contact points, breaking the circuit and demagnetizing the coils.
       • The armature - spring assembly completes the circuit again and the process is repeated again, creating the buzzing sound at the contacts for as long as the push button remains pressed.
15.          
    1.         
       1. 50 + (50 + 100) 100    
                  (50 + 100) + 100
          = 50 + ¹⁵⁰⁰⁰/₂₅₀
          =50 + 60Ω = 110Ω
       2. P.d across R4
          IT= E/RT
          = 22/110 = 0.2A
          IT=I2+ I4 where I2 is current through R2 and I4 is current through R3 and R4
          I4 = IT - I2 
          P.d across R2 = 22 - ITR1
          = 22 -(0.2 x 50) = 22 - 10 = 12 V
          =12V
          I2 = 12V/100Ω =0.12A
          I4=  IT - I2 = 0.2 - 0.12 = 0.08 A
          P.d across R4=I4R4 = 0.08 x 100
          = 8V