Electricity Paper 1 Questions and Answers - KCSE 2022 Past Papers

QUESTIONS

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

1.  
   1. Name four accessories used in a lighting circuit installation (2 marks)
   2.  
      1. List four types of materials used in electrical conductors. (2 marks)
      2. State two methods of disposing burnt out fluorescent tubes. (1 mark)
2. With the aid of a circuit diagram, show that for two parallel connected resistors R., and R, their total resistance is given by:
   R_T=R1+R2
          R1+R2
   (6 marks)
3. For a purely resistive AC circuit, draw the:
   1.  Circuit diagram   (2 marks)
   2. Phasor diagram  (2 marks)
   3. current voltage waveform (2 marks)
4. Figure 1 shows an electromagnet.
   
   
   1. Show the direction of current. (1 mark)
   2. Complete the direction of the field pattern. (2 marks)
   3. Indicate the polarity of the electromagnet. (1 mark)
5.  
   1. Table 1 shows the comparisons between a moving-coil instrument and a moving-iron instrument.
      Table 1
      

      Instrument	Used for measuring	method of control	Method of damping
      Moving-coil
      Moving-iron

      
      Complete the table. (3 marks)
   2. Outline the steps to be followed by an electrician when writing a business plan for starting a mobile phone repair workshop.
6.  
   1. Explain two types of single phase transformer losses (3 marks)
   2. Draw a labelled diagram of DC series motor. (3 marks)
7. Draw and label diagrams showing the formation of a P N junction when
   1. charges diffuse across the junction (3 marks)
   2. equilibrium state is reached (2 marks)
8. Figure 2 shows the diagram of a filament lamp.
   
   
   1. Name the parts labelled A to D. (2 marks)
   2. State two reasons for using each of the parts labelled B and C. (2 marks)
9. Draw each of the following symbols:
   1. Inductor (1 mark)
   2. Variable capacitor (1 mark)
   3. Electric heater (1 mark)
10.  
    1. Outline four checks to be done when trouble shooting a faulty lighting system. (2 marks)
    2. Figure 3 shows a metal template.
       
       List four tools and equipment used when fabricating the piece. (2 marks)

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

11.  
    1. Name one material used to make each of the following parts of electrical machines:
       1. Commutator segments
       2. Brushes.......
       3.  Slip rings
       4. Armature core
          (2 marks)
    2.  
       1. Figure 4 shows a single phase AC induction motor. Name the parts labelled A to E
          
          A      (1 mark)
          B      (1 mark)
          C      (1 mark)
          D      (1 mark)
          E      (1 mark)
       2. State the function of part labelled B
    3. A240 V/24V ideal transformer supplies a load of 120 W. Calculate:
       1. transformation ratio   (1 mark)
       2. supply current           (3 marks)
12.  
    1. 1. List two circuit conditions which can cause an electric shock. (2 marks)
       2. State the difference between electrical power engineering and communication ungineering. (2 marks)
    2. An electric kettle has a faulty cable. State two possible faults in the cable and for each fault state one remedy. (2 marks)
    3. Outline the procedure of installing socket outlets using a PVC conduit wiring system. (4 marks)
    4. Draw a labelled diagram of a cartridge fuse. (3 marks)
13.  
    1. Draw a sinusoidal AC waveform for two cycles and indicate each of the following:
       1. periodic time
       2. peak value
    2. Figure 5 shows R-C circuit.
       
       Determine:
       1. the branch currents, and I_R and I_C  (3 marks)
       2. supply current I (2 marks)
    3. Figure 6 shows diagrams of two experiments conductal in the laboratory to study characteristics of lines of magnetic flux.
        
       
       
       
       1. Indicate the polarities of the magnets. (2 marks)
       2. Outline three characteristics of the lines of magnetic flux observed in this experiment. (2 marks)
14.  
    1. State three differences between a semiconductor diode and a transistor. (3 marks)
    2. Sketch the IV characteristic curve of's rectifier diode and label:
       1. The axes
       2. V_F
       3.  V_R
    3. Figure 7 shows the combination of AND gate and NOT gate.
       
       
       1. Draw the truth table of the combination.      (4 marks)
       2. State the name of the gate formed after combination.  (1 mark)
       3. Draw the logic gate.     (1 mark)
15. Figure 8 shows three views of an object drawn in first angle projection. Make an isometric drawing of the object taking corner M as its lowest point. ( 13 marks)
    

MARKING SCHEME

1.  
   1.  
      • Distribution boards
      • Switch boxes
      • Fuses
      • Lamp holders
      • Lamps
      • C.C.U
      • Circuit breakers
   2.  
      1. Electrical materials
         • Copper
         • Aluminium
         • Brass
         • Zinc
         • Silver
         • Gold
           Any 4 x 1⁄2=2
      2. Disposing burnt out tubes
         • Crushing and burying deep in soil
         • Recycling for decorative use
         • Recycling and removing reusable parts
           Any 4x 1⁄2=2
2.  
   
   I=I₁+I₂
   But I = V    I₁=V       I₂=V
              R         R₁            R₂ 
   But V is common
   V    I₁=V       I₂=V  ⇒  1   =1   +  1
   R         R₁            R₂      R     R1    R2   
    1  =  1 +    1  = R₁+R₁
    R      R₁   R₂     R₁R₂                        
   
   hence R = R₁R₂
                    R1+R2
3.  
   1. Circuit diagram
      
   2. Phase diagram
      
   3. Current and voltage
      
4. 
   
   Direction of current 1
   Field direction-2
   Polarity-1
5.  
   1.  
      
      

      Instrument	Used for measuring	Method of control	Damping mathod
      Moving-coil	current and voltage D.C only	Hairsprings	Eddy current
      Moving-iron	Measure both d.d and a.c current and voltage	Hairsprings Gravity	Air

   2.  
      • Ideal screening
      • Legal aspects
      • Market assessment
      • Business planning
      • Implementation stage
        Any 4 (4x1⁄2)
6.  
   1. Transformer losses
      • Copper losses
        Losses due to winding resistance i.e. I²R losses
      • Iron losses
        Include Hysteresis and eddy current losses result due to heating of the core
   2.  
      
7.  
   1.   A simple p-type junction
      
   2. p-Ω junction equilibrium state
      
8.  
   1.  
      
   2.  
      1. Tungsten
         High melting point, can be drawn in thin wire
      2. Inert gas
         to prevent oxidation of, slows evaporation, strengthen life of filamen
9.  
   1. Inductor
      
   2. Variable capacitor
      
   3. Electric heater
      
10.  
    1. Faulty lighting system
       • Check for blown fuse   -Polarity test
       • Check for burnt globe  -Earth leakage
       • Check for broken wires -Insulation resistance test
       • Check for switch contacts-Continuity test
         Any (4 x 1⁄2)=2
    2. Tools and equipment
       • Drilling machine
       • Scriber
       • Drill bit 10 mm
       • Engineer's square
       • Hacksaw
       • Set of file
       • Engineer's steel rule
       • Punch
         Any (4x1⁄2)

SECTION B

11.  
    1.  
       1. Commutator segments - Copper
       2.  Brushes Carbon
       3. Slip rings - Brass
       4. Armature core - Iron lamination
    2.  
       1. A - A.C power supply
          B - Centrifugal switch
          C - Starting winding
          D - Running winding
          E - Rotor
       2. B - The centrifugal switch remains closed (1) when the motor starts when motor attains 75% of the speed it opens (1) and motor operates with running winding
    3. V₁=240V   V₂ 24   Output power = 120 W
       1. Transformation ratio
          = 240 = 10:1
              24
       2. Primary current
          V₁ = I₂
          V₂    I₁
          I₂= P = 120 = 5A
               V2    24
          
          
12.  
    1.  
       1. Electric shock
          • Poor insulation
          • Poor earthing
          • Exposed charged capacitors
          • Wires poorly terminated
            Accept any other suitable answer
            Any (2 x 1)=2
       2. Power deals with generation transmission and distribution of electricity while Communication deals with equipment and systems which transmit information from one point to another eg TV, mobile phones, satellites etc
    2.  
       

       Fault	Remedy
       i Blown fuse	Replace
       ii Open cable	Replace/Repair
       iii Loose terminals	Tighten
       iv Wrong connections	Wire corresctly

       
       Any 2 faults with the correct remedy (2×1)=2
    3.  
       • Mark out
       • Fix the socket boxes in their positions
       • Install the plastic conduit
       • Lay the PVC cables
       • Terminate the socket
         Any (4 x 1)=4
    4. Cartridge fuse
       
13.  
    1. i&ii 
       
    2. c=30μF, R=60Ω, V=240V, f=50Hz
       1.  I_R = V =240 = 4A
                   R    60
          IC = V = V = 2πfcV
                 Xc   2πfc 
          = 2π(50)(30×10^-6)(240)=2.262A
       2.  
          
    3.  
       1.   
          
       2. lines of magnetic flu
          • Form closed loops
          • Do not intersect
          • They behave like stretched elastic bands
          • They move from north to south
            Any other suitable answer (4x1⁄2)=2
14.  
    1. Diode and Transistor
       

       Diode	Transistor
       Has one junction	Has two junctions
       Is a rectifying device	It's an amplifying device
       Has two terminals	Has three terminals

    2. V-I curve for rectifle diode 
       
    3.  
       1. Truth table 
          

          Inputs		Outsputs
          A	B	AND'Y'	NAND Y
          0	0	0	1
          1	0	0	1
          0	1	0	1
          1	1	1	0

       2. NAND 
       3.   
          
15.