Electricity Paper 1 Questions and Answers - MECS Cluster Joint Pre Mock Exams 2021/2022

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ELECTRICITY
PAPER 1

INSTRUCTIONS TO CANDIDATES

Candidates should have the following for this examination

  • Answer booklet
  • Drawing instruments
  • Mathematical tables
  • Drawing paper size A3
  • This paper has TWO sections: A and B
  • Answer ALL the questions in section A and any FOUR questions from section B.
  • All dimensions are in millimeters unless otherwise stated.

Section A

  1.        
    1. List three classes of materials according to their electrical conductivity and for each give two examples. (3mks)
    2. State the meaning of the term career 1mks
    3. For each of the following state one safety precaution to be taken: 2mks
      1. Removing a 470µf/240v capacitor from TV circuit during repair
      2. Handling arc welding machine
  2.         
    1. Briefly explain why tungsten is used in making filament lamps. 2mark
    2. With a well labelled diagram show the difference between emitter follower amplifier and common emitter amplifier. 4mks
    3. The βdc of a transistor is 99
      Calculate;
      1. The αdc of the transistor 2mks
      2. The collector current when the transistor is connected in common emitter mode with a dc base current of 50µA. 2mks
  3. Draw a ring final diagram with four socket outlets and a spur. 5mks
  4. Study the figure below and use it to answer the questions below.
    MCEElecp1q4
    Find; 
    1. total capacitance in the circuit 1mks.
    2. Total quantity of charge 1mks
    3. Voltage across the 4µf capacitor. 2mks
  5. A 100kVA, 11kV/240V single phase transformer has 800 turns on the primary side.
    Calculate the:
    1. Primary current
    2. Number of secondary turns 4mks.
  6. Draw a well labelled diagram of a capacitor-start split phase motor 5mks
  7. Explain the reason for carrying out the following tests in a newly completed installation. 3mks
    1. Polarity test
    2. Insulation resistance test
    3. Ring continuity test
  8. The daily power consumption of a domestic consumer is as follows;
    1. Lighting 0.5kw for 6hrs
    2. Water heating 2kw for 2hrs
    3. Other appliances 3.5kw for 3hrs
      1. Determine the daily consumption in kWh. 3mks
      2. The monthly standing charge is Ksh. 100.00.calculate the bill for the of month of June if the charges are 16shs per unit. 3mks
  9. A moving coil instrument of internal resistance 100Ω requires a current of 0.5 MA to give a full scale deflection .If the instrument is modified to a voltmeter.
    1. Sketch the circuit of the modified instrument. 1mks
    2. Calculate the value of the instrument to measure up to 100 volts.2mks
  10. The figure below shows an arrangement of resistors in an electric circuit .If terminals x and Y are shorted.
    MECCompp1q10
    1. Sketch equivalent circuit.1mks
    2. Calculate total resistance of the circuit.1mks

Section B
Answer any four questions in this section

  1.          
    1. state the conditions necessary for resonance to occur. 2mks
    2. State relationship between current and voltage in the following circuits 2mks
      1. capacitive circuits
      2. Inductive circuits
    3. The figure below shows an RLC circuit. Calculate;
      MECCompp1q11c
      1. Current flowing through the circuit (2marks)
      2. Phase angle (2marks)
      3. Draw the phasor diagram representing the circuit currents.(3marks)
      4. The resonant frequency.(2marks)
  2.      
    1. Describe each of the following processes of making a permanent magnet. (4marks)
      1. Heating
      2. Stroking
    2. With an aid of a well labelled diagram ,explain how a relay operates in high current load.(9marks)
  3.        
    1. With the aid of a labelled diagram, explain the operation of a split-phase AC motor. (8marks)
    2. An ideal transformer is used to step down 480V to 240V.The 240 V output is connected to a 24KW resistive load.
      Determine the: (5marks)
      1. Load current
      2. Input current
      3. Transformer ratio
  4. In the amplifier circuit of figure 4, the voltages are measured with respect to ground and indicated currents due to the d.c voltages. Assume the Q1 is silicon transistor.
    MECCompp1q14
    1. State the functions of ;
      1. RB , RC, and RE 1 1/2mks
      2. C1,C2 and C3 1 1/2mks
    2. Calculate the value of ;
      1. RB and RE if the base current is 20 micro ampere.(3marks)
      2. Current gain HFE of Q1. 1mks
      3. VCE if the transistor is operating in cut off mode (2marks)
    3. What type of transistor is Q1?(1Mark)
  5.          
    1. Draw and label schematic symbol for each of the following two input logic gates. (3marks)
        1. OR
        2. NAND
    2. Construct two input table for the two input logic gates. (4marks)
    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. (6marks)

MARKING SCHEME

  1.            
    1.          
      1. conductors- silver, copper, gold, aluminum 1mks
      2. semiconductors- silicon, germanium 1mks
      3. insulators- plastic, wood, paper, glass 1mks

    2. career- it is an occupation that one engages in to earn a living. 1mks
    3.       
      1. capacitors should always be discharged by shorting the terminals for small capacitors and using a bleeder resistor for large capacitors. 1mks
      2.      
        1. avoid viewing ultraviolet lamps directly
        2. proper welding shield should be used to protect the eyes and the face
        3. prevent other people from looking the arc
  2.       
    1. tungsten has a high melting point of 3400°C and can be drawn easily in to wires.
    2.  
      MCEElecp1qa2b 2 
    3.        
      1.    α= β/β+1 
        ∴ α = 99/99+1
        α = 0.99 

      2.   β = Ic/IB
        99=Ic/50MA
        Ic=49.5mA
  3.      
    MCEElecp1qa3
  4.        
    1. Total cpacitance 
      6+4=10MF 
      1/10 + 1/2 = 1+5/10 = 6/10
      10/6 = 1.67 MF
    2. Total quantity of charge
      Q=CT
      QT= 1.67MF x 10
      QT=16.7MC
    3. Voltage across AMF capacitor
      V2MF=Q/C1
      Q=16.7MC
      V2M1=16.7/2
      V2M1=8.35
      10 -8.35 = 1.65V
      VAMF=1.67
  5.          
    1.   Primary current
      I=9.09
    2. Number of secondary turns
      Vp = Np
      Vs    Ms
      11000 = 800
        240       NS
      24 x 800 = NS
        1100    
      Ns=17.45
      Ns=18 turns
  6.         
    MCEElecp1qa6
  7.          
    1. polarity test-to ensure that all single pole switches, circuit breakers, fuses and thermostats are connected to the live conductors only.
    2. Insulation resistance-it’s done to ensure that no possible earth leakage currents between conductors or conductors with earth mass.
    3. Ring continuity test- it verifies the continuity of all conductors in ring circuit.
  8.          
    1.    Lighting 0.5 x 6 = 3
      Water heater 2 x 2 = 4
      Appliances 3.5 + 3 = 10.5
      total= 17.5kwh
    2. Standing charge =100
      1 unit = 16
      17.5= ?
      17.5 x 16 = 280
      280 + 100= 380Sh
  9.                  
    1.      
      MCEElecp1qa9
    2.  100v
      Rs = V max - Vm
                IFSD
      Rs= 100 - (0.5 x 10‾³ x 100)
                    0.5 x 10-3
      Rs=     99.95     
               0.0005
      Rs = 199.9
  10.                   
    1.      
      MCEElecp1qa10i
    2. Total
      R₁ x R₂ = 100 x 50 = 33.3Ω             
      R₁ + R₂        150
         R₃Rm     = 200 x 150 = 87.71Ω
       R₃ + Rm           350 
  11.                         
    1.  inductive and capacitive reactance must be equal
      • Z=0
      • in capacitive circuits currents leads voltage by 90° while in inductive ccts voltage leads current.
        IL           240           = 0.95A
                2TT x 50 x 0.8
        Ic =            240              = 7.5 A
                2TT x 50 x 100
               
        IR = 240/25 = 9.6A
        IT = √9.62 + (7.54 - 0.95)2
        IT= √135.59
        IT = 11.64A
  12.              
    1.                          
      1. heating-a ferromagnetic material is heated to red hot and the allowed to cool in a powerful magnetic field. 2mks
      2. a piece of hard steel is stroke with one pole of a magnet. Stroking is done along the length of the metal piece. At the end the magnet is lifted and taken through a complete curve before returning to the starting point. 2mks.
    2.             
      MCEElecp1qa12b
      MCEElecp1qa12bii
  13.            
    1. it consists of two sets of windings; starting windings and running windings. The starting windings have small diameters hence higher resistance and lower reactance. The running windings have larger diameters hence higher reactance and lower resistance. The two windings are connected in parallel to each other. When power is supplied the two windings causes a phase difference causing the rotor to rotate. The centrifugal switch disconnects the starting winding when the motor attains 75% of its running speed.
    2.        
          

      1. MCEElecp1qa13b
        load current 
        1/2mk P=VI 
        I= 24000/240
        I=100A
      2. Input current
          Is   Vp 
          Ip        Vs
          100   =   480   
            Ip        240
        Ip= 240 x 100
                    480
        Ip= 50A
      3. Transformer ratio
        480/240 = 2:1
  14.              
    1.          
      1. rb- regulates base current
        Rc- regulates collector current
        Re- stabilizes the amplifiers (limits leakage current)
      2. c1-blocks dc and allows ac to the amplifier
        c2 –ensures no ac flows through Re to maintain stability emitter de coupling
        c3- blocks dc and allows ac to flow to the output.
  15.        
    1.        
      1. or gate
        MCEElecp1qa15
      2. NAND gate
        MCEElecp1qa15b
    2. Truth table for NAND gate
      A B Q
      0
      0
      1
      1
       0
      1
      0
      1
       1
      1
      1
      0
    3.  Inspect the top plug, cable and kettle for burn-out signs. 1
      • Open the top plug 2
      • 1 and check for loose connections.
      • Check for fuse continuity. 1
      • Check for continuity of cables/conductors. 1
      • Remove the element and check continuity 1 of the element.

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