Electricity Paper 1 Questions and Answers - KCSE 2022 Prediction

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INSTRUCTIONS TO CANDIDATES

The paper contains two sections: Section A and Section B
Section A carries 52 marks while section B carries 48 marks
Answer ALL the questions in section A and ANY FOUR questions in section B
All the answers and working must be written on the question paper in the spaces provided below each question.
Non-programmable silent electronic calculators are allowed for use

SECTION A (52 Marks)
Answer all questions

1. Name four prime movers used in the process of generation electricity in Kenya (2 marks)
2. State two responsibilities of the following job groups (2 marks)
  1. Artisan
  2. Technician
1. State four main components of a business plan (2 marks)
2. State four causes of electric shock in a workshop (2 marks)
1. State two safety precaution that needs to be put in place to avoid accidents from overhead power lines (2 marks)
2. Draw a common base transistor configuration circuit (1 mark)
The figure 1 shows a logic gate
1. Give the name of the logic gate (1 mark)
2. Draw the truth table for the logic gate shown (2 marks)
1. Differentiate between a conductor and a semi-conductor. (1 mark)
2. State one area of application for each of the above materials (2 marks)
State four uses of ohmmeter function of a multimeter in trouble shooting circuits (2 marks)
1. Using a schematic diagram, show how a lamp can be controlled from two different position (2 marks)
2. Find the resistance of a resistor whose color code is: (2 marks)
  1. Grey, Red, Gold, Gold
  2. Green, White, Black, Gold
With aid of a diagram, differentiate between repulsion and attraction type moving iron meter (4 marks)
A single phase step-down transformer has a ratio of 8:1, a primary voltage of 3.3kV and a load of 6.6kVA. Ignoring losses, calculate; (6 marks)
1. Secondary voltage
2. Secondary current
Sketch the symbol for the following circuit symbol (2 marks)
1. Zener diode
2. Incandescent lamp
1. State the meaning of capacitance (1 mark)
2. State three factors that affect the capacitance of a capacitor (3 marks)
1. What is an autotransformer (1 mark)
2. An autotransformer has 1200 turns connected to 250V a,c to supply a 60V load. Calculate the number of turns in the secondary section (2 marks)
1. Three inductors of 60mH, 120mH and 75mH respectively, are connected together in a parallel combination. Calculate the total inductance of the parallel combination (1 mark)
2. Find the total energy in the circuit given that the total current is 2.5A. (2 marks)
1. State three parts of a machine (marks)
2. An alternating current completes ten cycles in 0.1 seconds. Calculate the period and frequency of the current. ()
In a 12V dc system a 40W solar panel is exposed to the sun for six hours daily. Calculate the number of days it will take to fully charge a 60 ampere-hour battery (4 marks)

SECTION B (48 Marks)
Answer any four questions

Figure 2 shows and RLC circuit. (12 marks)

Calculate the:
1. Inductive reactance
2. Capacitive reactance
3. Circuit impedance
4. Circuit current
5. Power dissipated in the circuit
1. With aid of a labelled diagram, explain the principle of operation of a trembler bell. (9 marks)
2. With aid of a diagram, differentiate between shunt generator and series d.c generator (3 marks)
Figure 3 shows the isometric view of a towing hook.
Draw, full size, in third angle projection: (12 Marks)
1. front elevation in the direction of arrow A;
2. end elevation in the direction of arrow B
1. With reference to Sinusoidal waveforms, explain each of the following terms (3 marks)
  1. Amplitude
  2. Period
  3. Frequency
2. A Sinusoidal voltage trace displayed on an oscilloscope has peak voltage of 48V and a period of 80 ms. (9 marks)
  Draw the waveform and calculate:
  1. Frequency
  2. Average value
  3. RMS value
1. Explain why a series motor develops high torque when subjected to a heavier load(5 marks)
2. A 240V/120V, 1.2kVA transformer delivers power to a load. Calculate the:
  1. Transformation ratio
  2. Rated secondary current
  3. Primary impedance at the rated load
  4. Number of turns in secondary winding if the primary winding induces 0.2V per turn (7 marks)

Marking Scheme

1. Name four prime movers used in the process of generation electricity in Kenya(2 marks)
  - Wind turbine
  - Gas turbine
  - Hydro-turbine
  - Steam turbine
    4x½=2 marks
2. State two responsibilities of the following job groups (2 marks)
  - Artisan
    - Perform skilled electrical work
    - Plant maintenance
    - Repair of electrical equipment
    - Control and modifying circuit
      Any 2x½=1 marks
  - Technician
    - To organize and manage workshop accordingly
    - To participate in the decision making process at the middle level management
    - To supervise, organize and instruct the work under them
      Any 2x½=1 marks
1. State four main components of a business plan (2 marks)
  - Self confident
  - Optimistic
  - Action oriented
  - Futuristic
  - Achievement motivation
  - Respond positively to challenges
    4x½=2 marks
2. State four causes of electric shock in a workshop (2 marks)
  - Carelessness
  - Poor electric circuit condition
  - Fatigue
  - Working under the influence of impairing substance such as alcohol
  - Incompetence in handling electric circuits
    Any 4x½=2 marks
1. State four safety precaution that needs to be put in place to avoid accidents from overhead power lines (2 marks)
  - Never attempt to climb electric power posts
  - Avoid climbing trees near overhead power lines
  - Do no throw objects at the power lines.
  - Do not erect buildings below power lines
  - Do not set fire near power line posts
    Any 4x½=2 marks
2. Draw a common base transistor configuration circuit (1 mark)
  
  1x1=1 mark
Figure 1 shows a logic gate
1. Give the name of the logic gate (1 mark)
  - NAND gate
2. Draw the truth table for the logic gate shown (2 marks)
  
  INPUT OUTPUT
  
  A B Z
  
  0 0 1
  
  0 1 1
  
  1 0 1
  
  1 1 0
  
  Correct input binary 1 mark
  Output binary 1 mark
  Total = 2 marks
1. Differentiate between a conductor and a semi-conductor. (1 mark)
  - A conductor is a material that has many free electrons and allows electric current to flow through them.
  - A semiconductor is a material that is a poor conductor at low temperature and good conductors at high temperatures.
    2 x ½=1 mark
2. State one area of application for each of the above materials (2 marks)
  - Conductors- power transmission and distribution, electronic devices such as radios, domestic installation e.t.c
  - Semiconductors- manufacture of diodes and integrated circuits, solar technology e.t.c
    1 x 2=2 marks
State four uses of the ohmmeter function of a multimeter in trouble shooting circuits (2 marks)
- Testing for open circuits
- Ascertaining polarities of diodes and transistors
- Checking short circuits
- Checking continuity of heating elements and motor winding.
- Identifying defective components
  Any 4 x ½=2 marks
1. Using a wiring diagram, show how a lamp can be controlled from two different position (2 marks)
  
  switch connected to live=1 mark
  L2 to L2 and L1 to L1 connection=1 mark
  Total= 2marks
2. Find the resistance of a resistor whose color code is;
  1. Grey, Red, Gold, Gold
    82 x 0.1 ± 5% = 8.2 ± 5%Ω
    1 mark
  2. Green, White, Black, Gold
    59 x 10º ± 5% = 59 ± 5%Ω
    1 mark
    Total (2 marks)
With aid of a diagram, differentiate between repulsion and attraction type moving iron meter (4 marks)

Repulsion type moving iron instrument consists of two vanes whereby one vane in stationary the other is fastened to the pivot which carries the pointer and the control spring.

The meter uses soft iron plunger projecting partway into the stationary field coil. The current in the field coil produces a magnetic force which pulls the plunger further into the coil.
A single phase step-down transformer has a ratio of 8:1, a primary voltage of 3.3kV and a load of 6.6kVA. Ignoring losses, calculate; (6 marks)
1. Secondary voltage
  voltage ratio = Vp/Vs = 81
  Vs = Vp/8 = ³³⁰⁰/₈= 412.5V
2. Secondary current
  Is = VpIps = ^{6.6 x 1000}/_412.5 = 16A
Sketch the symbol for the following circuit symbol (2 marks)
1. Zener diode
2. Incandescent lamp
1. State the meaning of the term capacitance (1 mark)
  - This the property of a circuit which opposes any changes in circuit voltage
2. State three factors that affect the capacitance of a capacitor (3 marks)
  - Area of the capacitor plates
  - Distance between the plates
  - The material of the dielectric
    3x1=3 marks
1. What is an autotransformer (1 mark)
  - It is a type of transformer that has its secondary winding tapped from the primary winding
  - Its primary and secondary winding is not electrically insulated from each
    Any 1x1=1 mark
2. An autotransformer has 1200 turns connected to 240V a,c to supply a 60V load. Calculate the number of turns in the secondary section (2 marks)
  - ^Vp/_Vs=^Np/_Ns
    Ns=^NpNs/_Vp
    Ns=^{1200 x 60}/₂₄₀=300 turns
1. Three inductors of 60mH, 120mH and 75mH respectively, are connected together in a parallel combination. Calculate the total inductance of the parallel combination (1 mark)
  - 1/L_T= ¹/₆₀ + ¹/₁₂₀ + ¹/₇₅ = ²³/₆₀₀
    LT=⁶⁰⁰/₂₃
    =26.09mH
2. Find the total energy in the circuit given that the total current is 2.5A. (2 marks)
  - E=¹/₂LI²=¹/₂ x 26.09 x 10^-3 x 2.5²
    =0.0815 joules
1. State three parts of a machine (112 marks)
  - Stator
  - Rotor
  - Commutator
  - Frame
  - End Shield
    3 x ¹/₂=1¹/₂ marks
2. An alternating current completes ten cycles in 0.1 seconds. Calculate the period and frequency of the current. (1¹/₂ marks)
  10 cycles=0.1 sec
  1 sec
  period = ^{1 x 0.1}/₁₀=0.01sec
  Frequency=¹/_T=¹/_0.01=100 H
In a 12V dc system, a 40W solar panel is exposed to the sun for six hours daily. Calculate the number of days it will take to fully charge a 60 ampere-hour battery (4 marks)
Current for the solar panel, I= 40/12=3.3333A
∴rate of charge=3.3333 x 6=20Ah
No. of days required for full charge is;
^60Ah/_20Ah=3 days
The figure below shows and RLC circuit.
Calculate the:
1. Inductive reactance
  - XL=2πfL
    =2 x π x 50 x 0.4
    =125.6Ω
2. Capacitive reactance
  - XC=¹/_2πfC
    = 1
    2 x π x 50 x 50 x 10^-6
    =63.7Ω
3. Circuit impedance
  - Z= √R²+(X_L-X_C)²
  - =502+(125.7-63.7)²
    =79.6Ω
4. Circuit current
  - I=^V/_Z
    =²⁴⁰/_79.6
    =3A
5. Power dissipated in the circuit
  - Power=I²R
    =3²x 50
    =450W
    (12 marks)
1. With aid of a labelled diagram, explain the principle of operation of a trembler bell. (9 marks)
  - When the push button is pressed, current flows through the circuit.
  - The coil becomes energized and attracts the armature which is attached to the striker hence strikes the gong. The armature moves away from the contact screw breaking the contact.
  - The coils are de-energized and the armature falls back to its original position and its circuit is completed once again.
  - The sequence of movement recurs causing a continuous trembling sound.
2. With aid of a diagram, differentiate between shunt generator and series d.c generator (3 marks)
  - DC Shunt generator
    - DC shunt field winding consists of high resistance coils which are made of a large number of turns of insulated copper wire.
    - The field winding is connected in parallel (shunt) with the armature. (any 1x1=1 mark)
  - DC Series generator
    - It has its field winding connected in series with the armature winding.
    - The windings are made of few turns of copper wire.
    - It does not build up the field if it is started with the external circuit open. (any 1x1=1 mark)
The figure below shows the isometric view of a towing hook.
Draw, full size, in third angle projection:
1. front elevation in the direction of arrow A;
2. end elevation in the direction of arrow B (12 marks)
  
  Hidden details=1
  First angle projection= 1
  Neatness=1
  Total =12 Marks
1. With reference to Sinusoidal waveforms, explain each of the following terms (3 marks)
  1. Amplitude
    - Is the maximum value, positive or negative of an alternating quantity
  2. Period
    - Is the time taken by an alternating quantity to complete one cycle
  3. Frequency
    - Is the number of cycles of an alternating quantity completed in a circuit
2. A Sinusoidal voltage trace displayed on an oscilloscope has peak voltage of 48V and a period of 80 ms. (9 marks)
  Draw the waveform and calculate:
  
  Axes - 1
  Peak values - 1
  Wave Form - 1
  Period - 1
  4 marks
  1. Frequency
    - f=¹/_T= 1 =12.5 Hz
      80x10^-3
  2. Average value
    - V_av=0.637xV_max
      =0.637x48
      =30.576V
  3. RMS value
    - V_rms=0.707xV_max
      =0.707x48
      =33.94V
1. Explain why a series motor develops high torque when subjected to a heavier load (5 marks)
  - Assuming the motor has been operating at moderate speed, when it is suddenly subjected to heavier load, the first effect is to slow down the motor.
  - Slowing the motor reduces back e.m.f generated since back e.m.f is directly proportional to speed.
  - Reduced back e.m.f makes the line voltage to push more current through the motors field coils
  - More current in the field coils produces a stronger magnetic field
  - Because torque depends on the product of both magnetic field and flux, both will therefore increase and so will the torque.
    (Any 5x1 =5 marks)
2. A 240V/120V, 1.2kVA transformer delivers power to a load. Calculate the: (7 marks)
  1. Transformation ratio
    - Transformer ratio=^Vp/_Vs=^240V/_120V=2
  2. Rated secondary current
    - I_S=^KVA/_Es=¹²⁰⁰/₁₂₀=10A
  3. Primary impedance at the rated load
    - Primary impedance=a²Z_L
      Secondary impedance=¹²⁰/₁₀₌12Ω
      a=z ∴ Z_P=2²x12=48Ω
  4. Number of turns in secondary winding if the primary winding induces 0.2V per turn
    - =^Es/_{Rate per turn}= ¹²⁰/_0.2 = 600 turns