Electricity Paper 1 Questions and Answers - Bunamfan Post Mock 2021 Exams

INSTRUCTIONS TO CANDIDATES
Candidates should have the following for this examination

1. Mathematical tables or non-programmable calculators maybe used
2. This paper has TWO sections: A and B
3. Answer ALL the questions in section A and ANY FOUR questions from section B.
4. All dimensions are in millimeters unless stated otherwise.

QUESTIONS

SECTION A (48 MARKS)
Answer all the questions in this section.

1.                      
   1. State three safety precautions to be observed when using electric power tools. (1 ½ marks)
   2. State three requirements for proper storage or electrical measuring instruments. (1 ½ marks)
   3. Arrange the following job titles in ascending order of seniority: craftsman, engineer, artisan and technician. (2 marks)
2.                    
   1. Name two electrical equipment at the consumer’s intake point in a domestic installation. (1 mark)
   2. Explain each of the following electric circuit conditions. (2 marks)
      1. Short circuit
      2. Overload
3. State the effect of each of the following on current in an electric circuit. (2 marks)
   1. Resistance
   2. Inductance
4.                                
   1. Give two reasons why aluminum is preferred to copper for overhead power line cables. (2 marks)
   2. In a 12V dc system a 40W solar panel is exposed to the sun for 6 hours daily. Calculate the number of days it will take to fully charge a 60 Ampere-hour battery. (3 marks)
5.                    
   1. Describe the energy conversion sequence in a hydro-electric power station. (2 marks)
   2. An alloy wire whose diameter is 1.0mm and resistivity is 75µΩm is used to make a 150Ω resistor. Calculate the length of the wire. (4 marks)
6.                          
   1. Name any two metals used to make alloy magnets. (1 mark)
   2. Explain how a permanent magnet can be demagnetized electrically. (2 marks)
7.                                        
   1. Explain why electric power is transmitted at high voltages. (2 marks)
   2. Explain the three functions of a switch gear at a domestic consumer’s intake point. (3 marks)
8.                      
   1. Name one material used to make each of the following parts of electric machines: (2 marks)
      1. Commutator segments
      2. Slip rings
      3. Brushes
      4. Armature core
9. With respect to dc motors, explain the risk involved in each of the following malpractices:
   1. Switching the motor to directly to full supply voltage. (3 marks)
   2. Running series motor without a load
   3. Operating a shunt motor while its field winding is open.
10. A moving coil instrument of internal resistance of 100 requires a current of 0.5mA to give full-scale deflection. If the instrument is modified to a voltmeter. Calculate the value of the multiplier for the instrument to measure up to 100V. (3 marks)
11. The figure 1 below shows an arrangement of capacitors connected to a 10V d.c supply.
    
    Figure 1
    Determine
    1. The combined capacitance (2 marks)
    2. The total charge in the circuit (1 mark)
    3. The total energy stored in the circuit. (2 mark)

SECTION B (52 MARKS)
Answer any four questions from this section.

12.                    
    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; (1 mark)
       2. Rated secondary current; (2 marks)
       3. Primary impedance at the rated load; (3 marks)
       4. Number of turns in secondary winding if the primary winding induces 0.2V per turn. (2 marks)
13.                        
    1. Sketch the voltage-current characteristic curve of a rectifier diode and label:
       1. The axis
       2. V_F
       3. V_R
    2. Figure 2 shows a Zener diode regulator circuit.
       
       Figure 2
       1. Given that the power dissipated by RS=2.125W, calculate the value of
          1. R_S
          2. R_L
          3. Power dissipated by R_L
       2.  If VIN is decreased to 11V, state what changes occurs in. (9 marks)
          1. V_R
          2. I_Z
14.                              
    1. Using a well sketched diagram show the symbols of the following logic gates. (2 marks)
       1. AND gate
       2. NOR gate
    2. Show the truth tables of the gates indicated below. (4 marks)
       1. NAND
       2. OR
    3.                      
       1. State any three considerations one has to make before settling for a certain business idea. (3 marks)
       2. Explain the purpose of each of the following features in the circuit in (b) (i) above.
          1. Choke (2 marks)
          2. Fluorescent powder. (2 marks)
15.                      
    1. Figure 3 shows an ac RC circuit.
       
       Figure 3
       1. Calculate the equivalent capacitance in the circuit. (2 marks)
       2. If at a frequency f, the reactive capacitance of the circuit is 6.0kΩ, calculate the magnitude of the current in the circuit. (5 marks)
    2. Figure 4 shows an incomplete circuit of a full wave rectifier.
       
       Figure 4
       1. Draw in the figure two more diodes to complete the circuit. (2 marks)
       2. Show on the circuit two points across which the rectified output should be obtained. (2 marks)
       3. State with reason the type of transformer shown above. (2 marks)
16.                            
    1. Differentiate between an Ohmic and non-ohmic conductor giving one example in each case. (2 marks)
    2. Figure 5 shows a circuit with resistors and voltmeter connected to a battery.
       
       Figure 5
       1. If each cell has an internal resistance of 0.7Ω, determine the total resistance in the circuit. (3 marks)
       2. What amount of current flows through the 3Ω resistor when the switch is closed? (3 marks)
       3. What is the reading of the voltmeter when the switch S is
          1. Open (1 mark)
          2. Closed (1 mark)
       4. Account for the difference between the answers in (I) and (II) above. (1mark)
    3. State one environmental effect of setting up a diesel run power station. (1 mark)

MARKING SCHEME

1.                    
   1.                                      
      • Never switch on an electric source if a person is leaning on the equipment.
      • Don’t use such tool in damp areas.
      • Ensure the tool is earthed.
      • Remove plug from the socket when equipment is not in use.
      • Ensure all electrical connections are safe. (Any 3x½ = 1 ½ marks)
   2.                          
      • Avoid storage in excessive heat.
      • Avoid dampness.
      • Remove dry cells.
      • Screen from magnetism.
      • Avoid dusty environment. (Any 3x½=1 ½ marks)
   3. Artisan, Craftsman, Technician, Engineer. (4x ½ =2 marks)
2.                                    
   1. File will remove tin coating thus causing copper to dissolve in hot solder. (1 mark)
   2.                        
      1. Short circuit:- load cannot draw current because it is bypassd by a conductor.
      2. Overload:- the current through the load is too large because resistance is too low for the rated load. (2x1 marks)
   3.                      
      1. Resistance:- impedes or reduces current in a cirtuit.
      2. Inductance:- opposes change of current. (2x1 marks)
3.                      
   1.                      
      • Are lighter than copper cables.
      • Are cheaper than copper cables.
      • Are more corrosion resistance than copper. (Any 2x1=2 marks)
   2. Current for solar panel is I = 40W
                                                    12V
      ∴rate of charge = 40 x 6AH/day
                                  12
      No. of days required for full charge is
      60A - h = 60 = 3 days
      40/2 x 6   20
      (4 marks)
4.                            
   1. potential energy →kinetic energy → mechanical energy → electical energy (4x½ =2 marks)
   2. Length of the wire is given.
      
      (5 marks)
5.                      
   1. Iron, Nickel, Cobalt, Aluminium, Copper. (Any 2 x½=1 mark )
   2. Done by placing the magnet inside a solenoid through which current is flowing. With the current still flowing, the magnet is slowly withdrawn from the solenoid. (2x1=2 marks)
6.                  
   1. Electric power is transmitted at high voltage in order to reduce current and therefore reduce cable sizes, power loss and cost. (2 marks)
   2.                        
      • Isolation:- switching off all conductors connecting to the supply.
      • Circuit protection:- automatically disconnects installation when current exceeds normal.
      • Protection against leakage of current to earth. (3 x1=3 marks)
7.                  
   1.                                    
      • Communicator segments:- copper.
      • Slip ring:- brass.
      • Brushes:- carbon or copper.
      • Armature core:- iron lamination. (4 x½=2 marks)
   2.                            
      • Due to low resistance excess current would flow damaging the motor.
      • There would be very low current in field coils causing the armature to race. This can lead to damage due to centrifugal forces.
      • The would be no back e.m.f. causing excess current to flow and possibly damaging the motor. (3x1=3 marks)
8.                        
   1. Find the terminal which shows continuity with the tab.
   2. With ohmmeter lead on collector, touch the negative lead on each of the other terminals. Terminal showing low resistance in the base.
   3. The emitter is the terminal showing open circuit condition with the collector.(3x1=3 marks)
9.                            
   1.                            
      • Increasing the number of its coil.
      • Using stronger magnet.
      • Using weaker hairspring or a wire suspension.
      • Using a long beam of light as a pointer. (3x1=3 marks)
   2.                                       
      1.                                                 
         • Loose control knob.
         • Faulty thermostat element.
         • Incorrect wiring. (2x ½ =1 marks)
      2.                      
         • Check the temperature control knob for tightness.
         • Open the iron box and check the thermostat.
         • Check the wiring for correctness.
         • Logical sequence. (4x ½=2 marks)
10.                          
    1. Detail drawing shows the parts with their sizes, materials, shapes etc.
    2. Exploded drawing shows the sequence in which all the parts are put together in the final assembly. (2x1=2 marks)
                          
       (5 marks)
11.                              
    1. Assuming the motor has been operating at moderate speed, when it is suddently subjected to a heavier load, the first effect is to slow down the motor.
       • Slowing the motor reduces back emf generated because back emf is directly proportional to speed.
       • Reduced back emf 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. (5x1=5 marks)
    2.                        
       1. Transformation ration (1 mark)
          Ep  = 240v = 2
          Eg    120v
       2. Is  = KVA = 1200 = 10A   (1 mark)
                   Es      120
       3. Primary impendence (Zp) = a²ZL (1 mark)
          Secondary impendence (ZL) = 120v = 12Ω  (1 mark)
                                                            10A
          a = z ∴ Zp = Z² x 12 = 48Ω
          (1 mark)
       4. No. of secondary coil turns
           =           Es           =          120         =  60 turns
                Rate per turn      0.2 per turn
          (2 marks)
12.                    
    1. V-1 curve of rectifier diode
       
       (4 marks)
    2.                    
       1. Value of resistor
          
       2.                                        
          • Since VZ remains constant VR decreases
          • IL remains constant
          • Iz decreases (3x1=3 marks)
13.                          
    1. Stroboscopic effect: is a phenomenon whereby reversing discharge of ions and electronics by the lamp concides with the speed of a revolving machine such that the machine appears to be stationary. (2x1=2 marks)
    2.                  
       1. Starter switch operated fluorescent lamp
          
          (7 marks)
       2. Choke: induces a high voltage which enables a discharge to be initiated between the electrodes of the tube when the starter contacts open. It also keeps the discharge and lamp current at a steady value when lamp is in operation.
          Flourescent Powder: converts the ultra violet light emitted by the discharge and melting mercury to drop to a shadowless white or coloured light.
          (2 x 1 ½ marks)
14.                                           
    1.                                                                      
       1. When S is open
          1. 
          2. 
       2.  No current flows because a balanced wheatstone bridge is formed (1 mark)
    2.                          
       1. Equivalent capacitance, C_T
          
          
       2. Current in the circuit, i
          
15.                              
    1.                    
       • Draw for schematic diagram of the circuit.
       • Draw the PCB artwork.
       • Transfer artwork to copper side of the board.
       • Etch the board.
       • Drill holes for the component.
       • Position the components and connectors.
       • Solder the components and connectors.
       • Dress the PCB that is, cut out the tails and close PCB surface. (8x ½ =4 marks)
         
         (8 marks)