Electricity P1 Questions and Answers - Wisdom Pre-Mock Form 4 Exams 2021/2022

INSTRUCTIONS TO CANDIDATES

• This paper has 12 printed pages.
• Answer all questions in section A and B.
• Mathematical tables or non-programmable electronic calculators maybe used

QUESTIONS

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

1.                            
   1. Outline the procedure of connecting an ammeter to take a measurement in a circuit. (2 marks)
   2. Figure 1 shows a resistor with color bands.
      
      Figure 1
      Determine its:
      1. Nominal resistance: (1 mark)
      2. Maximum resistance. (2 marks)
2.                              
   1. State two reasons for using a circuit diagram when troubleshooting n electrical equipment. (2 marks)
   2. State four components of a bill of materials in project fabrication. (2 marks)
3.                            
   1. Explain the effect of each of the following in a p-n junction: (2 marks)
      1. Forward bias;
      2. Reverse bias.
   2. State the meaning of each of the following ratings of a light emitting diode: (2 marks)
      1. I_F(max);
      2. V_F(typ);
4. Figure 2 shows a transformer whose primary resistance is 1kΩ.
   
   Figure 2
   Determine the: (5 marks)
   1. Primary current IP;
   2. Voltage across RL.
5. With the aid of labeled diagrams, illustrate the arrangement of magnetic domains in:
   1. Unmagnetised material;
   2. Magnetized material. (3 marks)
6.                                  
   1. Figure 3 shows a voltage divider circuit.
      
      Figure 3
      Determine the value of; (4 marks)
      1. E
      2. R₂;
      3. R₃.
   2. A consumer has the following loads connected to the supply.
      1. Five 60W lights for 4 hours;
      2. One 2kW kettle for ½ hour.
         Calculate the total:
         1. Energy consumed;
         2. Cost of energy used if the rate is 80 cents per unit. (3 marks)
7.                                        
   1. State three safety precautions to be observed by an operator using portable electric drill. (3 marks)
   2. Name four communication service provider companies currently operating in Kenya. (2 marks)
8.                            
   1. Name four types of insulating materials used in electrical circuits. (2 marks)
   2. State three advantages of PVC conduit wiring systems. (3 marks)
9.                      
   1. Calculate the inductance required to cause resonance at 150kHZ when the capacitance is 1.0pF. (2 marks)
   2. In a 240V circuit, the load current is 2.5A. If the power factor is 0.6, calculate: (3 marks)
      1. Apparent power;
      2. True power.
10. Make a free hand isometric drawing of a conduit saddle in the space below. (5 marks)

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

11. Figure 4, in the next page, shows the front elevation, end elevation and an incomplete plan of an object.
    1. Complete the plan.
       
       Figure 4
    2. On the isometric grid provided, draw the isometric projection of the object making X as the lowest point. (13 marks)
       
12. Figure 5 shows waveforms A and B. their vertical and horizontal scales are given.
    
    Figure 5
    1. Name each of the two waveforms. (1 mark)
    2. State the number of cycles displayed in each waveform. (2 marks)
    3. Calculate; (10 marks)
       1. Frequency of waveform A;
       2. Amplitude of each waveform;
       3. RMS voltage of waveform A
13.                              
    1. Draw a labeled diagram of a consumer unit with the following circuits. (11 marks)
       1. Lighting circuit
       2. Water heater circuit;
       3. Bell circuit
       4. Cooker circuit;
       5. Ring circuit.
    2.                                
    3. State the typical fuse ratings for any four of the final circuits in (a). (2 marks)
14. Figure 6 shows an amplifier circuit whose current gain is 50.
    
    Figure 6
    1.                                    
       1. Name the type of transistor.
       2. State the function of the capacitor C.
       3. Name the type of biasing. (3 marks)
    2. Calculate the values of; (10 marks)
       1. Voltage across R1;
       2. Base current IB;
       3. Collector current IC;
       4. Voltage VCE.
15. Figure 7 shows an R-L-C circuit.
    
    Figure 7
    1. Calculate the:
       1. Impedance of the circuit; (7 marks)
       2. Current. (3 marks)
    2. Draw the phasor diagram. (3 marks)

MARKING SCHEME

1.                            
   1. Procedure of connecting an ammeter to take measurements in a circuit
      • Turn - off the power
      • Ammeter should be connected in series with the load current.
      • Observe polarity.
      • Select the range starting from the highest.
   2.                    
      1. Nominal resistance
         Orange Black
         30
         Brown
         x 10¹ = 300
         Nominal = 300Ω
      2. Maximum resistance
         300 ± 5% = 315Ω
2.                          
   1. Circuit diagram
      • Shows connection of every component.
      • Shows values of components.
      • Shows the position of the components.
      • Shows functionality of the circuit.
   2. Bills of materials
      • Materials/parts.
      • Quantity.
      • Size.
      • Estimate costs.
3.                            
   1.                          
      1. Forward bias reduces the PN-junction (depletion layer) and hence the diode conducts
      2. Reverse bias increases the PN-junction (depletion layer) hence the diode does not conduct.
   2.                                    
      1. I_F(max) is the maximum forward current that the diode can pass without burning out.
      2. V_{F (typ)} is the forward voltage across the diode at the typical operating current.
4.                              
   1. I=V/R=100/1000=0.1A
   2.                                          
5.                                       
   
6.                                        
   1.                                  
      1. E=5+(1.2mAx2000)=7.4V
      2. R²=(5-1)/(1.2mA)=3,333.33Ω
      3. R³=1/1.2mA=833.33Ω
   2.                              
      1. Energy consumed
         Lights5x6x4/1000=1.2kwh
         Kettle1x2x0.5=1.0kwh
         Total energy = 2.2kwh
      2. Cost of energy
         = 2.2 x80 = 1.76 sh
7. Safety precautions to be observed
   1.                            
      • Ensure that the equipment is properly earthed.
      • Do not use it in damp areas.
      • Always remove the plug from the socket when the equipment is not in use.
      • When using extensions, ensure the joints are firm and insulated using the electricians insulation tape.
      • Hold it firmly.
      • Avoid loose clothing like ties.
   2. Communication service providers in Kenya
      • Telkom Kenya
      • Safaricom
      • Airtel
      • Yu
        or any other existing ones
8.                            
   1. Insulating materials used in electrical circuits
      • PVC .
      • Porcelain
      • Magnesium oxide
      • Paper
      • Rubber
      • Air
      • Formica
   2. Advantages of PVC
      • Ease of erection.
      • It is cheap.
      • It is resistant to corrosion.
      • It is light.
      • There is no risk to earth leaks.
9.                                    
   1. Inductance required
      L=         1              =                             1                        =1.12579H
          (4x∏x∏xfxfxC)    (4x∏x∏x150000x150000x1.0pF)
   2.                          
      1. Apparent power
         P=IV=2.5x240=600 VA
      2. True power
         = apparent power X power factor
         =600 X 0.6=360W
10.                                                      
    
11.                                          
12.                              
    1. Name of waveforms
       A - sine wave
       B - saw tooth
    2. Number of cycles
       A - 2 cycles (1)
       B - 3 cycles (1)
    3.                      
       1. Frequency of waveform A
          Where T=period
          T= 50µsx4
          =200µs
          f= 1  =             1         =5000Hz=5kHz
              T    (200/1000000)
       2. Amplitude
          A=V_peak=200mVx3=600mV=0.6V
          B=V_peak=0.5V X 2=1V
       3. RMS value of A
          =0.707xV_p=0.707x0.6=0.424V
13.                                                        
    
    Correct drawing (6 marks)
    Each subcircuit correctly labelled (½x6=3 marks)
14.                            
    1. Type of transistor
       NPN (1)
       Function of capacitor C
       To block D.C (1)
       Type of biasing
       Fixed bias (1)
    2.                            
       1. VR=VCC-VBE =60-0.6=5.4V
       2. I_B=   5.4    =0.54A
               10000
       3. I_C=βxIB=54μAx50=2.7mA
       4. R_CI_C= 2.7mAx1 x 103=2.7V
          V_CE=V_CC-V_R2
          = 6-2.7V
          =3.3V
15.                  
    1.                                          
       1. X_L=2∏fL
          =2x∏x50x50x10^-3
          =15.708Ω (2 marks)
          XC=   1    
                (2∏fC)
          =               1            
             (2x∏x50x2x10-6)
          =1591.5494Ω (2 marks)
          
          = 1866.36Ω (3 marks)
       2. I=E/Z=240/1866.36=0.1286A (3 marks)
    2.