Physics Paper 3 Questions and Answers - KCSE 2020 past papers

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Instructions to candidates

  • Answer all the questions.
  • You are supposed to spend the first 15 minutes of the 2% hours allowed for this paper reading the whole paper carefully before commencing your work.
  • Marks are given for a clear record of the observations made, their suitability, accuracy and use.
  • Candidates are advised to record their observations as soon as they are made.
  • Non-programmable silent electronic calculators and KNEC mathematical tables may be used.
  • Candidates should answer the questions in English.

Question 1

You are provided with the following:

  • two cells in a cell holder;
  • a switch;
  • a micrometer screw gauge;
  • a nichrome wire mounted on a millimetre scale;
  • a voltmeter;
  • an ammeter;
  • a jockey:
  • connecting wires with crocodile clips.

Proceed as follows:

  1. Using the micrometer screw gauge, measure and record the diameter d of the wire.
    d=........mm
    d=.........m                       (1 mark)
  2. Set up the apparatus as shown in Figure 1.
     vanier callipus
  3. Using the voltmeter, measure the potential difference E across the battery before closing the switch.
    E = ........volts. (1 mark)
  4. Adjust the length L of the wire to 0.1m (10cm). Close the switch, read and record the value of the current I in Table 1.
  5. Repeat (d) for the other values of L given in Table 1. Complete the table. (6 marks)
     Length L (m)  0.1   0.2   0.3   0.4   0.5   0.6   0.7 
     Current I (A)                  
     1/A−1              
  6. On the grid provided; plot the graph of 1/I (y axis) against L. (5 marks)
  7. From the graph, determine the:
    1. gradient S;    (3 marks)
    2. intercept C on the 1/axis.    (1 mark)
  8. Given that:
    1.  
      Formula of gradient S  determine the value of K1.    (2 marks)   

    2. Formula of current determine the value of K2

Question 2

You are provided with the following

  • a metre rule
  • a biconex lens.
  • a source of light (bulb in a bulb holder. cells in a cell holder and a switch).
  • a stand boss and clamp:
  • a lens holder.
  • a screen.
  • a half metre rule:
  • three pieces of plastic pipes A. B and C:
  • a vernier callipers (to be shared):
  • a stopwatch:
  • some plasticine.

Proceed as follows

         PART A

  1. Clamp the bulb holder onto the stand. Arrange the bulb, the lens and the screen along the metre rule as shown in Figure 2.
     Clamp and bulb holder
  2. Adjust the distance of the bulb from the lens to U = 25 cm. Put on the switch and adjust the position of the screen from the lens so that a sharp image of the bulb is observed. Record the distance V between the screen and the lens in Table 2.
  3. Repeat part (b) for the other values of U shown in Table 2. Complete the table.   (5 marks)
              Table  2
     U cm    25   30   35 
     V cm      
     M =
            U
         
     F =    V   
          M + 1 
         
  4. Determine the average value of F.   (2 marks)

    PART B

  5. Using the vernier callipers measure and record the diameters of the three pipes.
    dA, dB and dC
    dA =....cm....m  (1 mark)
    dB =....cm....m   (1 mark)
    dC =....cm....m   (1 mark)
  6. Measure and record the thickness X of the half metre rule. 
    X = .....cm.....m    (1 mark)
  7. Place the pipe marked A on the bench and use the plasticine to stop it from rolling.
    (see Figure 3 (a)).
    Pipe on bench
  8. Place the half metre rule onto the pipe such that it balances horizontally. Ensure that the halt metre rule is perpendicular to the axis of the pipe. (see Figure 3 (b)).
     Meter rule on pipe
  9. Push one end of the balanced half metre rule slightly downwards and release it so that it oscillates up and down. Measure and record in Table 3 the time for five complete oscillations.
  10. Repeat the procedure in (g). (h) and (i) for the other pipes B and C. Complete Table 3. (5 marks)
                                  Table 3
       Pipe A   Pipe B   Pipe C 
     Diameter d (m)       
     Time for five oscillations       
     Periodic time T (s)      
     formula for Z      
  11. Determine the average value of Z.          (2 marks)

Marking scheme

Question 1

You are provided with the following:

  • two cells in a cell holder;
  • a switch;
  • a micrometer screw gauge;
  • a nichrome wire mounted on a millimetre scale;
  • a voltmeter;
  • an ammeter;
  • a jockey:
  • connecting wires with crocodile clips.

Proceed as follows:

  1. Using the micrometer screw gauge, measure and record the diameter d of the wire.
    d=  0.29 ± 0.02mm  2 d.p a must      0.27↔0.31
    d= 2.9 × 10−4                      (1 mark)
    • Conversion of students value correctly.
    • Accept standard form to whatever no. of d.p.
  2. Set up the apparatus as shown in Figure 1.
     vanier callipus
  3. Using the voltmeter, measure the potential difference E across the battery before closing the switch.
    E = 3.1 ± volts. (1 mark)    1 d.p a must
  4. Adjust the length L of the wire to 0.1m (10cm). Close the switch, read and record the value of the current I in Table 1.
  5. Repeat (d) for the other values of L given in Table 1. Complete the table. (6 marks)
     Length L (m)  0.1   0.2   0.3   0.4   0.5   0.6   0.7 
     Current I (A) ± 0.10 
    -2 d.p a must
    -Trend decreasing from the 1st correct value
     0.72   0.53   0.44   0.38   0.33   0.29   0.27 
     1/A−1  -Correct reciprocals exact or to 4 s.f
     -All correctly done award 1 mark  
  6. On the grid provided; plot the graph of 1/I (y axis) against L. (5 marks)
    Graph of y axis against L
  7. From the graph, determine the:
    1. gradient S;   (3 marks)
      •  Δ ✓1
        ΔL✓1    Extracting correctly i.e correct interval 
        • Accept transfer of error corrected power of 10
        • No line mk no slope mk.
      • Accept correct interval and size. It can be deduced from the graph.
      • Correct evaluation exact or to 4 s.f (1mk)              (A−1 M−1)
      • Deny evaluation mark if the unit is wrong: Deny ½ mk for missing units
    2. intercept C on the 1/axis.    (1 mark)
      • Candidates value of C taking into account candidates error transfer either positive or negative with correct units award 1 mark.
      • deny ½ mark for missing unit
      • C- value should be within the grid.
      • Wrong units correct evaluation ½ mark
  8. Given that:
    1.  
      Formula of gradient S  determine the value of K1.    (2 marks)   
      • Correct substitution S,d and E (any) - Award 1mk
      • Correct evaluation exact or 4 s.f -Award 1mk
      • Ignore units

    2. Formula of current determine the value of K2   (1 mark)
      • Correct substitution of C and E  award ½mk
      • Correct evaluation exact or 4 s.f  award ½mk
      • Ignore units

Question 2

You are provided with the following

  • a metre rule
  • a biconex lens.
  • a source of light (bulb in a bulb holder. cells in a cell holder and a switch).
  • a stand boss and clamp:
  • a lens holder.
  • a screen.
  • a half metre rule:
  • three pieces of plastic pipes A. B and C:
  • a vernier callipers (to be shared):
  • a stopwatch:
  • some plasticine.

Proceed as follows

         PART A

  1. Clamp the bulb holder onto the stand. Arrange the bulb, the lens and the screen along the metre rule as shown in Figure 2.
     Clamp and bulb holder
  2. Adjust the distance of the bulb from the lens to U = 25 cm. Put on the switch and adjust the position of the screen from the lens so that a sharp image of the bulb is observed. Record the distance V between the screen and the lens in Table 2.
  3. Repeat part (b) for the other values of U shown in Table 2. Complete the table.   (5 marks)
              Table  2
     U cm    25   30   35 
     V cm 1d.p
    -± 10cm @ 1 mk
    -Values with decreasing trend
      45.0   35.0   30.0 
     M = V 
            U

    -Each value correctly evaluated. Exact or to 4 s.f
    -All correct award 1 mk.

     F =    V   
          M + 1 
    -correct evaluation exact or to 4 s.f
    -All correct award 1 mk.
  4. Determine the average value of F.   (2 marks)
    • Statement of principle of averaging students value shown
                           Averaging principle formula
    • Correct evaluation exact or to 4 s.f award 1 mk.
    • Ignore units

      PART B

  5. Using the vernier callipers measure and record the diameters of the three pipes.
    dA, dB and dC
    dA = 3.00-3.70cm 2d.p ✓1    correct coversion m  ½✓
    dB =3.60-4.90cm  2d.p  ✓1   correct conversion m  ½✓
    dC =5.10-5.80cm  2d.p ✓1    correct conversion  m  ½✓
  6. Measure and record the thickness X of the half metre rule. 
    X = 0.50-0.65cm  2d.p  ✓1    correct conversion   m ½✓   (1 mark)
  7. Place the pipe marked A on the bench and use the plasticine to stop it from rolling.
    (see Figure 3 (a)).
    Pipe on bench
  8. Place the half metre rule onto the pipe such that it balances horizontally. Ensure that the halt metre rule is perpendicular to the axis of the pipe. (see Figure 3 (b)).
     Meter rule on pipe
  9. Push one end of the balanced half metre rule slightly downwards and release it so that it oscillates up and down. Measure and record in Table 3 the time for five complete oscillations.
  10. Repeat the procedure in (g). (h) and (i) for the other pipes B and C. Complete Table 3. (5 marks)
                                  Table 3
       Pipe A   Pipe B   Pipe C 
     Diameter d (m)       
     Time for five oscillations ±1 to 2 d.p 1mk@   12.60  10.60  9.80
     Periodic time T (s) -correct evaluation exact or to 4 s.f
    -All correct  1 mk.  
     formula for Z -Each value correctly evaluated ½ mark to a max of 1 mark
    -4 s.f or exact
    -value of x must be in metres   
  11. Determine the average value of Z.          (2 marks)
    • Statement of principle of averaging of students value shown ✓1
           formula of principle averaging
    • correct evaluation exact or to 4 s.f (ignore unit) ✓1
  • If work is cancelled and replaced, mark the replaced one.
  • If work is cancelled and not replaced, mark the cancelled one.

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