PHYSICS Paper 2 Questions and Answers - KCSE 2022 Past Papers

SECTION A (25 marks)

Answer all the questions in this section in the spaces provided.

1. Figure 1 shows three cardboards A, B and C with holes placed between a source of light and an observer.
   
   Explain what is observed. (2 marks)
2. State how a polythene rod acquires a negative charge when it is rubbed by a piece of cloth. (1 mark)
3. State one device that can be used to detect microwaves. (1 mark)
4. Figure 2 shows an incomplete circuit.
   
   Complete the circuit by inserting a cell so that the current I flows in the direction shown when the switch S is closed. (1 mark)
5. State the basic law of magnetism.(1 mark)
6. Figure 3 shows a vertical object O placed in front of a concave mirror whose principal focus is at F.
   
   Draw a ray diagram to show how the image is formed. (3 marks)
7. State two properties of soft iron that makes it suitable for use as the core of the electromagnet of an electric bell. (2 marks)
8.  
   1. State one reason why sound travels faster at sea level than on high mountains. (1 mark)
   2. State one condition necessary for two progressive waves to form a standing wave. (1 mark)
9. Two students stand 300 m from a wall. One bangs two pieces of wood together and at the same time, the other starts a stop watch. They hear an echo after 1.8 seconds. Determine the speed of sound in air. (3 marks)
10. During an experiment to investigate the relationship between the angle of incidence i, and angle of refraction r for a ray of light travelling from air to glass, the values of sin i and sin r were determined.
    1. On the axes provided, sketch the graph of sin i against sin r for the values that were obtained. (1 mark)
       
    2. State how the refractive index of the glass can be obtained from the graph. (1 mark)
11. Figure 4 shows a circuit consisting of two resistors of 42 and 82, a cell and voltmeters V and V
    
    It is observed that voltmeter V, shows a higher reading than V,. Explain this observation. (2 marks)
12. A heating element is rated 3kW, 240 V. Determine the resistance of the element. (3 marks)
13. State two characteristics of images formed by diverging lenses. (2 marks)

SECTION B (55 marks)

Answer all the questions in this section in the spaces provided.

14. 1. State Lenz's law of electromagnetic induction. (1mark)
    2. Figure 5 shows a magnet held near a stationery solenoid.
       
       State what will be observed on the galvanometer when the:
       1. north pole end is pushed into the solenoid(1 mark)
       2. magnet is held stationary inside the solenoid (1 mark)
       3. north pole end is pulled out of the solenoid (1 mark)
    3. Explain what would be observed if the North pole of the magnet is now moved into the solenoid at a higher speed. (3 marks)
    4. State two causes of energy losses in a transformer. (2 marks)
15. 1. State the function of the ring main circuit in a domestic wiring system.(1 mark)
    2. Figure 6 shows a circuit consisting of switches S,, S., S, and three identical lamps L. L, and L, connected to the mains supply through a fuse.
       
       1. Identify two faults in the circuit. (2 marks)
       2. State the reasons for the answers in 15(b)(i). (2 marks)
       3. Describe how the brightness of lamps L, L, and L, compare when the switches S, and S, are closed. (2 marks)
       4. Explain the answer in 15(b)(iii). (2 marks)
16. 1. Figure 7 shows a circuit consisting of a cell in series with a galvanometer and two metal plates A and B.
       
       1. It is observed that when a beam of UV radiation falls on plate B, the galvanometer deflects. Explain this observation. (3 marks)
       2. Explain what would be observed on the galvanometer when a more intense beam of UV radiation is used. (2 marks)
    2. 1. State with a reason how the intensity of an X-ray beam can be increased in an X-ray tube. (2 marks)
       2. Figure 8 shows the trace of an AC signal on the screen of a Cathode Ray Oscilloscope (CRO).
          
          Given that the time base setting is 8.5 milliseconds per cm. Determine the:
          1. wavelength of the AC signal (1 mark)
          2. frequency of the AC signal (3 marks)
          3. State the functions of the following parts of a Cathode Ray Oscilloscope:
             1. The grid (1 mark)
             2. The filament(1 mark)
17. 1. State one hazard of radioactivity.
    2. Figure 9 shows a decay curve of a radioactive element.
       
       From the graph determine:
       1. the half life of the element (1 mark)
       2. the number of half lives it will have undergone when the count is 12.5 (2 marks)
    3. 1. State the effect of doping on a semiconductor. (1 mark)
       2. Explain how doping produces an n-type semiconductor from a pure semiconductor (3 marks)
       3. Figure 10 shows a circuit consisting of two galvanometers G, and G., two switches S, and S., a cell and two diodes D, and D..
          
          Explain what is observed when S, and S, are closed. (4 marks)
18. 1. Explain the effect on resistance of a diode when the forward bias voltage is increased. (2 marks)
    2. Figure 11 shows a circuit consisting of a 12V battery, 1.5k resistor, a Light Dependent Resistor (LDR) and a lamp of negligible resistance. The circuit can be used as a light detector.
       
       1. Explain what would be observed if the lighting conditions are changed from total darkness to bright light. (3 marks)
       2. Ifi the resistance of the LDR in the bright is 1×10³Ω, determine the potenrial difference acrose 1.5 lΩ resistor  (3marks)
          Determine the total change storry  by the capacitiot  (3mark)

MARKING SCHEME

SECTION A

1. The eye does not ✓ see the light since the holes are not ? in a straight line.  
2. The cloth loses some electrons ✓ to the polythene rod gains electrons from the clothe.
3.  
   • Crystal detectors
   • Aerial receiver
   • Solid state diodes
   • Radio receiver   (any one)
4.  
   
5. Like poles repel while unlike poles attract✓
6.  
   
7.  
   • Gives a stronger magnet (High concentration of magenetic flux) ✓
   • Easily demagnetized/ magnetized ✓
8.   
   1. Air pressure/density is higher ✓ at sea level than on high moutnain.
   2.  
      • The waves should have the same amplitude.
      • The waves should be out of phase
      • Moving in opposite directions   (any one)
9. S = ^d/_t ✓
       = 2 × 300   ✓
             1.8
       = 333.3 m/s ✓
10.  
    1.  
       
    2. By obtaining the gradient of the graph
       n = Δ sin i 
             Δ sin r
11. Same current flows ✓ through the resistors but given that V = IR, the higher the resistance, the higher the voltage.✓
12. P = V²  ✓
            R
       =  240²     ✓
           3000
       = 19.2Ω  ✓
13.  
    • Upright✓
    • Virtual ✓
    • Diminished 
    • Always between lens and mprincipal focus (any two)

SECTION B

14.  
    1. The direction of the induced current is such that is opposes the change producing it. ✓
    2.  
       1. Galvanometer deflects✓ momentarily
       2. There is no deflection ✓
       3. Galvanometer deflects in the opposite direction ✓
    3. More deflection ? due to more induced current ✓ because of the higher rate of change of flux)✓
    4.  
       • Hysteresis
       • Eddy currents✓
       • Heating effect/ copper losses
       • Flux leakage (any two)
15.  
    1. Supplies current ✓ to the sockets.
    2.  
       1.  
          • S₁ is connected ✓ to the neutral
          • S₃ is not necessary✓
          • L₂ and L₃ are in series
       2.  
          • Switch S₁ should be ✓ in live wire (Lamp is still live even with the switch open)
          • S₃ will short ✓ circuit the mains supply.
          • L₂ and L₃ should be connected in parallel.
       3.  
          • L₁ is brighter than ✓ L₂ and L₃
          • L₂ and L₃ have same ✓ brightness
       4. L₁ is on full✓ voltage from the mains while L₂ and L₃ share the voltage from ✓ the mains.
16.  
    1.  
       1.  
          • Uv dislodges electrons from plate B ✓
          • Electrons are attracted to plate A connected✓  to the positive terminal of the cell.
          • This causes the current to flow hence ✓ deflection.
       2. Galvanometer ✓ deflects more since more electrons are dislodged from plate B hence ✓ more current flows.
    2.  
       1. Increasing the ✓ heater current this increases the production ✓ of electrons hence more x-rays
       2.  
          1. λ = 1 × 2 = 2cm 
          2. f = ¹/_T =             1                 ✓
                            (8.5 × 10⁻³ × 2)
               = 58.82Hz✓
       3.  
          1. Grid: 
             • Controls the ✓ number of electrons reaching the screen.
             • Controls brightness of the spot on the screen.
          2. Filament; Heats the ✓ cathode to produce electrons.
17.  
    1.  
       • Burning of skin ✓
       • Causes of cancer  (any one)
    2.  
       1. Half life is 4.5s
       2. 3 half lives (Show working on graph)
    3.  
       1. Improves the conductivity of the semi conductor
       2. Doping an intrinsic ✓ semiconductor with atoms of group V elements, four electrons will bond with the ✓ electrons of intrinsic semiconductor. The remaining ✓ electron is donated for conduction.
       3.  
          • Both G₁ and G₂ ✓ deflect but G₂ has a greater deflection than G₁ ✓
          • D₂ is reverse biased while D₁ is forward ✓ biased hence conducts some of the main current ✓ but G₂ carries the total current.
18.  
    1. Resistance ✓ decreases with increase in the applied voltage because the size of the depletion layer reduces ✓ allowing more current to flow.
    2.  
       1.  
          • Initially the lamp does not light ✓ but when bright light falls on the LDR, the lamp lights. ✓
          • When light falls on LDR, its resistance decreases hence current flows causing ✓ the lamp to light.
       2. R_T = 1000 + 500 = 2500Ω ✓
          I_T = V_T =   12    = 4.8 × 10⁻³A ✓
                 R_T    2500
          V = IR
             = 1500 × 4.8 × 10⁻³
              = 7.2V  ✓
    3. Smoothens the output signal ✓
    4. Q = CV ✓
           =  10 × 2.5  × 12 ✓
               10 + 2.5
           = 24μC ✓