Physics P2 Questions and Answers - Form 3 Term 3 Opener Exams 2022

INSTRUCTIONS TO CANDIDATES

1. Write your name and Index No. in the spaces provided above.
2. Sign and write the date of the examination in the space provided above
3. This paper consists of Two sections; A and B
4. Answer ALL the questions in Section A and B in the spaces provided.
5. All working MUST be Clearly shown
6. Non-programmable silent electronic calculators and KNEC Mathematical tables may be used for calculations

QUESTIONS

SECTION A (25 MKS)

1. Figure 1 shows a pencil lying in front of a plane mirror. The pencil is moved 2cm towards the mirror in the same orientation.
   
   Figure 1
   Determine the distance between the new position of the tip of the pencil and its image. (2mks)
2.                      
   1. State the basic law of magnetism. (1mks)
   2. Figure 2 shows two bar magnets, one whose poles are labelled and a second one whose poles are labelled X and Y. Iron nails are attracted to the lower ends of the magnets as shown. (1mk)
      
      Figure 2
      Identify pole X ……………………………………………….……………………………………
3. State the reason why convex mirror is preferred over a plane mirror for use as a driving mirrors in cars. (1mk)
4. Figure 3 shows the displacement-time graph for a certain wave.
   
   Figure 3
   1. Determine the frequency of the wave. (2mks)
   2. On the same diagram, draw a wave with half the amplitude and twice the frequency of the one shown. (1mk)
5.                  
   1. State the main difference between primary chemical cells and secondary chemical cells. (1mk)
   2. State how the design of a dry Lechlanche cell reduces polarization. (1mk)
6. Figure 4 shows a wave incident on a narrow opening.
   
   Figure 4
   Draw the appearance of the wave after passing through the opening. (1mk)
7. A student stands between two classroom walls and claps. After 0.6 seconds, she hears the first echo and hears the second echo after 0.8 seconds. Determine the distance from the student to the further wall. Take speed of sound in air = 320m/s. (3mks)
8. In the fig. 4 shown below (not drawn on scale) sketch the path of a ray till it emerges from the prism.(1mk)
   
9. Describe the changes that can be observed during discharging process of lead-acid accumulator. (2mks)
10. Figure 5 shows the cross-section of two bar magnets and a current carrying conductor held between them. The direction of current is into the paper.
    
    Figure 5
    1. indicate with an arrow the direction of force experienced by the conductor. (1mk)
    2. State one way in which the force on the conductor above can be reduced. (1mk)
11. Distinguish between transverse and longitudinal waves. (2mks)
12.              
    1. State Ohm’s law. (1mk)
    2. Figure 6 shows an electrical circuit.
       
       Figure 6
       Determine the Ammeter reading in a closed circuit. (3mks)

SECTION B (55 MARKS)

13.                            
    1. Figure 7 below shows a narrow beam of white light onto a glass prism.
       
       Figure 8
       1. What is the name of the phenomenon represented in the diagram? (1mk)
       2. Name the colour at X and Y. Give a reason. (3mks)
       3. What is the purpose of the slit? (1mk)
    2. Figure 8 below shows the path of ray of yellow light through a glass prism.
       The speed of yellow light in the prism is 1.8 x 10⁸m/s.
       
       Figure 8
       1. Determine the refractive index of the prism material (speed of light in vacuum C = 3.0 x 10⁸m/s). (3mks)
       2. Show on the same diagram, the critical angle, c, and hence determine its value. (3mks)
       3. Given that r = 31.2°, determine the angle . (3mks)
14.                              
    1. The figure below shows resistors in a circuit. The internal resistance of the battery is negligible.
       
       
       1. Calculate the effective resistance of the circuit. (3 marks)
       2. Find the total current in the circuit. (3 marks)
       3. P.d between X and Y (2 marks)
    2. Define the term “e.m.f” of a cell. (1 mark)
    3. Why is repulsion the surest way for polarity of a magnet. (1 mk)
15. Some plain water waves were produced in a ripple tank. They pass from a region of deep water into aregion of shallow water. The figure shows what the waves look like from above
    
    
    1. State what happens at the boundary to:
       1. The frequency of the waves (1 mark)
       2. The speed of the waves (1 mark)
       3. The wave length of the waves (1 mark)
    2. The waves have a speed of 0.12m/s [in the deep water. Wave crests are 0.08m apart to the deep water. Calculate the frequency of the sources producing the waves. (3 marks)
    3. State two differences between a stationary wave and a progressive wave. (2 marks)
    4. The wave shown in the figure below has a velocity of 200ms^-1
       Determine:
       
       
       1. The period T of the wave. (2mks)
       2. The frequency of the wave. (2mks)
       3. The wavelength of the wave. (2mks)
    5. State two difference between electromagnetic waves and mechanical waves. (2 mks)
16.                        
    1. State two factors that affect the strength of an electromagnet. (2 mks)
    2. Figure below shows suspended metre rule in equilibrium balanced by the magnet and weight shown. The iron core is fixed to the bench.
       1. State and explain the effect on the metre rule when the switch S is closed. (3 marks)
       2. State and explain the effect on the metre rule when the terminals of battery are reversed. (2 marks)
       3. Suggest how J on the set up can be varied to have metre rule tilt anticlockwise faster. (1 mark)
       4. Explain your suggestion in b(iii) above. (3 marks) 

MARKING SCHEME

1. New object distance = 5 – 2 = 3cm
   Distance between tip and its image = 3 + 3 ✔
   = 6cm✔
2.                      
   1. Like poles repel, unlike poles attract. ✔
   2. Pole X is South pole ✔
3. Convex mirror gives a wider field of view of✔the rear (behind) compared to plane mirror.
4.                  
   1. Period, T = 8.0 x 10^-4S✔
      Frequency =   1      =           1         
                          period      8.0 x 10^-4
      = 1250
      = 1.25 x 10³ Hz
   2.                        
      
5.                  
   1.  Primary cells cannot be recharged after use while secondary cells can be recharged after use.✔
   2. Polarization is reduced by adding a depolarizer
      or
      Adding Manganese (VI) oxide✔
6.                                
   
7. 2 Distance = Speed x time✔ (2d = vt)
   2x = 320 x 0.8 ✔
   x = 320 x 0.8
                2
   x = 128m ✔
8.                  
   
9.                
   • Hydrogen gas bubbles at the cathode.
   • - White deposit forms at the plates.
   • - Relative density of the electrolyte drops.
10.                            
    1.                       
       
    2. Reduce the amount of current. ✔
       or
       Reduce magnetic field strength.
11.                  
    • Transverse waves forms crests and troughs as they propagate while longitudinal waves forms part of compression and rarefaction. ✓¹
    • Transverse waves moves perpendicular to the direction of wave motion while longitudinal waves moves parallel to the direction of wave motion. ✓¹
12.                        
    1. The current flowing through a conductor is directly proportional to the potential difference across the conductor provided Temperature and other physical conditions are kept constant.✔
    2. Effective Resistance = (3 x 6)/(3+6)
       = 18/9
       = 2
       V = IR            I = V/R✔               = 5/2 = 2.5A ✔
13.                
    1.                    
       1. Dispersion of light.
       2. X – Red
          Y – Violet
          - Red has the lowest frequency/longest wavelength hence least deviated while violet has the highest frequency/shortest wavelength hence most deviated.
       3. Act as point source of light.
    2.                                  
       1.                                    
          
       2. C on the diagram. ✓¹
          
       3. Sin θ = _αη_g
          = 1.6667 x 31.2 ✓¹
          = 0.8634
          = 59.7° ✓¹
14.                        
    1.                          
       1. 3 + 1 = 4Ω
          4 + 2 = 6Ω
          R = 6 x 4
          6 + 4 ✔
          = 2.4Ω
          RE = 2.4Ω + 5.6Ω ✔
          = 8Ω ✔
       2. V = IR ✔
          I = 6 ✔
               8
          = 0.75A ✔
       3. V_5.6Ω = 0.75 x 5.6
          = 4.2V
          6V - 4.2V✔ = 1.8V✔
          
          Alt
          RXY = 2.4Ω
          V = 2.4 x 0.75 ✔
          = 1.8V ✔
15.                              
    1.                           
       1. Frequency not affected
       2. Speed reduces
       3. Wavelength reduces
    2. f=v/ƛ=0.12/0.08=1.5Hz
    3.                        
       • Stationary wave Progressive wave
       • No energy is transferred from source - energy is transferred from source
       • Wave form does not appear to move - wave form moves away continuously
    4.                          
       1. Time taken to make one complete oscillation
          10 x 10^-2 seconds
       2. F = I 
                T
                   I        = 10Hz         
          10 x 10^-2 
       3. V = λf
          X = v
                f
          = 200 = 20m
              10
    5.                              
       • Mechanical waves require material medium for transmission but electromagnetic waves do not
       • Mechanical waves are longitudinal or transverse but electromagnetic waves are only transverse in nature. 2mks
16.                    
    1.                       
       • Amount of current
       • Number of coils / turns
       • Shape of the core
    2.                          
       1. The metre rule tilt anticlockwise ✔when switch is closed current flows in coil magnetising bar ✔. End of coil facing magnet become south pole ✔hence attract magnet to which ruler is attached ✔
       2. Metre rule tilt clockwise or weight comes down while magnet moves upwards ✔. Reversing terminals reverses direction of current hence polarity of electro magnet.✔ End facing magnet become north pole hence repulsion
       3. Move jockey J from B toward A / right to left of R. ✔
       4. It reduces resistance and increase current hence stronger electromagnet