KCSE 2015 Physics Paper 2 with Marking Scheme

Section A (25 marks)
Answer all the questions in this section in the spaces provided.

1. Figure 1 shows three mirrors arranged at right angles to each other . A ray of light is incidenton one of the mirrors. 
   Complete the diagram to show the path of the ray after reflection on each of the mirrors.  (3 marks )
2. It is observed that when a charged body is brought near the cap of a positively charged electroscope, the divergence of the leaf increases . State the type of charge on the body. (1 mark)
3. State a reason for topping a lead-acid accumulator with distilled water. (1 mark)
4. Figure 2 shows a soft iron bar AB placed in a coil near a freely suspended magnet.
    
   Explain the observation made when the switch is closed. (2 marks)
5. State the reason why a convex mirror is preffered over a plane mirror for use as a driving mirror.(1 mark)
6. State two ways in which the strength of the electromagnet can be increased.(2 marks)
7. State two differences between electromagnetic waves and mechanical waves.(2 marks)
8. Figure 3 shows straight waves incident on a divergent lens placed in a ripple tank to reduce its depth.
   Complete the diagram to show the wavesthe waves in both shallow regions and beyond the lens.(2 marks)
9. A ship in an ocean sends out an ultra sound whose echo is received after 3 seconds,.If the wavelength of the ultra sound in water is 7.5cm and the frequency of the transmitter is 20 kHz, determine the depth of the ocean. (3 marks)
10. A nail at the bottom of a beaker containing glycerine appears to be 6.8 cm bellow the surface of glycerine. Determine the height of the column of glycerine in the beaker. (take the refractive index of glycerine as 1.47) (3 marks)
11. State one application of thermionic emission. (1 mark)
12. Figure 4 shows a cathode ray entering into a region between two charged plates.
    
    Complete the diagram to show the path of the ray in the field. (1 mark)
13. When a transformer is connected to an ac source, the output voltage is found to be 24V. If the power input is 200 W, determine the output current.(Assume the transformer is 100% efficient) (3 marks)

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

14.    
    1. State two factors that affect photoelectric emission. (2 marks)
    2. Light of a wavelength 4.3 x 10^-7 m is incident on two different metal surfaces, nickel and potassium.(Take speed of light as 30 x 10⁸ ms^-1 and planks constant h as 6.63 x 10^-34Js)
       1. Determine the energy of the incident radiation. (3 marks)
       2. If the work function of nickel is 8.0 x 10^-19J and that of potassium is 3.68  x 10^-19J , state with a reason from which of the two metals the given light will eject electrons.  (2 marks)
       3. Determine the velocity of the emitted electrons from the metal surface in b(ii).(take the mass of an electron as 9.1 x 10^-31 kg)(3 marks)
15.     
    1. State two factors that determine the resistance of a metallic conductor.(2 marks)
    2. Explain how a fuse safeguards electrical appliances against excessive currents. (2 marks)
    3. A hair dryer is rated 2.5 kW, 240V.
       
       1. Determine whether a 10 A fuse may be suitable for the dryer. (3 marks)
       2. Determine the cost of using the hair dryer for 3 hours if the cost of electricity is ksh 0.80 per kilowatt hour. (2 marks)
16.         
    1. It is observed that the alpha (α) particles have a lower penetrating power than beta (β) particles. Explain this observation. (2 marks)
    2. A radioactive substance has a half life of 12 years. Determine the time it will take to decay to 12.5% of its original value. (2 marks)
    3. A Geiger Müller (GM) tube is used for detecting radiations from a radioactive source.
       State the function of:
       1. the mica window (1 mark)
       2. bromine gas in the tube(1 mark)
    4.        
       1. In a diffusion chamber, explain why some of the tracks formed are observed to be:
          1. short (2 marks)
          2. straight ( 2 marks)
       2. State two advantages of GM tube instead of a diffusion cloud chamber to detect radiations from radioactive substances. (2 marks)
17.      
    1. State three factors that affect the capacitance of a parallel plate capacitor.(3 marks)
    2. Figure 5 shows the circuit used to charge a capacitor C.
       
       1. State what would be observed on the following when the switch is closed
          1. the milliammeter (1 mark)
          2. the voltmeter (1 mark)
       2. Explain how the capacitor gets charged (3 marks)
       3. State the purpose of the resistor R. (1 marks)
       4. On the axes provided, sketch the graph of voltage (v) against time(t) (2 marks)
          
18.      
    1. Three resistors 2Q, 3Q, and 4Q are to be connected to a cell such that they have the least effective resistance.
       1. Draw a circuit diagram to show how they can be connected to achieve this.(2 marks)
       2. Determine the least effective resistance of the three resistors.(3 marks)
    2. A real object of height 1 cm is placed 50 mm from a converging lens forms a virtual image 100 mm from the lens.
       1. Determine the:
          1. focal length of the lens (3 marks)
          2. magnification. (2 marks)
       2. On the grid provided draw to scale the ray diagram for the setup, to show how the image is formed.

MARKING SCHEME

1. 
   
   Correct angle at every surface
   Arrow on rays (3 marks)
2. Positive charge (1 mark)
3. To maintain the relative density of the electrolyte. (1 mark)
4.    
   • The suspended magnet is repelled
   • End B of the electromagnet attains a north pole when current flows.(2 marks)
5. Has a wide field of view. (1 mark)
6.    
   • Increase the magnitude of current.
   • Increase the number of turns per unit length.
   • Increase cross sectional area.
   • Use of soft iron core. (2 marks)
7.    
   • Electromagnetic waves do not require a material medium while mechanical waves require a material medium for transmission.
   • Electromagnetic waves travel at the speed of light while mechanical waves travel at slower speeds. (2 marks)
8. 
      
   • decreased wavelength in shallow region
   • diverging after refraction to the deep region. (2 marks)
9. V = λf
   V= 7.5  x 20 x 1000
        100
   Depth = 7.5  x 20 x 1000 x 3 
               100                       2
   = 2250m (3 marks)
10. η =    real depth    
         apparent depth
    1.47 =real depth= 9.996 a
                 6.8
    real depth = 10 cm (3 marks)
11. Production of cathode rays / x-rays. (1 mark)
12.  
    
    deflection towards positive plate (1 mark)
13. V_pI_p = V_sI_s
    200 = 24 I_s
    I_s = 8.33A (3 marks)
14.    
    1.    
       • Energy of incident radiation
       • Work function of the metal
       • Intensity of the radiation
         Any 2 = 2 marks)
    2.    
       1. E = hc 
                 λ
          = 6.63 x 10^-34 x 3.0 x 10⁸
                     4.3 x 10^-7
          = 4.626 X 10^-19 J (3 marks)
       2.    
          • Potassium
          • The work function of potassium is less than the energy of the incident radiation (2 marks)
       3. E=Wo+K.E
          4.626 x 10^-19 J=3.68 x 10^-19 J + K.E
          KE =9.4558x10^-20 J
          ½ MV² = 9.4558 x 10^-20
          V = 9.4558 x 10^-20 x 2
                       9.1 x 10^-31
          V = √9.4558 x 10^-20 x 2
                        9.1 x 10^-31
          = 4.56 x 10⁵ ms^-1 (3 marks)
15.    
    1.      
       • length of conductor
       • area of cross-section
       • temperature
       • resistivity of conductor
         (Any 2 = 2 marks)
    2. When excessive currents flow through the circuit, the wire gets heated and melts hence breaking the circuit. (2 marks) 
    3.    
       1. I =  P  
                V 
          = 2500
              240
          =10.42 A
          Fuse not suitable since current through the appliance is higher than the fuse rating. (3 marks)
       2. Cost = 0.8 x 3 x 2.5
          = Ksh. 6.00
16.      
    1. Alpha particles are heavier and move at lower speeds hence less penetrating power than Beta particles which are lighter and move faster (2 marks)
    2. 100% →12→ 50%→12→25%→12→12.5%
       3 half-lifes
       36 years (2 marks)
    3.      
       1. allows the radiations into the tube (1 mark)
       2. absorbs kinetic energy of positive ions so that they do not cause secondary ionization in the tube (1 mark)
    4.    
       1.    
          • Short - They ionize heavily loosing most of the energy hence cannot travel far.
          • Straight - They are massive compared to air molecules hence collision with air molecules cannot change their path.
       2.    
          • GM is easily portable than a cloud chamber. (1 mark)
          • GM is more sensitive.
          • GM tube detects radiation at very low intensity and cloud chamber cannot detect radiation at very low intensity. (1 mark)
17.      
    1.      
       • distance of separation between plates
       • area of overlap of plates
       • type of dielectric between plates (3 marks)
    2.    
       1.    
          1. Current rises to maximum and then drops to zero (1 mark)
          2. Potential difference between the plates increases to a maximum (1 mark)
       2. Negative charges flow from the negative terminal of the battery to one plate of the capacitor. Negative charges flow from the other plate of the capacitor to the positive terminal of the cell hence equal positive and negative charges gather on the plates, opposing further flow of electrons when fully charged (3 marks)
       3. Resistor - to slow down the charging process so that current and voltage are is observed. (1 mark)
       4. 
                
          • Parallel arrangement
          • Circuit symbols
18.    
    1.    
       1.  
          
       2. 1  = 1  + 1  + 1 
          R     2     3     4
           R = 12 
                 13
          = 0.923Ω (I)
    2.    
       1.    
          1. 1  + 1  = 1 
              u     v      f
             v = -100mm
             1  -  1   = 1 
             50 100     f
              1 =  1  
              f    100
          2. m=  v  = 100 
                    u      50
       2.