Physics Paper 2 Questions and Answers - Sukellemo Joint Mock 2020/2021

SUKELLEMO JOINT MOCK
Kenya Certificate of Secondary Education
PHYSICS PAPER 2
2 hours 

Instructions to Candidates

• Answer all the questions in the spaces provided.
• This paper consists of 2 sections: Section A and B
• All working must be clearly shown.
• Non-programmable silent electronic calculators and KNEC mathematical tables may be used.

SECTION A: (25 MARKS)
Answer ALL questions in this section in the spaces provided:

1. State two conditions under which a pinhole camera may form an image on its screen which has the same size as the object. (2mks)
2. The figure below shows a ray of light incident on the surface of one plane mirror.
   
   Sketch the path of the ray on the diagram after striking mirror 2 indicating all the angles.(2 marks)
3. A steel is to be magnetized by electrical method as shown below. Identify the pole P and Q of the resulting magnet. (1mk)
   
   P:
   Q:
4. A small chain is often seen hanging at the back of a petrol carrying lorry. State and explain its significance. (2mks)
5. The figure below shows two waveforms representing the same wave motion.
   
   Determine the velocity of the wave. (3mks)
6. An object O is placed in front of a concave mirror and on the principal axis, as shown in the figure below. Complete the light ray diagram to locate the position of the image. (3mks)
   
7. Arrange the following radiations in order of increasing wavelengths. (1mk) Infrared, blue light, ultraviolet, radiowaves, x-rays.
8. The figure below shows a block diagram of a p-n junction diode.
   
   On the same diagram, show how a cell may be connected so that it is reverse biased. (1mk)
9. A girl standing at a distance claps her hands and hears an echo from a tall building 2 seconds later. If the speed of sound in air is 340m/s, determine how far the building is. (3mks)
10. What do you understand by polarization as used in a simple cell? (1mk)
11. State how the defect mentioned in question 10 above is minimized in a simple cell. (1mk)
12. A current-carrying conductor AB is in a magnetic field as shown in the figure below.
    
    
    1. Indicate the direction of force F acting on the conductor. (1mk)
    2. State two factors that determine the direction of the force F. (2mks)
13. You are given three resistors of values 5Ω, 8Ω and 12Ω. Show in a circuit diagram how you would connect them so as to give:
    1. an effective resistance of 9.8Ω. (2mks)
    2. the least effective resistance. (2mks)

SECTION B: (55 MARKS)
Answer ALL questions in this section in the spaces provided.

14.    
    1. Define refractive index. (1mk)
    2. The critical angle of a certain material medium is 43.2º. Determine the refractive index of the material. (2mks)
    3.    
       1. What do you understand by the term accommodation? (1mk
       2. The diagram below shows a certain defect of vision. Name the defect. (1mk)
          
       3. On the figure below show how the defect can be corrected. (2mks)
          
    4. An object is placed 40cm in front of a concave lens of focal length 20cm; determine the position of the image. (3mks)
15.    
    1.    
       1. State Lenz’s a law of electromagnetic induction. (1mk)
       2. A bar magnet is moved into a coil of insulated copper wire connected to a centre-zero galvanometer, as shown in the figure below.
          
          1. Show on the diagram the direction of induced current in the coil. (1mk)
          2. State and explain clearly what is observed on the galvanometer when the S-pole of the magnet is moved into and then withdrawn from the coil. (4mks)
    2. A transformer has 800 turns in the primary and 40 turns in the secondary winding. The alternating e.m.f connected to the primary is 240V and the current is 0.5A.
       1. Determine
          1. the secondary e.m.f (2mks)
          2. the power in the secondary if the transformer is 95% efficient. (2mks)
       2. Explain how energy losses in a transformer are reduced by having:
          1. a soft-iron core. (2mks)
          2. a laminated core. (1mk)
16.    
    1.    
       1. Distinguish between thermionic emission and photoelectric emission. (2mks)
       2. State one factor which affects the rate of each of the above types of emission.
          1. Thermionic emission. (1mk)
          2. Photoelectric emission. (1mk)
    2. Sodium has a work function of 2.3eV. Given that: Planck’s constant h = 6.63 x10^-34JS, velocity of light in vacuum, C = 3.0 x 10⁸m/s, 1 electron-volt (1eV) = 1.6 x 10^-19 C and mass of an electron, m_e = 9.1 x 10^-31kg, calculate:
       1. its threshold frequency. (2mks)
       2. the maximum velocity of the photoelectrons produced when the sodium is illuminated by light of wavelength 5.0 x 10^-7m. (4mks)
       3. the stopping potential V, with the light of this wavelength. (2mks)
17.    
    1. State two advantages of using a Cathode Ray Oscilloscope (C.R.O) as a voltmeter over the ordinary voltmeter. (2mks)
    2. An X-ray operates at 30000V and the current through it is 2mA. Given that the charge of an electron is 1.6 x 10^-19C, h = 6.63 x 10^-34JS, speed of light, C = 3.0 x 10⁸m/s, calculate:-
       1. the maximum kinetic energy of the electrons when hitting the target. (2mks)
       2. the number of electrons hitting the target per second. (2mks)
       3. the minimum wavelength of the X-rays emitted. (2mks)
18.    
    1. A radioactive carbon-14 decays to nitrogen by beta particles as shown below.
       ¹⁴₆C = ^x₇N+0^e_y
       Determine the values of x and y. (2mks)
    2. The graph below shows radioactive decay of iodine.
       
       Use the graph to determine the:-
       1. Fraction of the amount remaining after 16.2 days. (2mks)
       2. Determine the half – life of iodine. (2mks)
       3. Mass remaining after 17 days.
    3. The figure below shows the cross-section of a diffusion cloud chamber used to detect radiation from radioactive sources.
       
       1. State the function of the following:
          1. Alcohol. (1mk)
          2. Solid CO₂. (1mk)
       2. Explain briefly how the diffusion cloud chamber can be used to detect and identify alpha particles. (3mks)

MARKING SCHEME

SECTION A: (25 MARKS)
Answer ALL questions in this section in the spaces provided:

1. State two conditions under which a pinhole camera may form an image on its screen which has the same size as the object. (2mks)
   • When the object distance from the pinhole is equal to the image distance.
     
   • When the screen is as large as the object.   
             
2. The figure below shows a ray of light incident on the surface of one plane mirror.
   
   Sketch the path of the ray on the diagram after striking mirror 2 indicating all the angles.(2 marks)

   • X= 2Ө
     = 2 × 15
     30º 

3. A steel is to be magnetized by electrical method as shown below. Identify the pole P and Q of the resulting magnet. (1mk)
   
   P:South Pole
   Q:North Pole
4. A small chain is often seen hanging at the back of a petrol carrying lorry. State and explain its significance. (2mks)
   • Friction generates charges on the lorry the chain discharges the lorry to prevent sparks which may lead to explosion
     
     
5. The figure below shows two waveforms representing the same wave motion.
   
   Determine the velocity of the wave. (3mks)
   • λ= 0.4m
   • T = 0.06m                                                                                                               
     V = λf   
     = 0.4 × 0.06
     = 0.24m/s  
     
     
6. An object O is placed in front of a concave mirror and on the principal axis, as shown in the figure below. Complete the light ray diagram to locate the position of the image. (3mks)
   
   Answer
   
   
7. Arrange the following radiations in order of increasing wavelengths. (1mk) Infrared, blue light, ultraviolet, radiowaves, x-rays.
   
   • X-ray → Ultraviolet → Blue light → Infrared → Radio waves
     
     
8. The figure below shows a block diagram of a p-n junction diode.
   
   On the same diagram, show how a cell may be connected so that it is reverse biased. (1mk)
   Answer
   
9. A girl standing at a distance claps her hands and hears an echo from a tall building 2 seconds later. If the speed of sound in air is 340m/s, determine how far the building is. (3mks)
   • V=^2s/_t
     S=^Vt/₂
     =^{340 x 2}/₂
     340m
     
     
10. What do you understand by polarization as used in a simple cell? (1mk)
    • Process by which hydrogen gas bubbles form an insulating layer on the positive copper plate
      
      
11. State how the defect mentioned in question 10 above is minimized in a simple cell. (1mk)
    • Adding potassium dichromate powder/depolarizing agent/oxidizing agent which oxidizes hydrogen to water
      
      
12. A current-carrying conductor AB is in a magnetic field as shown in the figure below.
    
    
    1. Indicate the direction of force F acting on the conductor. (1mk)
       
    2. State two factors that determine the direction of the force F. (2mks)
       • Direction of magnetic field
         
13. You are given three resistors of values 5Ω, 8Ω and 12Ω. Show in a circuit diagram how you would connect them so as to give:
    1. an effective resistance of 9.8Ω. (2mks)
       
    2. the least effective resistance. (2mks)
       

SECTION B: (55 MARKS)
Answer ALL questions in this section in the spaces provided.

14.    
    1. Define refractive index. (1mk)
       • Refractive index is ratio of sine of angle of incidence to the sine of angle of refraction for a given pair of media
         
         
    2. The critical angle of a certain material medium is 43.2º. Determine the refractive index of the material. (2mks)
       • 1.4608
    3.    
       1. What do you understand by the term accommodation? (1mk)
          • Accommodation is the fine adjustment of focal length of the eye so as to fit images of objects of different distances on the retina
            
            
       2. The diagram below shows a certain defect of vision. Name the defect. (1mk)
          
          
          • Long sightedness or (hypermetropia) 
            
            
       3. On the figure below show how the defect can be corrected. (2mks)
          
          Answer
          
    4. An object is placed 40cm in front of a concave lens of focal length 20cm; determine the position of the image. (3mks)
       • ¹/_-f = ¹/_V + ¹/_U
         ^-1/₂₀ = ¹/_V + ¹/_U
         ¹/_V = ¹/₂₀ - ¹/₄₀ = ^-2-1/₄₀ = ³/₄₀
         V= ^-40/₃ = 13.3cm
15.    
    1.    
       1. State Lenz’s a law of electromagnetic induction. (1mk)
          • Lenz’s law – The direction of the induced current is such that the induced current which it causes to flow produces a magnetic effect that oppose the change producing it. 
            
            
       2. A bar magnet is moved into a coil of insulated copper wire connected to a centre-zero galvanometer, as shown in the figure below.
          
          
          1. Show on the diagram the direction of induced current in the coil. (1mk)
             
          2. State and explain clearly what is observed on the galvanometer when the S-pole of the magnet is moved into and then withdrawn from the coil. (4mks)
             • S-Pole of magnet moved into the coil.
               -Galvanometer deflects to the left (on one side) momentarily. -Induced current in the coil flows so as to form a South Pole at the  end of the coil near the magnet.
             •  S-Pole of magnet withdrawn.
               -Galvanometer reverses its direction of deflection/deflects to the right  momentarily.
               -Changing the direction of motion  reverses the direction of induced  current so that the end of the coil tends to form a N-Pole so as to oppose the motion of the magnet out of the coil.  
                     
    2. A transformer has 800 turns in the primary and 40 turns in the secondary winding. The alternating e.m.f connected to the primary is 240V and the current is 0.5A.
       1. Determine
          1. the secondary e.m.f (2mks)
             
             Vp = 240V
             Ip = 0.5A
             ^Ns/_Np = ^Vs/_Vp
             ⁴⁰/₈₀₀ = ^Vs/₂₄₀
             Vs= ^{(40 x 240)}/₈₀₀
             =12V
             
             
          2. the power in the secondary if the transformer is 95% efficient. (2mks)
             • V_sI_s = 95% V_pI_p
               ⁹⁵/₁₀₀ x 240 x0.5
               =114W
               
               
       2. Explain how energy losses in a transformer are reduced by having:
          1. a soft-iron core. (2mks)
             • Soft iron is easily magnetized and easily demagnetized; this reduces heat loss due magnetization and demagnetization  (hysteresis loss) 
               
               
          2. a laminated core. (1mk)
             • Laminated core increases resistance of the core which reduces the size of eddy currents thus reducing heating due to the eddy  currents.
16.    
    1.    
       1. Distinguish between thermionic emission and photoelectric emission. (2mks)
          • Thermionic emission – Is the escape or evaporation of electrons from a metal surface when heated
          • Photoelectric emission – Is the escape or evaporation of electrons from a metal surface when light of suitable frequency falls on the metal surface. 
            
            
       2. State one factor which affects the rate of each of the above types of emission.
          1. Thermionic emission. (1mk) - Temperature of metal/ type of metal
          2. Photoelectric emission. (1mk) - Frequency of incident light/ Work function/ threshold frequency of the metal
             
             
    2. Sodium has a work function of 2.3eV. Given that: Planck’s constant h = 6.63 x 10^-34JS, velocity of light in vacuum, C = 3.0 x 10⁸m/s, 1 electron-volt (1eV) = 1.6 x 10^-19 C and mass of an electron, m_e = 9.1 x 10^-31kg, calculate:
       1. its threshold frequency. (2mks)
          • hf_o = w_o
            6.63 x 10^-34f_o=2.3 x 1.6 x10^-19
            f_o = 5.55 x 10¹⁴HZ
       2. the maximum velocity of the photoelectrons produced when the sodium is illuminated by light of wavelength 5.0 x 10^-7m. (4mks)
          • hf = w_o + ¹/₂MV^₂
            h^c/_λ = W_o + 1/₂MV²
            6.63 x 10^-34 x      3.0x10⁸   = 2.3 x 1.6 x 10^-19 + ¹/₂MV²
                                     5.0 x 10^-7
            3.978 x 10^-19 = 3.68 x 10^-19 + ¹/₂MV^{2
            1}/₂MV² = 0.298 x 10^-19
            
            
       3. the stopping potential V, with the light of this wavelength. (2mks)
          • eV_s = ¹/₂MV²
            1.6 x 10^-19V_s = 0.298 x 10^-19
            V_s= 0.298 x 10^-19
                    1.6 x 10^-19
            =0.186Volts
17.    
    1. State two advantages of using a Cathode Ray Oscilloscope (C.R.O) as a voltmeter over the ordinary voltmeter. (2mks)
       • Has no coil to burn out  
       • Has instantaneous response.
       • Has nearly infinite resistance, therefore draws very little current.
       • Can measure both AC and DC voltages.
         
         
    2. An X-ray operates at 30000V and the current through it is 2mA. Given that the charge of an electron is 1.6 x 10^-19C, h = 6.63 x 10^-34JS, speed of light, C = 3.0 x 10⁸m/s, calculate:-
       1. the maximum kinetic energy of the electrons when hitting the target. (2mks)
          • Ke = ½MV² = eV
            = 1.6 × 10^-19 × (30,000)²
            = 1.44 × 10^-10J
            
            
       2. the number of electrons hitting the target per second. (2mks)
          • I=ne
            2 x 10^-3 = n x 1.6 x 10^-19
            n= 2x 10^-3
                1.6 x 10^-19
            =1.25 x 10¹⁶ electrons
            
            
       3. the minimum wavelength of the X-rays emitted. (2mks)
          • hf_max = eV But f_max= ^e/λ_min
            h ^e/_^λmin = eV
            λmin =hc/eV 
            = 6.63 x 10-34 x 3 x 10⁸
                  1.44 x 10^-10
            = 1.38 x 10^-15m
18.    
    1. A radioactive carbon-14 decays to nitrogen by beta particles as shown below.
       ¹⁴₆C = ^x₇N+0^e_y
       Determine the values of x and y. (2mks)
       • 14=x+0
         x=14
         6=7+y
         y=-1
         
         
    2. The graph below shows radioactive decay of iodine.
       
       Use the graph to determine the:-
       1. Fraction of the amount remaining after 16.2 days. (2mks)
       2. Determine the half – life of iodine. (2mks)
          • T_½ = 7500 -2500 = 5000 years
            
            
       3. Mass remaining after 17 days.
          • No of halflifes = ¹⁵⁰⁰⁰/₅₀₀₀
            N1 =No(½)³
            100(½)³
            =12.5g
            
            
    3. The figure below shows the cross-section of a diffusion cloud chamber used to detect radiation from radioactive sources.
       
       
       1. State the function of the following:
          1. Alcohol. (1mk)
             • Alcohol condenses around the ions formed by the radiation to form the narrow tracks
               
               
          2. Solid CO₂. (1mk) 
             • Cools the bottom of the chamber and also condenses alcohol below its normal temperature
               
               
       2. Explain briefly how the diffusion cloud chamber can be used to detect and identify alpha particles. (3mks)
          • Sold CO₂ cools the bottom of the chamber alcohol vapourises and spreads through the chamber. It is cooled below its normal temperature in case of a radiation, air around the path of the radiation is ionized. Alcohol condenses around these ions to form a narrow cloud.