Physics Questions - Alliance Boys High School Post Mock KCSE 2020

INSTRUCTIONS TO THE CANDIDATES:

• Write your name and index number in the spaces provided above.
• Answer all the questions both in section A and B in the spaces provided below each question
• All workings must be clearly shown; marks may be awarded for correct steps even if the answers are wrong.
• Mathematical tables and silent electronic calculators may be used.

SECTION A: (25 marks)
Answer all the questions in this section in the spaces provided

1.   
   1. State two characteristic of images formed by a pin hole camera. (2 marks)
   2. What is the effect on the image when the distance between the pinhole and screen is increased? (1 mark)
2. State one common property of electromagnetic waves. (1 mark)
3. The figure below shows a coiled wire connected in series with a dry cell, a switch and a rheostat AB. If the switch is closed and the jockey J on the rheostat is moved towards B what is the effect on: -
   
   
   1. The resistance of the circuit (1 mark)
   2. The current through the coil (1 mark)
4. The heating element of an electric hot plate has a resistance of 80022. What is the energy dissipated when the element is kept on for 20 minutes on a 240V supply (3 marks)
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5. The figure below shows the displacement-time graph for a certain wave. If both the frequency and the amplitude of the wave is doubled, sketch on the same axes the new waveform. (2 marks)
   
6. Arrange the following in the table below in the order of increasing wavelength; infrared, microwave, gamma rays, violet light.(1 mark)
   
7. The potential difference across an X-ray tube is 200V. If the charge on an electron is 1.6x10-1°C, calculate the kinetic energy of the electron. (3 marks)
8. State an adjustment that can be made in a x-ray tube to increase the quantity of x-rays produced (1 mark)
9. An electric heater is switched on for 10hrs. If the total cost for running the heater for the ten hours is Ksh600 Calculate the power rating of the heater in Watts given that the cost of electricity is ksh30 per kWh (3 marks)
10. A current of 0.8A flows through an electric circuit. Determine the quantity of charge that passes a point in the circuit in minutes. (2 marks)
11. The figure below shows a coil rotated in a magnetic field to generate an ac voltage
    
    
    1. State two ways in which the induced ac voltage can be increased(2 marks)
    2. State the energy changes that take place as the coil rotates (1 markis)
    3. State one modification that can be made in figure above for the output to be de voltage (1 mark)
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12. State one difference between X-rays and gamma rays based on their production (1mark)
    
    SECTION B: (55 marks)
    Answer all the questions in this section in the spaces provided
13.  
    1. Give the difference between the thermionic emission and photoelectric effect. (1 mark)
    2. The figure below shows a diagram of a Cathode Ray Tube. Study it then answer the questions that follow:
       
       
       1. Explain why the cathode ray tube is highly evacuated (2 marks)
       2. When an opaque object is placed on the path of the electron beam, a shadow is formed on the screen. State the property of cathode rays deduced from this observation. (1 mark)
       3. State one function of the anode (1 mark)
       4. The cathode ray tube can be modified to be a cathode ray oscilloscope by connecting the input terminals and time base circuit. State the plates across which the time-base circuit is connected to (1 mark)
    3. State one difference between Cathode rays and X-rays (1 mark)
    4. The figure below shows the display of an ac signal from on the Cathode Ray Oscilloscopes screen.
       
       Given that the smallest square is lcm by lcm and that the time-base setting is 220 millisecond/division, determine the frequency of the signal (3 marks)
14.  
    1. Distinguish between refraction and diffraction of light (1 mark)
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    2. The critical angle for a liquid-air interface is 40°. Calculate the refractive index of the liquid.(3 marks)
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    3. State one reason why glass prisms are preferred to plane mirrors for use in periscopes (1 mark)
    4.  
       1. A converging lens can be used as a magnifying glass. State where the object should be placed for this to be possible. (1 mark)
       2. An object placed 12 cm from a convex lens is magnified 4 times. Determine the focal length of the lens. (4 marks)
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       3. State one difference between the human eye and the camera (1 mark)
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15.  
    1. Boron has a half-life of 3 days. A sample of Boron has a mass of 2g. What mass of the sample will have decayed after 15 days? (3 marks)
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    2. The figure below shows a Geiger Muller tube.
       
       
       1. State the purpose of the mica window (1 mark)
       2. What is the purpose of the bromine (1 mark)
       3. Briefly explain how the Geiger Muller tube it works to detect radiations. (2 marks)
    3. When beta and alpha radiations are passed through a magnetic field, the beta radiations are deflected more than the alpha radiations. Give a reason for this observation. (1 mark) 
16. The figure below shows a transformer. The primary coil is connected across 1000V and has a current of 2A while the output voltage from the secondary coil is 200V.
    
    
    1. State whether this is a step-up or step-down transformer (1 mark)
    2. How is hysteresis loss of energy minimized in the transformer? (1 mark)
    3. Calculate the number of turns in the primary coil if number of turns in secondary coil is 10 (2 marks)
    4. Calculate the efficiency of the transformer if the current in the secondary coil 1.5A (3 marks)
    5. Define the term work function of a metal surface. (1 mark)
    6. State how the work function affects photoelectric emission (1 mark)
    7. A certain metal has a work function of 3.1eV. Determine the threshold frequency.
       (Take leV = 1.6 x 10^-19 J, Planck's constant h = 6.63x10 ^-34, and velocity of light in vacuum C = 3.0 x10⁸m/s) (2 marks)
       .. ............................................................................
17.  
    1. State one way of how the capacitance of a parallel plate capacitor can be reduced (1 mark)
    2. The figure below shows a power source of 6.0V and switches P and Q connected to two capacitors of capacitances 4uF and 6uF and a voltmeter
       
       
       1. Given that switch Q is closed and P open, calculate the charge on the 4uF capacitor. (3 marks)
       2. Determine the effective capacitance when both switches are closed. (2 marks)
    3. Describe what is observed on the voltmeter when:
       
       1. Switch Q is closed and switch P open. (1 mark)
       2. Switch Q is closed and opened, and then P is closed. (1 mark)
       3. Explain your answers in part (1) and (ii). (2 marks)
    4.  
       1. Define doping as used in electronics (1 mark)
       2. Differentiate between a p-type and n-type semiconductors in terms of charge carriers. (2 marks)
       3. Explain what happens to the depletion layer when a diode is reverse biased. (2 marks)