PHYSICS PAPER 2 Marking Scheme - 2019 KCSE Prediction Answers Set 1

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SECTION A (25 Marks)
Answer all the questions in this section 

  1. Figure 1 shows a plane mirror, a point object (P) and the position of the observer's eye. Show whether the observer will see the image of the object point P or not. (1 mark)
    p2 ans 1
  2. State one difference between a lead-acid battery cell and a Leclanche' cell (1 mark)
     Lead acid battery  Leclanche' cell
     Can be recharged after use  Cannot be recharged after use
     Polarization and local action are not defects  Polarization and local action are defects
     Produces more current since it has low internal resistance  Produces less current since it has higher internal resistance 
  3. A form one student at Elimu Girls' High School connected a simple electric circuit as shown in Figure 2 below.
    p2 fig 2
    State and explain the observation made on bulbs B1 and B2 when switches S1 and S2 are both closed (2 marks)
    Bulb Blights while B2 does not; current flows through B1 since it is in a closed circuit while B2 does not light since it is short circuited
    Figure 3 shows an X- ray tube used to produce X - rays. Use it to answer questions 4 and 5
    p2 fig 3
  4. Give a reason why;
    1. The metal target is made to rotate (1 mark)
      The metal target rotates during operation to change the point of impact, thereby reducing the wear and tear on it.
    2. Lead metal is used to shield the X-ray tube (1 mark)
      Lead is a metal of very high density hence absorbs all the stray X-ray radiation 
  5. The X-ray tube in Figure 3 produces elctrons which are accelerated by a p.d of 12kV. Assuming all the enrgy goes to produce X - rays, determine the maximum frequency of the X-rays produced. (Planck's constant h= 6.62 × 10-34 Js) and charge on an electron, e = 1.6 × 10-19 C) (3 marks)
    p2 ans 5
  6. Figure 4 (not drawn to scale) shows a bright electric bulb placed behind a screen which has a hole with an object cross-wire. A concave mirror of focal length 30 cm is placed in front of the screen. The position of the mirror is adjusted until a sharp image of the cross-wire is formed on the screen
    Determine the distance between the mirror and the screen (1 mark)
    p2 fig 4
    Radius of curvature, r = 2f = 2 × 30 =60cm
  7. In a house, there is a cooker rated 6 kW. The mains potential is 240 V and the fuses available are 35A, 30 A, 15 A and 13 A. Determine the fuse that would be suitable for the cooker (3 marks)
    Operating current I = P/V = 6000/240
    =25 A
    Suitable fuse : 30 A fuse
  8. When X- rays are passed above the cap of a positively charged electroscope it is observed that the leaf divergence decreases. Explain this observation (2 marks)
    X-rays ionize the air above the cap; Electrons are attracted towards the cap hence neutralizing the posituvely charged electroscope
  9. Several 200 Ω carbon resistors are to be connected in a circuit so that a current of 2A flows from a 50 V source. Determine how many resistors are required. (3 marks)
    p2 ans 9
  10. Figure 5 shows part of a wiring circuit for a house
    p2 fig 5
    Correct two faults made in the wiring (2 marks)
    • Fuse should be connected to the live wire anad not the neutral wire
    • Ceiling lights should be connected in parallel and not in series
    • Switch operating the ceiling lights should be connected to live wire and not neutral wire
    • The lower bulb should not be short circuited
  11. The current of electrons hitting the screen of a C.R.O is 15 mA. Given that the charge of one elctron is 1.6 × 10 -19 C, determine the number of electrons that hit the screen per second (2 marks)
    p2 ans 11
  12. Figure 6 shows an electromagnetic spectrum in the increasing order of wavelength from X to Y
    p2 fig 6
    1. Identify the region  of the spectrum labeled C (1 mark)
      C - ultraviolet rays rej. UV
    2. Give one similarity between the spectrum labeled B and the visible light (1 mark)
      Any of the properties of elctromagnetic waves e.g. they travel in straight line at the speed of light (3× 108 m/s); Can be detected by photocells
  13. Give a reason why electrical power is transmitted over long distances at high voltage (1 mark)
    High Voltage transmission ensures that small amount of current flows in the transmission line which reduces heating effect of an elctric current hence minimizing power loss due to transmission

SECTION B (55 Marks)
Answer all the questions in this section

    1. State what is meant by thermionic emission (1 mark)
      It is the production of electrons from a metal surface by heating the metal
    2. Figure 7 shows a beam of cathode rays entering the space between two charged metal plates. Continue the dotted line to show the path of the cathode rays as they travel between the plates and into the space beyond the plates
      p2 ans 14b
    3. Figure 8 shows a cathode ray tube of cathode ray oscilloscope (C.R.O)
      p2 fig 8
      1. Name the parts labeled A and D (2 marks)
        A- Hot filament D-Y plates or Y deflection plates
      2. What property does the part labeled B have for its efficient functioning (1 mark)
        B is made of a metal of low work function
      3. State the function of the part labeled C (1 mark)
        Controls the intensity of the elctron beam (or brightness of the spot) on the screen
    4. The figure below shows the trace on the screen of an ac signal connected to the Y plates of a CRO with time base on
      Given that the time base control is 50 ms/div, detrmine the frequency of the a.c signal (3 marks)
      p2 q14d
      f =1/T
      f = 1/(4× 0.05)
      f= 5 Hz
    1. A piece of light object (cork floats on the surface of water as shown in Figure 9 below. A straight edge vibrator placed at point A of the ripple tank generates water waves which travel towards point B
      p2 ans 15 aii
      1. Successive waves pass the cork every 0.4 seconds and the speed of the wave is 0.5 m/s. Determine the wavelength of the waves (2 marks)
        p2 ans 15ai
      2. Indicate on the diagram the direction in which the card moves as the waves pass on it (1 mark)
    2. Figure 10 shows a ray of light travelling from water towards its interface with air. The critical angle, C of water is 49o
      p2 ans 15b
      1. Skecth the path of the ray on the diagram above after striking the interface at 
        1. An angle if incidence, θ = C. Label the ray as R(1 mark)
        2. An angle of incidence θ >C. Label the ray as R2 (1 mark)
      2. Calculate the absolute refractive index, n, of water (3 marks)
        n = 1/sin C
        n = 1/sin 49o
        n = 1.325
    3. A student struck the prongs of a tuning fork agaisnt a hard surface in the laboratory. The prongs vibrate producing a longitudinal wave passing through air as shown in the Figure 11
      p2 ans 15 c
      1. Label the compression and rarefaction regions in the longitudional wave. (1 mark)
      2. Using a lline with a double arrow, indicate on the diagram a distance d equal to one wavelength of the wave (1 mark)
      3. On the diagram, show with an arrow the direction of motion of the air particle P as the waves pass (1 mark)
    4. State one advantage of optical fibre cable over conventional copper cables as used in telecommunication (1 mark)
      • Have higher carrying capacity
      • They are thinner and lighter than copper cables hence easier to install
      • They are secure since the information on transit cannot be tapped
      • Free from interference from electromagnetcic waves
    1. Figure 12 shows ammeters and resistors connected to a battery of em.f. 15.0 V and negligible internal resistance
      p2 fig 12
      1. Find the reading of the meter P
        P = 5 − 2 = 3 A
      2. If the resistance R1 is 1.2 Ω , determine as shown in figure 1 the values of R2
    2. Figure 13 shows two charged plates P and Q one is earthed and the other is connected using a copper wire to the cap of an elctroscope which was initially unchanged. 
      p2 fig 13
      State what happens to the leaf of the eletrocope when plate P is moved sideways while keeping plate Q and distance of separation constant. (1 mark)
      Leaf divergence increases
    3. Figure 14 shows a circuit where a battery of emf 4.5 V, switches A and B, two capacitors C1 = 0.4 μF and C2 = 0.6 μF and a voltmeter are connected.
      p2 figure 14
      Determine the voltmeter reading when
      1. Switch A is closed and switch B is open (1 mark)
        V= 4.5 V
      2. Switch A is closed and opened, and then B is closed (3 marks)
        p2 ans 16cii
    4. Figure 15 shows a conductor C placed on a negatively charged polythene rod. Identify the charges on conducto C and sphere S when C is connected to S using a wire
      p2 fig 15
      Conductor C Positive charge (1 mark)
      Sphere S  Negative charge(1 mark)
    1. Figure 16 below shows a steel bolt being magnetized
      p2 fig 16
      Identify the pole P and Q of the resulting magnet
      P- South pole
      Q - North pole (both correct one mark)
    2. Figure 17 shows an electric device running on a battery
      p2 ans 17bii
      1. Name the parts labelled X and Y (2 marks)
        X- Split ring or commutator
        Y - Carbon brushes
      2. Given the direction of rotation of the coil is as shown in the diragram, indicate the poles of the magnet on the diagram
      3. If the battery was replaced with a copper wire, sketch on the axes below a graph of induced current against time flowing through the variable resitors if the coil is made to rotate as it is in the figure above (1 mark)
        p2 ans 17biii
      4. Suggest an improvement that can be made to increase the magbitude of induced current in  (iii) above
        • Increasing the amount of current through the coil
        • Using stronger magnets
        • Winding the rectangular coil on a laminated soft iron core
        • Increasing the number of turns
    3. The input voltage of a transformer is 220 V and its output voltage is 12 V. When a 60 W bulb is connectd across the secondary coil, the current in the primary coil is 0.32 A. Determine the efficiency of the transformer (3 marks)
      p2 ans 17c
    4. One of the causes of energy loss in a tranformer is through fromation of eddy currents. State one way in which eddy currents lead to energy loss in a transformer (1 mark)
      • Eddy currents produce magnetic flux that opposes the change causing it (according to Lenz's law)
      • Eddy currents produce heating effect of an electric current as they flow through the tranformer core
    5. Figure 18 shows a connection to a three pin plug
      p2 fig 18
      1. Identify the leads labelled B and C (2 marks)
        B- Neutral wire/cable
        C - Live lead/cable
      2. Give a reason why the pin onto which lead A is connected is normally longer than the other two pins (1 mark)
        Earth pin is longer since it is the first one to enter hence opens the socket pushing the shutter/ bliner for the other two pins to enter
    1. Figure 19 shows a human eye with a defect
      p2 fig 19
      1. Name the defect (1 mark)
        Long sight/Long sightedness/Hypermertropia
      2. State one possible cause of this defect (1 mark)
        • Too short eye ball
        • Eye lens having very long focal length 
    2. A lens forms an image that is four times the size of the object on a screen. If the distance between the object and the screen is 150 cm, determine;
      1. State with reasons what type of lens was used (2 marks)
        Convex/converging lens
        Because it forms a real image on the screen
      2. The focal length of the lens (3 marks)
        p2 ans 18bii
    3. A barber holds a concave mirror a short distance from his client's face. Given that the described arrangement is as shown on Figure 20 and the radius of curvature is 40 cm
      p2 ans 18c
      1. Draw on the same figure, a ray diagram to show the position of the image of the client's face (2 mark)
      2. Use the ray diagram to determine the magnification of the image (1 mark)
        m= v/u = 20/10 = 2 ± 0.2 
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