KCSE 2012 Physics Paper 2 with Marking Scheme

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KCSE 2012 Physics Paper 3 with Marking Scheme

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SECTION A (25 marks)

Figure 1, shows a plane mirror XY placed equidistant from two parallel lines AB and PT.

Four students stand at P, Q, R and T infront of the mirror
1. Indicate the positions of the images of students Q, R and T on line AB. (1 mark)
2. State which of the images are visible to the student standing at P. (1 mark)
3. Using rays indicate on the figure, how (b) above is possible. (1 mark)
Figure 2 shows two mirrors PQ and QR inclined at an angle of 110°. A ray of light is incident on mirror PQ at an angle of 60°.

Complete the diagram to determine the angle of reflection of the ray in the mirror QR. (3 marks)
Figure 3, shows three identical light bulbs connected to a 15 volts battery whose internal resistance is negligible.

Determine the reading of the voltmeter V. (2 marks)
Figure 4, shows a negative point charge placed near a positively charged rod.

Draw on the diagram the resulting pattern. (2 marks)
Figure 5, shows an object O at the bottom of a beaker full of a liquid. An observer above the beaker sees its image at point X inside the liquid.

$refractive index kcse 2012$

Determine the refractive index of the liquid. (2 marks)
Figure 6 shows a narrow beam of radiation from a radioactive source, incident to post a card. The emergent radiation passes through a magnetic field which is perpendicular to the plane of the paper, and into the paper.

A detector moved along line AC only detects radiations B and C. State the types of radiation detected.
(1 mark)
Figure 7, shows two similar coils P and Q around the end L and M of a piece of soft iron. A steady current passes through the coils.

State the polarity of the resulting magnet at end L. (1 mark)
Light from a lamp falls on the cap of a negatively charged electroscope. It is observed that the divergence of the leaf decreases. Explain the observation. (2 marks)
Figure 8, shows an object O placed infront of a diverging lens whose principal focus is F.

On the figure, draw a ray diagram to locate the image formed. (2 marks)
Figure 9, shows the cross-section of an optical fibre made of two types of glass A and B. The refractive index of B is lower than that of A.

A ray of light enters the optical fibre at P and emerges from Q.
1. Sketch the path of the ray through the fibre. (1 mark)
2. State the reason why light travels through the fibre in (i) above. (1 mark)
Figure 10, shows the cross section of a conductor held between two magnets and carrying a current out of the paper.

Indicate with an arrow the direction in which the conductor move when it is released. (1 mark)
State why the alternating current (a.c) is used for transmitting electricity over long distances. (1 mark)
Figure 11, shows an alternating current (a.c) connected across a diode D and resistor R.

On the axes provided sketch the output as observed in the CRO connected across R. (1 mark)

SECTION B: (55 marks)
Answer ALL questions in the spaces provided.
1. Figure 12, shows a displacement time graph for a progressive wave.
  1. State the amplitude of the wave. (1 mark)
  2. Determine the frequency of the wave. (4 marks)
  3. Given that the velocity of the wave is 20 ms^-1, determine its wavelength. (3 marks)
2. Figure 13 shows two identical dippers A and B vibrating in water in phase with each other. The dippers have the same constant frequency and amplitude. The waves produced are observed along line MN:
  
  It is observed that the amplitudes are maximum at points Q and S, maximum points P and R.
  1. Explain why the amplitude is maximum at Q. (2 mark)
  2. State the amplitude is minimum at R. (1 mark)
  3. State what would happen if the two dippers had different frequencies. (1 mark)
Figure 14, shows a circuit in which a battery, a switch, a bulb, a resistor P, a variable resistor R, a voltmeter V and two ammeters A₁ and A₂ of a negligible resistance are connected.

P has a resistance of 10Ω. When the switch is closed A₁ reads 0.10A and voltmeter reads 1.5V.
1. Determine;
  1. The current passing through P. (3 marks)
  2. The resistance of the bulb. (2 marks)
2. The variable resistor Q is now adjusted so that a larger current through A₂.
  1. State how this will affect the resistance of the bulb. (1 mark)
  2. Explain your answer in b(i). (2 mark)
3. A house has one 100W bulb, two 60W bulbs and one 30W bulb. Determine the cost of having all the bulbs switched on for 70 hours, given that the cost of electricity is 40 cents per kilo-watt hour. (3 marks)
1. Figure 15, shows two coils A and B placed close to each other. A is connected to a steady D.C supply and a switch, B is connected to a sensitive galvanometer.
  1. The switch is now closed. State the observation made on the galvanometer. (2 marks)
  2. Explain what would be observed if the switch is then opened. (2 marks)
2. The primary coil of a transformer has 1000 turns and the secondary coil has 200 turns. The primary coil is connected to a 240V a.c mains, supply.
  1. Explain how the e.m.f is induced in the secondary coil. (2 marks)
  2. Determine the secondary voltage. (3 marks)
  3. Determine the efficiency of the transformer given that the current in the primary coil is 0.20A and in the secondary coil is 0.80A. (3 marks)
1. Figure 16, shows a graph of magnification against object distance, for an object placed infront of a lens of focal length 20cm.
  
  Using the graph:
  1. State the effect on the size of the image as the object distance is increased from 25cm. (1 mark)
  2. Determine the distance between the object and the lens when the image is the same size as the object. (2 mark)
  3. Determine the image distance when the object distance is 25 cm. (3 marks)
2. Figure 17, shows an object O placed in front of a converging mirror of focal length 15cm.
  
  Draw on the figure a ray diagram to locate the image formed. (3 marks)
3. State why parabolic reflectors are used in car headlights. (1 marks)
Figure 18 shows parts of an x-ray tube.
1. Explain why:
  1. A potential difference is applied to the filament. (2 marks)
  2. A high potential difference is applied between the cathode and the anode. (2 marks)
  3. Most of the tube is surrounded by lead. (1 mark)
2. State how the resulting x-rays are affected by increasing the potential difference between the anode and the cathode. (1 mark)
3. Light of frequency 7.5×10¹⁴ Hz strikes a metal surface whose work function is 4.0×10^-19J. Determine the kinetic energy of the emitted photoelectrons.
  (take planks constant h = 6.63 × 10^-34 Js)

MARKING SCHEME

1. (1 mark)
2. T and R (1 mark)
3. - Reflected ray from T and R moves towrds P; (1 mark)
(3 marks)
V + V + ^V/₂ = ^5V/₂^5V/₂ = 15V ✓
V = 6V
∴ ^v/₂ = ⁶/₂ = 3V✓ (2 marks)
Check correct direction of field lines (2marks)
Refractive index = real depth
apparent depth
= ⁴⁰/₃₀
= 1.33 ( 3 marks)
β and γ rays;
L - south pole;
UV light ejects electrons by photo electric;
emission reducing the negative charges; (2 marks)
(3 marks)
2. Rectilinear propagation
  F; correct direction (1 mark)
(1 mark)
Alternating current can be stepped up, or enhances reduced power losses; (1 mark)
(1 mark)

SECTION B

1. 1. amplitude = 5cm ✓ (1 mark)
  2. T = 20s ✓
    f = ¹/_T ✓
    ^{f = 1}/₂₀ = 0.05H₂ ✓ ( 4marks)
  3. V = λf ✓
    λ = 20 ✓
    0.05
    =400m ✓ (3marks)
2. 1. Waves at Q are in phase✓ so there is constructive interference. ✓ ( 2marks)
  2. Waves are out of phase hence destructive interference. ✓ (1 mark)
  3. Interference pattern would disappear. ✓ (1 mark)
1. 1. V=IR ✓
    10I = 1.5✓
    I = 0.15A✓ (3 marks)
  2. bulb = 0.1A✓
    R × 0.1 = 1.5✓
    R=15Ω✓ (2 marks)
2. 1. the resistance of the bulb would increase;
  2. Current is higher hence increases; temperature increased temperature results in increased resistance; (2 marks)
3. Number of units = (0.1 x 10 + 0.06 × 10+ 0.03 x 10)
  = 1.9 units;
  Cost = 1.9 x 40 x 7;
  = Ksh 5.32; (3 marks)
1. 1. Pointer deflects upto a certain;maximum value and then returns to zero; (2 marks)
  2. There is a deflection in the opposite direction then back to zero; As Flux in A falls, flux in B also falls and causes induced e.m.f in the opposite directions; (2 marks)
2. 1. Current in the primary is constantly changing its direction; so that the resulting flux (which link coils) is constantly changing its direction. Therefore alternating e.m.f is induced in the secondary coil; (2 marks)
  2. Vs = Ns
    Vp Np
    Vs = 200
    240 1000
    Vs = 48V; (3 marks)
  3. Efficiency = power output × 100%
    Power input
    = Is Vs × 100%
    IpVp
    = 0.8 × 48 × 100%
    0.2 × 240
    = 80% (3marks)
1. 1. The image diminishes ( becomes smaller); (1 mark)
  2. m = 1 ⇒ ^V/_u = 1
    V = u = 40cm; (2 marks)
  3. u = 25
    m = 4
    m = ^v/_u
    ^v/₂₅ = 4
    V = 100cm; (3marks)
2. ( 3 marks)
3. A bulb/ lamp placed at principal focus will give a wide parallel beam; (1 mark)
1. 1. To produce electrons; by thermionic emission; (2 marks)
  2. To accelerate the electrons to give them enough K.E. to produce X-rays at the anode;
    (2 marks)
  3. To absorb stray X-rays, thus protecting the operator from those rays; (1 mark)
2. Increases K.E. of electrons and hence causes X-rays of higher frequency; (1mark)
  OR .
  - X-ray are more penetrative
  - X-rays of shorter wavelength.
3. E=hf;
  = 6.63 x 10^-34 x 7.5 x 10¹⁴
  = 4.97 x 10^-19J;
  K.E = 4.97 x 10^-19 − 4.0 x 10^-19;
  = 0.97 x 10^-19J; (4 marks)