Physics Paper 2 Questions and Answers - Form 4 End Term 1 Exams 2022

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Questions

INSTRUCTIONS

This paper consists of two sections, Section A and B. Answer ALL the questions in both sections in the spaces provided in this paper.
ALL working must be clearly shown.
KNEC mathematical tables and non-programmable silent electronic calculators may be used.
Note: Take acceleration due to gravity g = 10m/s2

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

State any one condition under which a pinhole camera may form an image on its screen which has the same size as the object. (1 mark)
The source of sound and two listeners are positioned close to a tall building as shown in the
figure below.
1. State the property of sound by which Listener A is able to hear the sound produced. (1 mark)
2. Listener B is moving directly towards Listener A and has a problem hearing the sound produced. Explain. (2 marks)
Give two differences between a transformer and induction coil. (2 marks)
The figure below shows some region of part of the electromagnetic spectrum
1. Name the region that is detected by a blackened bulb thermometer. (1 mark)
2. State one use of the E-M wave in the region labelledB. (1 mark)
The figure below shows ultraviolet light striking a clean Zinc plate on a positively charged leaf electroscope.

Explain the following observations:
1. The leaf does not fall. (1 mark)
2. When the same experiment is carried with a negatively charged electroscope, the leaf falls. (1 mark)
A girl observes her face in a concave mirror of a focal length 90cm. If the mirror is 70cm away, state two characteristics of the image observed. (2 marks)
The ammeter in the circuit in figure 3 has negligible internal resistance. The cell has an internal resistance of 0.5Ω and an electromotive force of 3.0V.

Figure 3
Determine the value of current the ammeter registers when switch S is closed. (2 marks)
An electric heater is rated 3 kW. Find the electrical energy in kWh(kilo watt- hours) consumed by the heater when used on a 240V supply for 180 minutes. (2 marks)
An echo sounder produces a pulse and an echo is received from the seabed after 0.4 seconds.
If the speed of sound in water is 1500m/s. Calculate the depth of the sea bed. (2 marks)
The set up in figure 4 below can be used in a laboratory for lifting and releasing a steel ball.
1. State the material which is suitable for use in the core. (1mark)
2. If a slightly larger ball is to be lifted, it is necessary to make an electromagnet stronger.
  Name two ways of increasing the strength of the magnet. (2 marks)
The figure 5 below shows the path of a ray of light passing through a rectangular block of Perspex placed in air.

Calculate the refractive index of Perspex. (2 marks)
The sketch shown below is a displacement-time graph of a wave traveling at 320ms-1

Find the wavelength of the wave. (2 marks)

SECTION B (55 MARKS)
Answer ALL the questions

1. The mains electricity is transmitted through high tension voltage (H.T).
  State one danger of this transmission. (1 mark)
2. The figure shows a section of a domestic wiring.
  1. Identify two serious mistakes in the circuit. (2 marks)
  2. Explain why the circuits in domestic wiring should be connected in parallel with the main supply. (2 marks)
3. Study the figure shown below
  1. State the name of the machine shown in figure above. (1 mark)
  2. What are the names of the parts labelled A and B? (2 marks)
    A ___________________________________________________________________________
    B ___________________________________________________________________________
  3. What would be the effect of doubling the number of turns of the coil if the speed of rotation remained unchanged? (1 mark)
4. The voltage-time signals obtained is as shown in the diagram below in graph A
  1. Find the frequency of the alternating E.m.f shown by graph A. (2 marks)
  2. What alterations have been made to the machine in c) above to produce the emf represented by graph B? (1 mark)
The figure below shows the features of an X- ray tube
1. Name the parts labelled A and B. (1 mark)
  A ___________________________________________________________________________
  B ___________________________________________________________________________
2. Explain how a change in the potential across PQ changes the intensity of the X-rays produced in the tube. (2 marks)
3. What property of lead makes it suitable for use as shielding material? (1 mark)
4. In an X-ray tube operating at 100kV,the tube current is 20mA. Determine the number of electrons hitting the target every second. (2 marks)
  (Charge of an electron = 1.6 10−19C)
5. The figure below shows a cell. Use it to answer questions that follow:
  1. Name the type of cell. (1 mark)
  2. Label the cathode. (1 mark)
  3. The maximum kinetic energy of the photo-electrons emitted from a metal surface is 9.95 x 10^-20J. If the threshold frequency of light required to cause photo-electric emission with this metal is 5 x 10¹⁴ Hz. Calculate the frequency of the incident radiation.
    (Plank’s constant = 6.63 x 10^-34 JS) (3 marks)
1. 1. Explain why carbon -14 (614 C) is radioactive while carbon 612 C is not. (1 mark)
  2. A radioactive isotope showed a count rate of 82 counts per second initially. After a time of 210 seconds, the count rate dropped by 19 counts per second. The average background count remained constant at 10 counts per second. What is the half-life of the material? (2 marks)
2. The figure below shows features of a diffusion cloud chamber used for detecting radiations from a radioactive source.
  
  Explain how the chamber works when a radioactive particle is introduced at the source. (2 marks)
3. 1. What is the purpose of solid carbon (IV) oxide. (1 mark)
  2. State one advantage of the cloud chamber over a G.M tube as a detector of radioactive radiations. (1 mark)
4. P-type and n-type semiconductors are made from a pure semiconductor by a process known as “doping”.
  1. State what is meant by doping. (1 mark)
  2. Explain how the doping produces an n-type semi-conductor. (2 marks)
5. In the circuit in the figure below, when the switch S is closed, the voltmeter shows a reading.
  When the cell terminals are reversed and the switch is closed, the voltmeter reading is zero. Explain these observations. (2 marks)
1. The figure below shows the features of a cathode ray oscilloscope;-
  1. Name the parts A and B. State the role played by each of the parts A and B. (2 marks)
    A ___________________________________________________________________________
    B ___________________________________________________________________________
  2. Explain how electrons are produced. (2 marks)
  3. State the factors considered when choosing the material for the cathode. (1 mark)
2. The figure below shows the trace on the screen of an a.c signal connected to the y-plates of a C.R.O with time base on.
  
  Given that the time base control is 100ms/div and the y-gain is at 120V/division.
  Determine:
  1. The frequency of the a.c signal. (2 marks)
  2. The peak voltage of the input signal. (2 marks)
1. The figure below shows straight wavefronts incident on convex reflector.
  Complete the diagram to show the reflected wavefronts. (1 mark)
2. Narrow slits cannot be used to demonstrate interference of sound waves. Explain. (1 mark)
3. The figure below shows sound waves emitted by a drum struck.
  Explain why the wave fronts are directed to the ground. (2 marks)
4. Figure below shows an experimental set up consisting of a mounted lens, I, A screens, a metre rule and a candle
  1. Describe how the set up may be used to determine the focal length f, of the lens. (3 marks)
  2. State why the set up would not work if the lens were replaced with a diverging lens. (1 mark)
5. The graph figure below shows the relationship between 1u and 1v for a converging lens where u and v are the object and image distances respectively.
  
  From the graph, determine the focal length, f of the lens. (3 marks)

Marking Scheme

When the object distance from the pinhole is equal✔ to the image distance.
When the screen is as large as the object✔1(any 1) Give 1 mk
1. Diffraction✔1
2. Interference.✔1
  The two opening acts as two source✔1 hence the two waves interfere producing soft and loud sound along the way✔1
1. Transformer uses a.c only while induction coil uses d.c✔
2. Transformer produces a humming sound while induction coil is quiet✔1
1. Region C ✔1
2. Microwaves – for cooking in microwave ovens.✔ (any 1x1 = 1 mk)
3. For communication – satellite communication
1. The electrons emitted from the zinc plate✔1 are attracted by the positive charge on the electroscope, hence the leaf does not fall.✔1
2. Electrons emitted from the zinc plate are repelled by the ✔1 negative charge on the electroscope hence the leaf falls.
1. Enlarged ✔
2. Virtual ✔
3. Erect/ upright✔
E = IR + Ir
3.0 = I 3.5 I 0.5✔1
∴ I = 0.75A;✔
E = p x t
3 x 180
60✔1
= 9kWh✔1
V = d/t
1500 = d0.2✔1
d = 150 2 = 300m✔1
1. Soft iron✔1 since it is easily magnetized and demagnetized.
2. Increasing the amount of current ✔1
  Increasing the number of turns on the coil✔1
Refractive index of perplex
= Sin 90°
Sin 42.50✔1
= 1
Sin 42.50✔
1 = 1.48✔1
Period = 0.03/3 = 0.01✔1
f = 1/T = 1/0.01✔1
f = 100Hz✔
1. Risk of electric shock in case pole collapses✔
  Can cause fires to vegetation and structures✔
  Harmful effects from e-m radiations due to effects of electric field.
2. 1. Fuse is connected to the neutral wire✔ instead of live wire.
    Bulbs are connected to the neutral wire which is at zero potential instead of the live✔1 wire
  2. So as to receive the full voltage ✔1 from the source.✔1
3. 1. An a.c generator (dynamo)✔1
  2. A – armature✔1
    B – slip rings✔1
  3. The voltage of the induced e.m.f ✔1 doubles✔1
4. 1. f = 1T = 10.1✔1
    = 10Hz✔1
  2. The slip rings have been replaced by split ✔1 rings (commuters)
1. A – Cathode rays/ electrons/ electron beam✔
  B – Anode / copper Anode✔ (both correct 2 x ½ = 1 mk)
2. Change in p.d across PQ cause change in filament ✔1 current OR temperature of cathode increases. This changes the number of electrons released by the cathode✔1 hence intensity of X- rays.
3. High density✔1
4. Q = 1t
  = 20 10^-31
  Total change of electrons = ne
  20 10^-3 = n 1.6 10^-19✔1
  n = 1.25 10¹⁷ electrons✔1
5. 1. photo emmisive✔1
  3. E = hf
    hf = hfo + K.E max
    6.63 x 10-34 x f = 6.63 x 10-34 x 5x1014 + 9.95x 10-20
    = 3.315 x 10-19 + 9.95 x 10-20✔1
    6.63x10-34f = 13.265 x 10-19✔1
    f = 2.001 x 1015 Hz✔1
1. 1. In 612C, the neutron to proton ratio is one✔1 thus stable, while 614C N/P>1 thus unstable.
  2. Actual rate = 82–10= 72c/s,19-10 = 9c/s✔1
    72 →36 →18 →9
    2/0→3 half lies
    t½ = 2103 = 70 sec ✔1
2. The radiations ionize argon gas✔1 along their path. The alcohol vapour condense✔ on the ions formed creating tracks.
3. 1. Lower the temperature✔ in the chamber thus making it possible for the alcohol vapour to condense.
  2. The nature of radiation✔1 can be identified
4. 1. Doping in the introduction of an impurity✔1 into a pure semiconductor to improve its conductivity.
  2. Formed by adding a pentavalent atom (phosphorous) to a group 4 semi conductor (silicon) and an extra electron is left unpaired and is available for conduction. ✔1 It’s called n-type because the majority carriers are electrons.
5. Diode is forward- biased; current flows✔1
  Diode- reverse- biased; no current flows✔1
1. 1. A – Grid….. Controls the brightness of the spot ✔
    B – Y- plates….. deflects the electron beam vertically.✔
  2. When the cathode is heated, electrons are emitted from it through thermionic emission which are then accelerated and focused by✔1 the anode to the fluorescent screen.
  3. Low work function✔1
2. 1. T =4 X 100
    = 400MS✔1
    f = 1/T = 1/(400 × 10^-3) = 2.5Hz✔1
  2. = n Y-gain
    = 2 120✔1
    = 240V✔1
2. Sound waves have longer wavelengths✔1 thus they cannot be diffracted by narrow slip. (2mks)
3. There is hot air high above the ground✔1 sound travels faster in hot air than in cold air ✔1 hence change in wavelength (2 mks)
4. 1. Adjust the position of the lens until a sharp image of the flame is observed✔1
    Record the object distance (u) and the image distance (V)
    Repeat with different object positions✔1
    Use the relation f = uvu+v to determine f ✔1
  2. Diverging lens produces a virtual image which cannot ✔1 be formed on a screen
5. At 1/u intercept, 1/v = 2.5 10^-2 cm^-1✔1
  1/f= 1/u+ 1/v
  = 2.5 10^-2∴ f = 12.5 10^-2 ✔1
  1/0.025=40cm¹