## Physics Paper 2 Questions and Answers - Meru Central Cluster Exam 2020

PHYSICS
PAPER 2
TIME: 2  HOURS

INSTRUCTIONS

• This paper consist of TWO sections; A and B.
• Answer ALL the questions in section A and B in the spaces provided.
• ALL working MUST be clearly shown.

SECTION A – 25 MARKS (ANSWER ALL THE QUESTIONS)

1. Figure 1 below shows an object O placed in front of a plane mirror. A ray of light is drawn coming object O and striking the mirror at P. After striking the mirror, the ray of light is reflected.
1. Which of the four dots represent correct position of the image of O? Label this dot Q (1mk)
2. By drawing a line on the diagram above to represent the reflected ray at P, mark the angle of reflection and label it r. (1mk)
2. An echo sounder of a ship received the reflected waves from a sea bed after 0.20s. Determine the depth of the sea bed if the velocity of sound in water is 1450m/s (2mks)
3. Figure 2 below shows a simple experiment using a permanent magnet and two metal bars A and B
Put close to the iron filings.

State with a reason which bar is made from a soft magnetic material. (2mks)
4. The figure below shows a highly negatively charged rod being brought slowly near the cap of a positively charged leaf electroscope. It is observed that the leaf initially falls and then rises.

Explain this observation (2 marks)
5.
1. A generator capable of producing 100kw is connected to a factory by a cable with a total resistance of 5 ohms. If the generator produces the power at a potential difference of 5kv. What would be the maximum power available to the factory? (2 marks)
2. State one cause of power loss in transmission of the main electricity (1 mark)
6. The figure below shows eight resistors forming a network in circuit between X and Y.

Calculate the effective resistance of the network. (3 marks)
7. State:
1. One application of ultraviolet radiation (1 mark)
2. One detector of the radiation in (a) above. (1 mark)
8. The figure below shows a rectangular coil in a magnetic fields rotating in a clockwise direction.
1. Indicate the poles X and Y of the magnets. (1 mark)
2. Suggest one way of increasing the magnitude of the force in such a coil. (1 mark)
9. A battery is rated at 30Ah. For how long will it work if it steadily supplies a current of 3A.(2 marks)
10. An element R decays by giving off an alpha particle. Complete the equation below showing the valves of a and b (2mk)

a = ____________________ b = ____________________
11. The circuit diagram in figure13 below shows four capacitors connected between two points A and B.
Determine the capacitance across AB. (3mks)

Section B (55 marks)

1.
1. The figure below shows and image I formed by a concave mirror

Determine its magnification M. (3 marks)
2. The figure below shows lenses of a compound microscope. The focal length of the objective is 2 cm and that of eyepiece is 4cm. The two lenses are 9cm apart. An object 1 cm high is placed 3cm from the objective lens.
1. Construct rays to show the position of the final image seen by the eye. (4 marks)
2. Find the magnification obtained by this arrangement (2 marks)
2. The figure below shows water wave fronts
1. Approaching a boundary between a shallow and deep region. The speed of the waves in the shallow region is less than in the deep region.

On the same diagram complete the fare to show the wave fronts after crossing the boundary. (2 marks)
2. A vibrator is used to generate water waves in a ripple tank. It is observed that the distance between the first crest and the midpoint to the fifth trough is 237.5cm. The waves travel 224.0cm in 6.0 seconds.

Determine:
1. The wavelength of the waves (3 marks
2. The speed of the waves (2 marks)
3. The frequency of the vibrator (2 marks)
3. The plane water wave front are incident onto a concaved barrier as show in the figure below.

Show on the same diagram the nature of the reflected wave fronts. (2 marks)
3. The figure below shows the parts and circuit of a model X-ray tube.

1. Name the parts labeled Q and R (2marks)
Q
R
2. State the suitable material for use in Q and give a reason for your answer (2marks)
3. State the function of part R (1 marks)
4. Describe how electrons, hence X-rays, are produced in the tube (2 marks)
5. Explain why the glass tube is evacuated (2 marks)
6. What property of lead makes its suitable material for shielding (1 mark)
7. State how the following changes affect the nature of X-rays produced
1. Increasing in potential across MN (1mark)
2. Increasing the filament current (1 mark)
4.
1. What is photoelectric emission? (1 mark)
2. A radiation falls on photosensitive material state how the following changes affect the emitted photoelectrons:
1. Increase in intensity of incident radiation. (1 mark)
2. Increase in the frequency of incident radiation (1 mark)
3. The figure below shows a graph of stopping potential (voltage)V, against frequency f, of a radiation falling on a photosensitive surface.
Given that eVs = hf – hfo where h= plants constant, fo = threshold frequency i.e frequency when Vs = 0 and e is the charge on an electron = 1.6 x 10-9C. Use the graph to determine;
1. The threshold frequency for the surface (1 mark)
2. The gradient of the graph, hence the value of plank’s constant h. (3 marks)
3. The work function Wo of the surface given that Wo = Hfo for the surface (2mrk)
5. A student connected a circuit as shown in figure 16 below hoping to produce a rectified out put

1. Sketch the graph of the output on the CRO screen (1mk)
2. Explain how the output above is produced (2mks)
3. Name other two uses of a junction diode (2mks)
6. Figure 7 shows two coils A and B placed close to each other. A is connected to a steady dc supply and a switch B is connected to a sensitive galvanometer.
1.
1. The switch is now closed. State the observation made on the galvanometer 2mks)
2. Explain what would be observed if the switch is then open (2mks)
2. the primary coil of a transformer has 1000 turns and secondary coil has 200 turns the primary coil is connected to a 240v ac supply
1. Determine the secondary voltage (3mks)
2. Determine the efficiency of the transformer given that the current in the primary coil is 0.2A and in the secondary coil is 0.7A (3mks)

MARKING SCHEME

1.
2. Speed of sound =(2 ×distance)/time
depth = 1450x 0.20/2 =145 m
3. Loses its magnetism faster thereby becoming weaker hence attracting fewer iron filings√ (2mrks
4. Leaf initially falls due to negative charges neutralizing the positive charges. Excess negative charges on the cap are repelled towards the leaf and the stem of the electroscope, causing the leaf to rise.
5.
1. Current 1 power/(p.d)=100000/(500 V)=20
Power loss = 12 R = 202 X 5 = 2000w
Power available for factory = 100,000 – 2,000 = 98,000W or 98KW
2. Resistance of cables/supply of high currents.
6. Resistors in parallel
RT =5 X 5/5 X 5=25/10=2.5 4∩ =10A=∩
Resistors in series
RT=3.5+2.5+4∩ =10∩
7.
1.
• By the human skin to synthesize/manufacture vitamin D
• In bacteriology to kill harmful organisms
• In manufacture of washing powder and paints that fluoresce
• To detect forged documents such as cheques and fake currency notes in banks
• In burglar alarms
• In automatic door openers (any 1 mark)
2.
• Photographic plates/ films
• Photocells
• Light dependant resistors (LDR)
8.
1.
• X – south poole Y – noth pole : using Flemings
• left-hand rule
2.
• Increasing current
• Strengthening magnetic field
• Turns of the coil (any 1 mark)
9.
• Q = it
But 1 = 3A and Q = 30Ah
30 = 3 X t
t= 10hours
10. a= 236-4 = 232
b = 72 – 2 = 70
11.
12.
1.

Magnification = image height = 2.8√ = 2.545√
NB: Image and object heights must be measured accurately.
2.
1.

2. Magnification= M0 x Me
=2.2 x 2.59
13.
1.
2.
1. Q2 < Q1 or wavefronts refracted Away from normal or larger spacing in deep shallow region
4 3/4 Æ =237.5 CM
1 Æ = 237.5cm/ 50.0 cm

2. Speed = distance/time= 224.0cm/2.8=V=80.0cm/s

3. V = f Æ
80 = f x 50; f = 1.6Hz
3.
• Wave length spacing maintained
• Reflected wave fronts takes the shape of the reflector and more towards focal point F
14.
1. Q: target/anode
R; concave focusing cathode

2.
• Q – target/anode; tungsten or molybdenum
• It has a very high melting point
(hence not likely to melt due to very high temperatures of the target)
3. R- Concave focusing cathode. To concentrate (focus) the electron beam onto the target/anode
4.
• Current flows through the filament in the cathode and electrons are produced through thermionic emission.
• When the electrons at very high speed are suddenly stopped by the target their kinetic energy is transformed into X-rays and heat.
5. To remove air molecules so that the electrons do not lose some of their kinetic energy through collision
6. High density/ its ability to absorb most of the X-rays
7.
1. Harder/more penetrating/higher quality X-rays are produced
2. Intensity/quality of X-rays produced increases
15.
1. The setting free of electrons from a surface of a metal when illuminated/irradiated with radiation of suitable frequency.
2.
1. Increase in intensity increases the number of emitted photoelectrons (photoelectrons increases)
2. increase in frequency of incident radiation increases the kinetic energy of photoelectrons emitted threshold frequency fo = 4.6 X 1014 Hz (graph line extended to cut the frequency axis)
NB: must be from the graph
3.
1. h/e=ΔVs/Δf
ΔVs/Δf=        1.3-0.65
(7.8-6.2) × 1014
0.65
1.6 × 1014
=4.0625 ×10-15

=1.6 ×10-19 ×4.0625 1015
=6.5 × 10-34J5

3. work function W0= hfo
Wo=6.5 ×10-34 × 4.6 × 1014
=2.99 × 10-19 JA=πr2
16.
1.
2.
• During the first half –cycle D1 is forward biased while D2 is reverse biased. Hence , current takes the path A , D1 RT .√
• During the next half –cycle, D2 is forward biased while D1 is reverse biased and the path of the current is BD2RT.√
3.
• Protects a circuit from damage √
• As a switch √ Any 2
• In charging a battery using solar panels.
17.
1.
1.  The galvanometer deflects on one side and then back to zero
2. A greater deflection will be obtained in the opposite direction as current takes less time to die off than to build up. The changing current in the primary coil induces a current in the secondary coil due to charging magnetic field of the primary current.
2.
1. VS = NS = VS = 240 x 200 = 48V
VP    NP               1000
2. E = Power output x 100
Power input
= 48 x 0.7 x 100
240 x 0.2
= 70%