Kenya Certificate of Secondary Education
2020 Physics paper 2
Section A (25 Marks)
Answer all the questions in this section.
1. State the observation made on the image in a pin hole camera when the distance between the object and the pin hole is reduced.(2 marks)
The image size increases/ becomes larger/ bigger/ magnified.
2. Figure 1 shows a gold leaf electroscope
(a) Name the part labelled A.(1 mark)
Protect the surrounding of the metal rod and leaf from damage or draughts /external effects external effects. The e.m.f. across it’s terminals
(b) State the function of the part labelled B.(1 mark)
3. State two measurements that should be taken for one to decide whether a lead acid accumulator is due for charging. (2 marks)
The emf across it’s terminals/Voltage/ pod across The relative density of the electrolyte/ density of acid/electrolyte (not liquid)
4. Explain what happens to the speed of a water wave as it moves from the shallow to the deep end in a ripple tank.(2 marks)
the speed increases since the wavelength λ increases but the frequency is same because source is the same (no mark for contradiction)
5. The critical angle for a ray travelling from glass to air is 42°. Determine the refractive index of the glass.(3 marks)
π = 1⁄sin c
= 1⁄sin 42°
= 1⁄0.669
= 1.495 (accept 2 dp – 1.49/ 1.50/ 1.494)
6. Figures 2(a) and 2(b) show two circuit diagrams with identical lamps and identical cells.State with a reason which of the bulbs will be brighter(2 marks)
B.The two cells series provide a higher electromagnetic force/ potential difference/ current/ voltage 7.
7. Figure 3 shows an annular ring
(uniformly distributed and must be on the surface)
8. When iron filings are sprinkled onto a bar magnet, it is observed that there are more iron filings at the ends than in the middle. Explain this observation.(2 marks)
There is a greater magnetic force at the ends due to increased field lines at the ends of the bar magnet than at the centre of the bar magnet./ high flux density/ or due to higher concentration of field lines at the ends.
9. Draw a diagram to show a p-n junction connected in the reverse bias mode.(2 marks)
(reject incomplete circuit)
10. A broadcasting station produces radio waves of wavelength 800 m. Determine their frequency (speed of air is 3 x 108ms-1)(2 marks)
f = 3 x 108 λ
3 x 108/800 = 0.00375 x 108 Hz
= 3.75 x 105Hz
11. Explain how x-rays are produced in the x-ray tube(2 marks)
Electrons are produced by thermionic emission
Electrons are accelerated by high voltage
Electrons are suddenly stopped to produce x-rays/ stopped by hard surface
(marks not tied. They are independent)
12. State the purpose of a fuse in an electrical circuit.(1 mark)
To disconnect circuit when excess current flows( break)/ protect/ guard appliances from excess current/ electrical fire
13. Figure 4 shows circular water waves incident on a plane reflector placed at an angle to the path of the waves.Complete the diagram to show the reflected waves.(2 marks)
curve correctly
angle of reflection
(check for correct shapes)
(correct wavelength and angles)
SECTION B (55 MARKS)
14.(a) State two ways of minimising power losses during the transmission of electric power.(1 mark)
Stepping up the voltage/ current
Use of good conductor cables/ thick cables
(b) An electric cooker is rated 2.5 kW, 250 V. State the meaning of these values. (1 mark)
The electric cooker has power output of 2500W when it operates at a potential of 250v, P = V1
(c) A consumer has the following appliances in the house:
An electric iron rated 1500 W
A water heater rated 500 W
An electric cooker rated 2500 W
Three bulbs each rated 60 W.
The house is fitted with a 12 A fuse. Determine:
(i) whether the consumer can connect all the appliances to the 240 V power
supply at the same time;(4 marks)
Total power = 1500 + 2500 + 500 +( 60×3)
= 4680 W
Total current required = 4680/240 = 19.5A
(ii) the resistance of the heating element used in the electric cooker.(3 marks)
Hence fuse blows and disconnects the current when it exceeds 10A ie, all appliances can’t be connected at the same time/ max current for all appliances connected is higher than the fuse rating.
15.(a) Describe how the focal length of a concave mirror can be determined using a screen and a metre rule.(1 mark)
Using the mirror focus a distant object on to the screen
Adjust the distance the screen & mirror to obtain a sharp image
Measure the distance between the screen & the mirror – this is the focal length of the mirror
(using the metre rule measure the distance between the screen and the mirror)
(b) An object is placed 8cm from a concave mirror whose radius of curvature is 20cm. Determine the position of the image.(3marks)
1/f = 1/u + 1/v
1/v = 1/10 – 1/8
1/v = 4 – 5/10
v= 40cm
(c) An object of height 10 cm is placed 30 cm from a converging lens of focal length 18 cm.
(i) On the grid provided. draw a ray diagram to locate the position of the image formed.(3 marks)
For diagram, check:
Correct scale
Correct rays
Correct image post
Correct tilt
(ii) From the diagram in part
(i), determine the:
(I) image height;(2 marks)
Image height = 15cm
(II) image distance.(2 marks)
Image distance = 45cm
16.(a) State two uses of radioactivity in medicine.(1 mark)
Sterilization of medical equipment
Treatment of malignant growths/ radio therapy/ killing cancerous cells / removing thyroid glands
(b) The following is a nuclear reaction equation: 63Li + 10n → 31H + xyP
Determine the values of x and y.(2 marks)
x = 4
y= 2
(c) Figure 5 shows a radioactive element placed in an evacuated glass chamber. The element produces alpha, beta and gamma emissions.
The three emissions pass through an electric field.
(i) Complete the diagram to show the path of each of the emissions.(3 marks)
Don’t award when passing through the plate
Deflected earlier and moves further
Deflected later and doesn’t move further
(ii) State the reason why:
To shield the radiation from moving to the other direction
Prevent collision of clear vision on the effect of the field/ minimise loss of / prevent ionisation
(I) the radioactive element is kept inside a lead box:(1mark )
(II) the chamber is evacuated.(1 mark)
(d)(i) Arrange the following electromagnetic waves in the order of decreasing frequency: Microwaves; Gamma rays; Radiowaves; X-rays.(1 mark)
Gamma rays, X- rays, microwaves , radio waves
(ii) The half life of a certain radioactive substance is 24 days. Given that the initial
(i) sample of the substance has a mass of 64 g, determine the mass which is left after 72 days.(2 marks)
64 → 32 → 16 → 8
24 day 48 day 72
→ 3 half lives → 8g left
alternatively,
64 x (1/2)3
17.(a)(i) Name the three components of the electron gun in a cathode ray tube.(3 marks)
The heating coil/ cathode
Grid
The anodes
(ii) State one difference between the deflection systems of a cathode ray tube and the television tube.(1 mark)
The cathode ray tube uses plates for deflection while a television tube uses coils ( CRT – electric fields while TV uses magnetic field)
(b) Figure 6 shows a graph of stopping potential against the frequency for a certain photo emissive surface, drawn by a student from the data collected when carrying out an experiment on photoelectric effect.
From the graph, determine the:
(i) threshold frequency of the surface:(3 marks)
eVs = hf – hfo
at Vs = 0
hf = hfo
f = fo which is obtained by extrapolating the graph to obtain the value of fo when Vs=0
4.6 x 1014 1 +2 ± 0.1
(ii) plank’s constant h, given that the energy of the incident photon is 1.6X 10-19 J;
(3 marks)
(iii) work function of the surface.(3 marks)
18.(a) Figure 7 shows a soft iron ring placed between the poles of two bar magnets. On the figure, draw the magnetic field lines between the poles. (2 marks)
(b) Figure 8 shows two straight conductors P and Q connected to a battery and a variable resistor.
(i) Using arrows, indicate on the diagram the direction of current that flows through P and Q when the switch is closed.(1 mark)
(ii) State what is observed as the current flows through the conductors.(1 mark)
The two conductors repel/ move away/ distance below conductors increases
(iii) Explain the observation in (ii).(3 marks)
As the current flows, a magnetic field develops around each conductor such that the field between the conductor reinforce each other creating a stronger force.
(c)i. State how eddy currents are minimised in a transformer.(1 mark)
By laminating the core
ii. A step down transformer has 600 turns in the primary coil. The input voltage is 120 V while the output voltage is 24 V. Determine the number of turns in the secondary coil.(3 marks)
