INSTRUCTIONS TO CANDIDATES:
- This paper consists of two Sections; A and B.
- Answer ALL the questions in sections A and B in the spaces provided.
- All workings must be clearly shown.
- Non-programmable silent electronic calculators and KNEC Mathematical tables may be used.
SECTION A: (25 MARKS)
Answer all questions in this section in the spaces provided:
- State two conditions under which a pinhole camera may form an image on its screen which has the same size as the object. (2mks)
- The figure shows a ray of light incident along the normal. The mirror is rotated at an angle of 15º in a clockwise direction without changing the position of the incident ray,
Determine the angle between the reflection ray and the incident ray. (2mks) - A steel is to be magnetized by electrical method as shown below. Identify the pole P and Q of the resulting magnet. (1mk)
P:
Q: - A small chain is often seen hanging at the back of a petrol carrying lorry. State and explain its significance. (2mks)
- The figure below shows two waveforms representing the same wave motion.
Determine the velocity of the wave. (3mks) - An object O is placed in front of a concave mirror and on the principal axis, as shown in the figure below. Complete the light ray diagram to locate the position of the image. (3mks)
- Arrange the following radiations in order of increasing wavelengths. (1mk)
Infrared, blue light, ultraviolet, radiowaves, χ-rays. - The figure below shows a block diagram of a p-n junction diode.
On the same diagram, show how a cell may be connected so that it is reverse biased. (1mk) - A girl standing at a distance claps her hands and hears an echo from a tall building 2 seconds later. If the speed of sound in air is 340m/s, determine how far the building is? (3mks)
- What do you understand by polarization as used in a simple cell? (1mk)
- State how the defect mentioned in question 10 above is minimized in a simple cell. (1mk)
- A current-carrying conductor AB is in a magnetic field as shown in the figure below.
- Indicate the direction of force F acting on the conductor. (1mk)
- State two factors that determine the direction of the force F. (2mks)
- You are given three resistors of values 5Ω, 8Ω and 12Ω. Show in a circuit diagram how you would connect them so as to give:
- An effective resistance of 9.8Ω. (2mks)
- The least effective resistance. (1mk)
SECTION B: (55 MARKS)
Answer question in this section in the spaces provided.
-
- Define refractive index. (1mk)
- The critical angle of a certain material medium is 43.2º. Determine the refractive index of the material. (2mks)
-
- What do you understand by the term accommodation? (1mk)
- The diagram below shows a certain defect of vision. Name the defect. (1mk)
- On the figure below show how the defect can be corrected. (2mks)
- An object is placed 40cm in front of a concave lens of focal length 20cm; determine the position of the image. (3mks)
-
-
- State Lenz’s a law of electromagnetic induction. (1mk)
- A bar magnet is moved into a coil of insulated copper wire connected to a centre-zero galvanometer, as shown in the figure below.
- Show on the diagram the direction of induced current in the coil. (1mk)
- State and explain clearly what is observed on the galvanometer when the S-pole of the magnet is moved into and then withdrawn from the coil. (4mks)
- A transformer has 800 turns in the primary and 40 turns in the secondary winding. The alternating e.m.f connected to the primary is 240V and the current is 0.5A.
- Determine
- The secondary e.m.f (2mks)
- The power in the secondary if the transformer is 95% efficient. (2mks)
- Explain how energy losses in a transformer are reduced by having:
- A soft-iron core. (2mks)
- A laminated core. (1mk)
- Determine
-
-
-
- Distinguish between thermionic emission and photoelectric emission. (2mks)
- State one factor which affects the rate of each of the above types of emission.
Thermionic emission. (1mk)
Photoelectric emission. (1mk)
- Sodium has a work function of 2.3eV. Given that: Planck’s constant h = 6.63 × 10-34JS, velocity of light in vacuum, C = 3.0 × 108m/s, 1 electron-volt (1eV) = 1.6 × 10-19 C and mass of an electron, me = 9.1 × 10-31kg, calculate:
- Its threshold frequency. (2mks)
- The maximum velocity of the photoelectrons produced when the sodium is illuminated by light of wavelength 5.0 × 10-7m. (4mks)
- The stopping potential V, with the light of this wave length. (2mks)
-
-
- State two advantages of using a Cathode Ray Oscilloscope (C.R.O) as a voltmeter over the ordinary voltmeter. (2mks)
- An X-ray operates at 30000V and the current through it is 2mA. Given that the charge of an electron is 1.6 × 10-19C, h = 6.63 × 10-34JS, speed of light, C = 3.0 × 108m/s,
Calculate:-- The maximum kinetic energy of the electrons when hitting the target.(2mks)
- The number of electrons hitting the target per second. (2mks)
- The minimum wavelength of the X-rays emitted. (2mks)
-
- A radioactive carbon-14 decays to nitrogen by beta particles as shown below.
Determine the values of χ and y. (2mks) - Study the graph then answer the questions
- What is the half-life of carbon-14? How do you know? (2mks)
- What percentage of acarbon-14 remains after 17,190 years? (3 marks)
- The figure below shows the cross-section of a diffusion cloud chamber used to detect radiation from radioactive sources.
- State the function of the following:
- Alcohol. (1mk)
- Solid CO2. (1mk)
- Explain briefly how the diffusion cloud chamber can be used to detect and identify alpha particles. (3mks)
- State the function of the following:
- A radioactive carbon-14 decays to nitrogen by beta particles as shown below.
MARKING SCHEME
-
- When the object distance from the pinhole is equal to the image distance. ✓ (1mk)
- When the screen is as large as the object. ✓ (1mk)
- χ = 2Ө
= 2 × 15 ✓ (2mks)
= 30º ✓ -
- P: South Pole ✓
- Q: North Pole ( 1mark for both)
- Friction generates ✓ charges on the lorry the chain discharges ✓ the lorry to prevent sparks which may lead to explosion. (2mks)
- λ = 0.4m✓
T = 0.06m ✓ (3mks)
V = λf = 0.4 × 0.06
= 0.24m/s ✓ -
- X-ray → Ultraviolet → Blue light → Infrared → Radio waves (1mk)
-
(1mk) -
V = 2S/t
S = Vt
2 ✓
= 340 x 2
2 ✓
= 340m ✓ (3mks) - Process by which hydrogen gas bubbles form an insulating layer on the positive copper plate. ✓ (1mk)
- Adding potassium dichromate powder/depolarizing agent/oxidizing agent which oxidizes hydrogen to water. (1mk)
-
-
(1mk) - Direction of magnetic field. ✓ (1mk)
-
-
(1mk)
(1mk)
SECTION B: (55 MARKS) -
- Refractive index is ratio of sine of angle of incidence to the sine of angle of refraction for a given pair of media. ✓ (1mk)
n = 1/Sin C -
= 1/Sin 43.2° ✓
= 1.4608 (2mks)
= 1.46 ✓ -
- Accommodation is the fine adjustment of focal length of the eye so as to fit images of objects of different distances on the retina. ✓ (1mk)
- Long sightedness or (hypermetropia) ✓
Correct lens ✓
Correct rays ✓ (2mks)
-
1/f = 1/V + 1/U
−1/20 = 1/V + 1/40 ✓
1/V = −1/20 − 1/40 ✓ = −2 − 1/40 = 3/40 (3mks)
V = − 40 cm
3 ✓ = − 13.3cm
- Refractive index is ratio of sine of angle of incidence to the sine of angle of refraction for a given pair of media. ✓ (1mk)
-
-
- Lenz’s law – The direction of the induced current is such that the induced current which it causes to flow produces a magnetic effect that oppose the change producing it. ✓ (1mk)
-
-
(1mk) - S-Pole of magnet moved into the coil.
- Galvanometer deflects to the left (on one side) momentarily ✓
- Induced current in the coil flows so as to form a South Pole at the end of the coil near the magnet. ✓ (2mks)
S-Pole of magnet withdrawn. - Galvanometer reverses its direction of deflection/deflects to the right momentarily.
- Changing the direction of motion ✓ reverses the direction of induced current so that the end of the coil tends to form a N-Pole so as to oppose. ✓ the motion of the magnet out of the coil. (2mks)
-
-
-
-
NP = 800 NS = 40
VP = 240V VS =
IP = 0.5A IS
Ns = Vs
Np Vp
40 = Vs
800 240 ✓ (2mks)
Vs = 40 x 240
800
= 12V ✓ -
VS IS = 95% VP IP ✓
= 95 x 240 x 0.5
100 (2mks)
= 114W ✓
-
-
- Soft iron is easily magnetized and easily demagnetized; ✓ this reduces heat loss due magnetization and demagnetization (hysteresis loss) ✓ (2mks)
- Laminated core increases resistance of the core which reduces the size of eddy currents thus reducing heating due to the eddy currents. ✓ (1mk)
-
-
-
-
- Thermionic emission – Is the escape or evaporation of electrons from a metal surface when heated ✓
Photoelectric emission – Is the escape or evaporation of electrons from a metal surface when light of suitable frequency falls on the metal surface. ✓ (2mks) - Thermionic emission – Temperature of metal ✓
OR - Type of metal ✓ (1mk)
Photoelectric emission - Frequency of incident light. ✓
OR - Work function/threshold frequency ✓ of the metal (1mk)
- Thermionic emission – Is the escape or evaporation of electrons from a metal surface when heated ✓
- hfo = Wo ✓
6.63 × 10-34 fo = 2.3 × 1.6 ×10-19 (2mks)
fo = 5.55 × 1014HZ ✓ -
-
- hfo = Wo ✓
-
-
-
- Has no coil to burn out ✓
- Has instantaneous response. ✓
- Has nearly infinite resistance, therefore draws very little current.
- Can measure both AC and DC voltages. Any two (2mks)
-
- Ke = ½MV² = eV
= 1.6 × 10-19 × (30,000)²
= 1.44 × 10-10J - I = ne
= 2 × 10ˉ³
1.6 × 10-19 ✓ (2mks)
n = 1.25 × 1016 electrons ✓ - hfmax = eV But fmax = e/λmin
(2mks)
- Ke = ½MV² = eV
-
-
- 14 = χ + 0
χ = 14 ✓
6 = 7 + y
y = −1 ✓ (2mks) -
- T½ = 7500 – 2500 ✓ = 5000 years ✓ (2mks)
- No of half life’s = 15000/5000 = 3
N1 = No (½)³
= 100 (½)³ (3mks)
= 12.5g ✓
OR
-
-
- I Alcohol condenses around the ions formed by the radiation to form the narrow tracks ✓ (1mk)
- Sold CO2 – Cools the bottom of the chamber and also condenses alcohol below its normal temperature. ✓ (1mk)
- Sold CO2 cools the bottom of the chamber alcohol vapourises and spreads through the chamber. ✓ It is cooled below its normal temperature incase of a radiation, air around the path of the radiation is ionized. ✓ Alcohol condenses around these ions to form a narrow cloud. ✓ (3mks)
-
- 14 = χ + 0
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