## Physics Paper 2 Questions and Answers - Form 3 Mid Term 2 Exams 2021

INSTRUCTIONS TO CANDIDATES

• This paper consists of TWO sections: A and B
• Answer ALL the questions in the sections A and B in the spaces provided.
• ALL working MUST be clearly shown.
• Non-programmable silent electronic calculators and KNEC mathematical tables may be used.

Section A (25marks)

1. State any one law of reflection (1mrk)
2. The figure below shows a piece of wood placed in front of a north pole of a bar magnet. Complete the diagram to show how the field pattern appears. (2mrks) 3. A material of unknown electrical properties is placed on the cap of a positively charged electroscope. The leaf is observed to fall. Giving a reason state the type of the material. (2mrks)
4. The speed of sound in air is 340ms-1, calculate its wavelength when the frequency is 256Hz. (3mrks)
5. A man standing between two walls fires a gun and hears two echoes. If he stands 200m closer to one of the walls and the time difference between the two echoes is 0.176 seconds, determine the distance between the walls.(speed of sound in air is 340ms-1
(3mrks)
6. The figure below shows a section of a counter made using an electromagnet that punches holes on a moving paper tape. 1. Describe how a hole is punched into the tape (2mrks)
2. State two ways of improving the strength of the electromagnet (2mrks)
7. A pin is placed at the bottom of a beaker filled with water to a height of 13cm. when observed from above the water it appeared to be 10cm deep. Determine the refractive index of water. (3mrks)
8. Define electrical resistance (1mrk)
9. Polarization is one of the defects of a simple cell, state one way of minimizing this defect. (1mrk)
10. Complete the ray diagram below to locate the position of the object for the image given. (3mrks) 11. Determine the work done when a charge of 16Coulomb flows across a conductor when the potential difference across it is 12V. (2mrks)

SECTION B (55 MARKS)

1. Define refractive index (1mrk)
2. A ray of light is incident in water at an angle of 30° on water – glass plane surface. Calculate theangle of refraction in the glass. ( ang= 1.5, anw= 1.33) (3mrks)
3. The graph below shows the relationship of sines of the angles of incidence and refraction for crown glass material. Use it to answer the questions that follow. 1. Determine the refractive index of the glass (3mrks)
2. Find the angle of incident in air for which the angle of refraction is 31.33°
(2mrks)
4. State one condition for total internal reflection to occur. (1mrk)
1.
1. The figure below shows wavefronts travelling towards an obstacle. Complete the path followed by the waves after being reflected. (3mrks) 2. Explain how sound travels farthest at night than in the day. (2mrks)
3.
1. Explain what is meant by coherent source (1mrk)
2. In young’s double slit experiment, light from a source passes through slit S to slits S1 and S2 before they reach the screen as shown below. 1. An alternate pattern of bright and dark fringes is observed on the screen. Describe how the pattern forms. (2mrks)
2. State the observation made on the screen if two ordinary sources were placed at S1 and S2. (1mrk)
3. State the effect of reducing the slit separation on the fringes. (1mrk)
2.
1. define potential difference (1mrk)
2. The figure below shows a circuit with three identical bulbs connected to a 6V source supplying 3A current. Determine the potential difference across bulb C. (3mrks)
3.
1. When a battery is connected to a bulb of 2.4Ω, a current of 3A flows but when it is connected to a 6Ω bulb, only 1.5A can be driven across. Find the emf and the internal resistance of the battery.  (4mrks)
2. Given that the battery is made up of three cells connected in series, determine the internal resistance of a single cell. (2mrks)
3.
1. Sketch the electric field pattern around the body shown in the figure below. (1mrk) 2. Define capacitance (1mrk)
3. The figure below shows a circuit with four capacitors connected to a 50V source. Determine, 1. The effective capacitance in the circuit (3mrks)
2. The charge on the 2μF capacitor (3mrks)
4.
1. State two ways of obtaining a magnified image from a pin-hole camera. (2mrks)
2. Complete the ray diagram below to locate the position of the image. (3mrks) 3. Determine the number images formed when two plane mirrors are inclined at an angle of 72° to each other. (2mrks)
4. State two characteristics of images formed by plane mirrors. (2mrks)
5.
1. You are provided with a watch glass and an Aluminium foil. Describe how one can make a concave mirror from these. (2mrks)
2. By construction, locate on the grid provided below the position of the image and the object for a concave mirror of focal length 10cm. given that the image is inverted and twice the size of the object. Both the image and the object are perpendicular to the principal axis. (5mrks)
3. State one application of a concave mirror with an arrangement of (b) above. (1mrk) ## MARKING SCHEME

Section A (25marks)

1.
• The angle of incidence equals to the angle of reflection
• The incident ray, the reflected ray and the normal all lie in the same plane at the point of incidence(1mrk)
2. (2mrks)
3. A material of unknown electrical properties is placed on the cap of a positively charged electroscope. The leaf is observed to fall. Giving a reason state the type of the material. (2mrks)
• Conductor
• It is capable of discharging the electroscope
4. The speed of sound in air is 340ms-1, calculate its wavelength when the frequency is 256Hz. (3mrks)
v = fλ
λ =340 = 1.328m
256
5. t1 + t2 = 0.176s ∴ 340 = 200 × 2
t1
340 = x − 200,  t1400 = 1.176s,
t2                340
t2 = 0.176 + 1.176 = 0.352s,
340 × 0.352 = x − 200                     x = 119.68 + 200 = 319.68m
6. The figure below shows a section of a counter made using an electromagnet that punches holes on a moving paper tape.
1. Describe how a hole is punched into the tape (2mrks)
• Counter activates the circuit causing the electromagnet to attract the soft iron that strikes the tape
2. State two ways of improving the strength of the electromagnet (2mrks)
• Increasing the number of turns, increasing the amount of current
7. n =   real depth        =13    =1.3
apparent depth      10
8. Define electrical resistance (1mrk)
• Opposition offered to the flow of current.
9. Polarization is one of the defects of a simple cell, state one way of minimizing this defect. (1mrk)
• Using depolarizer like manganese IV oxide or potassium dichromate
10. Complete the ray diagram below to locate the position of the object for the image given. (3mrks) 11. Determine the work done when a charge of 16Coulomb flows across a conductor when the potential difference across it is 12V. (2mrks)
W = QV
= 16 × 12
= 192J

SECTION B (55 MARKS)

1.
1. Define refractive index (1mrk)
• The ration of the sine of the angle of incidence to the sine of the angle of refration
2. A ray of light is incident in water at an angle of 300on water – glass plane surface. Calculate the angle of refraction in the glass. ( ang= 1.5, anw= 1.33) (3mrks)
n1 sin θ1 = n2 sin θ2 1.33 sin 30°= 1.5 sin θ2
sin θ21.33 sin 30° = 0.4290         θ2 = 25.40°

1.5
3. The graph below shows the relationship of sines of the angles of incidence and refraction for crown glass material. Use it to answer the questions that follow. 1. Determine the refractive index of the glass (3mrks)
n = 0.82 − 0.5   = 0.32  = 1.39 − 1.52
0.56 − 0.33  = 0.23
2. Find the angle of incident in air for which the angle of refraction is 31.33° (2mrks)
sin i = 0.78 i = 51.26°
4. State one condition for total internal reflection to occur. (1mrk)
• Light must be travelling from a denser medium to a less dense medium
• The angle of incidence in the denser medium must be greater than critical angle
2.
1. The figure below shows wavefronts travelling towards an obstacle. Complete the path followed by the waves after being reflected. (2mrks) 2. Explain how sound travels farthest at night than in the day. (2mrks)
• The difference in the density (warm and cold layers) of air at night causes the sound to be gradually refracted with most of the sound remaining near the earth’s surface.
3.
1. Explain what is meant by coherent source (1mrk)
• a source that has same frequency and constant phase
2. In young’s double slit experiment, light from a source passes through slit S to slits S1 and S2 before they reach the screen as shown below.
1. An alternate pattern of bright and dark fringes is observed on the screen. Describe how the pattern forms. (2mrks)
• The two waves interfere constructively to form bright fringes and destructively to form dark fringes
2. State the observation made on the screen if two ordinary sources were placed at S1 and S2. (1mrk)
• There will be a uniform illumination
3. State the effect of reducing the slit separation on the fringes. (1mrk)
• Distance between the fringes increased
3.
1. define potential difference (1mrk)
• Work done in moving a unit charge across a conductor
2. The figure below shows a circuit with three identical bulbs connected to a 6V source supplying 3A current.
Determine the potential difference across bulb C. (3mrks)
current across B and C is 3A,
if resistance is R,effective resistance = R + R/2
= 6/= 2Ω = 3R/2 R = 4/3
VC = 1.5 × 4/3 = 2V
3.
1. When a battery is connected to a bulb of 2.4Ω, a current of 3A flows but when it is connected to a 6Ω bulb, only 1.5A can be driven across. Find the emf and the internal resistance of the battery.     (4mrks)
E = IR + Ir
E = 3 × 2.4 + 3r,
E = 1.5 × 6 + 1.5r ∴
E = 7.2 + 3r, E = 9 + 1.5r
1.8 = 1.5r
r = 1.2Ω,
E = 9 + 1.5 × 1.2 = 10.8V
2. Given that the battery is made up of three cells connected in series, determine the internal resistance of a single cell. (2mrks)
r = 1.2 ÷ 3 = 0.4Ω
4.
1. Sketch the electric field pattern around the body shown in the figure below. (1mrk) 2. Define capacitance (1mrk)
• Charge stored per unit voltage
3. The figure below shows a circuit with four capacitors connected to a 50V source. Determine,
1. The effective capacitance in the circuit (3mrks)
Cs = 8 × 6 = 3.429,
14
Cp = 2 + 3.429 = 5.429μF,
Cs5.429 × 4 = 2.303μF
9.429
2. The charge on the 2μF capacitor (3mrks)
total charge = 2.303 × 50 = 115.15μC;
Pd on 4μF =115.15 = 28.7875V;
4
Pd on parallel = 50 − 28.7875 = 21.2125V
charge on 2μF = 2 × 21.2125 = 42.425μC
5.
1. State two ways of obtaining a magnified image from a pin-hole camera. (2mrks)
• Take camera closer to the object
• Increase the length of camera
2. Complete the ray diagram below to locate the position of the image. (2mrks) 3. Determine the number images formed when two plane mirrors are inclined at an angle of 72° to each other. (2mrks)
n = 360 −1 = 4 images
72
4. State two characteristics of images formed by plane mirrors. (2mrks)
• Virtual, same size as object, equidistant from mirror with object, laterally inverted
6.
1. You are provided with a watch glass and an Aluminium foil. Describe how one can make a concave mirror from these. (2mrks)
• The foil is cut then wrapped on the watch glass. One views in the depression for a concave mirror
2. By construction, locate on the grid provided below the position of the image and the object for a concave mirror of focal length 10cm. given that the image is inverted and twice the size of the object. Both the image and the object are perpendicular to the principal axis. (5mrks) 3. State one application of a concave mirror with an arrangement of b) above. (1mrk)
• Shaving mirror, dentists’ mirror

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