# Physics Paper 3 Questions and Answers with Confidentials - 2021 KCSE Eldoret Diocese Mock Exams

## Questions

INSTRUCTIONS TO CANDIDATES

• Mathematical tables and non-programmable calculators may be used.
• This paper consists of section A and section B.
• ALL working MUST be clearly shown.

Question 1

1. You are provided with the following apparatus:
• 2 size D dry cells
• 100cm nichrome wire on a mm scale, labelled P at one end.
• A bulb (2.5V) and a bulb holder.
• 8 connecting wires (at least 4 with crocodile clips)
• Cell holder
• A voltmeter (0-5V)
• An ammeter (0-1A)
A jockey
1. Connect the apparatus provided as shown in the diagram. 2. Place the jockey at L = 20cm from P, then close the switch.
Record the ammeter reading and the voltmeter reading in the table below.
3. Repeat the experiment by placing the jockey at L = 30, 40, 50, 60 and 80cm from P.
 Length l (cm) I (A) Pd, V(V) I(mA) Pd, v(MV) log I log V 20 30 40 50 60 80 3mks 3mks -½mk -½mk -½mk -½mk
4. Plot a graph of log I (y-axis) against log V (5mks)
5. Determine the slope of the graph. (3mks)
6. Give that log I = nlog V + log K where n and k are constants of the lamp. Determine using your graph the value of:
1. K (2mks)
2. n (2mks)

Question 2

Part A
You are provided with the following:

• A retort stand, boss and clamp.
• 2 boiling tubes
• A thermometer
• Some distilled water in a beaker labelled W
• Some liquid in a beaker, labelled L
• A 250ml beaker containing some water.
• A measuring cylinder
• A stop watch
• A tripod stand and wire gauze
• A card board with a hole in the middle
• A burner.

Proceed as follows
1. Clamp one boiling tube on the retort stand. Measure and pour 45ml, of the distilled water, W
into a boiling tube. Set up the apparatus as shown in the figure below. 2. Heat the water in the large beaker (250ml) until the temperature of the distilled water reached 850C. Remove the boiling tube from the hot water by lifting up the retort stand and placing it away from the burner.
3. Stir the water in the boiling tube using the thermometer. Record in the table below the temperature of the distilled water at intervals of 30 seconds starting at 800C until it drops to 600C (stir the distilled water before taking any reading).
 Time in minutes 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Temperature of W(0C) Temperature of L(0C)

 Time in minutes 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 Temperature of W(0C) Temperature of L(0C)

(4mks)
4. Using the second boiling tube, repeat the procedure in b and c using 45ml of liquid L instead of distilled water. Record your results in the same table.
5. Using the same axis on the grid provided, plot a graph of temperature (y-axis) against time for
1. Distilled water, W
2. Liquid L
(Label the graphs of L and W. (7mks)
6. From the graph, determine:
1. the time, t taken for the distilled water to cool from 750C to 650C.
tw = _____________ minutes (1mk)
2. the time, t taken for liquid L, to cool from 750C to 650C
tL= _____________ minutes (1mk)
7. Determine the constant r given that where d, density of liquid, L = 0.8g/cm3. (2mks)

PART B
You are provided with the following:

• Copper wire of length 130cm.
• Test tube of diameter 1.5cm (ordinary)
• Metre rule.

Procedure

• By using the wire provided, make 20 closely packed turns around the said ordinary test tube as shown. 1. Measure the length x = ______ cm (1mk)
2. Use the result “X” to determine the thickness of the wire, d = ______ cm. (1mk)
3. Given that the volume of the wire 1/4πd2L, determine the volume, V of the wire if
L = 120cm. (3mks)

## Confidential

1.
• 2 size D dry cells.
• 100cm nichrome wire on a mm scale, labelled P at one end, 0 cm mark.
• A bulb (2.5V) and a bulb holder.
• 8 connecting wires (at least 4 with crocodile clips).
• Cell holder
• A switch
• A volt Meter ( 0 – 5V)
• An ammeter ( 0 – 1A)
•  A jockey

Section B

•  A liquid labelled L (50 ml of paraffin).
• A measuring cylinder (50ml or 100ml).
• 2 boiling tubes.
• A thermometer.
• 50ml of distilled water in a beaker labelled W.
•  A 250ml beaker containing some water.
• A stop watch
• A tripoid stand and wire gauze
• Clamp and stand
• A card board with a hold at the middle (3cm by 3cm)
• Copper wire of length 130cm. (0.28mm)
• Test tube of diameter 1.5cm (ordinally)
•  Metre rule

## Marking Scheme

Question 1

 Length l (cm) I (A) Pd, V(V) I(mA) Pd, v(MV) log I log V 20 0.14 0.25 140 250 2.146 2.398 30 0.16 0.35 160 350 2.204 2.544 40 0.18 0.50 180 500 2.255 2.699 50 0.19 0.65 190 650 2.279 2.813 60 0.20 0.85 200 850 2.301 2.929 80 0.24 1.30 240 1300 2.380 3.114 3mks 3mks -½mk -½mk -½mk -½mk

I(A) = x ± 0.2A ; 2dp (4sf) (4sf)
V(v) = x ± 0.10V ; 2dp

1. Graph attached

2.  slope =  Δ  log I   (from graph not the table)
Δ log V
(238 – 214) x 10-2 Correct substitution – (1 mk)
(310 – 240) x 10-2 Correct evaluation – (1 mk)
= 0.3429 (4 sf) ans to 4 sf or exact – (1 mk)
3. log I = n log v + log k
y = mx + C
1. log k = y intercept
log k = 211 x 10-2 = 2.11 (1mk)
k = 128.82 (4sf or exact)
2. n
n = slope
= 0.3429 ( exact as in e)

Question 2

PART A

1.

 Time in minutes 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Temperature of W(0C) 80 79 77.5 76 75 74 72.5 71 70 69 Temperature of L(0C) 80 76 75 72 70 68 66 64.5 62.5 61

 Time in minutes 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 Temperature of W(0C) 68.0 67.0 66.0 65.0 64.5 62.5 62.0 61.5 61.0 60.0 Temperature of L(0C)
Temp (0C) + 2.00C (4mks)
2. Graph (as attached) 3.
1. tw = 6.5 – 2.0
= 4.5 minutes
Correct readings from graph – 1mk
2. tl = 2.0 – 0.6
= 1.4 minutes
Correct readings from the graph – 1mk
4. r = 4.2tl
dtw
4.2 x 1.4   = 1.6333
0.8 x 4.5
Correct substitution of tl , tw and d – 1mk
Correct evaluation of 4 SF or exact - 1mk

Part B

1.  Length, x = 0.8 + 0.2cm - 1mk
2.  thickness    d     = x
20
0.8cm = 0.04cm
20
Correct substitution of x – 1/2mk
Correct evaluation to 2 dp or exact – 1/2mk
3.  V = 1/4 πd2 L
= 1/4 x 3.142 x(0.04)2 x 120
= 0.1508cm3 1
Correct substitution of  = 3.142 , d2 , L – 1mk
Correct evaluation to 4 s.f or exact – 1mk
Correct unit – 1mk

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