# PHYSICS PAPER 3 - KCSE 2019 STAREHE PRE MOCK EXAMINATION (WITH MARKING SCHEME)

Question 1
PART A

You are provided with the following:

• A metre rule.
• Knife edge raised 20cm above the bench.
• One 50g mass and 100g mass.
• Some water in a beaker.
• Tissue paper.
• Liquid L in a beaker.

Proceed as follows

1. Balance the metre rule on the knife edge and record the readings at this point.
Balance point ………………………………………………….. cm                                          (1 mark)
For the rest of the experiment, the knife edge must be placed at this point.
2. Set up the apparatus as shown in figure below. Use the threads to hung the masses such that the positions of support can be adjusted.

The balance is attained by adjusting the position of the 100g mass. Note the distance X cm and D cm are measured from the knife edge and 50g mass is fully immersed in water. Record the values of X and D.
X    =     ………………………………………………………… cm                                         (1mark)
D    =     ……………………………………………..………… cm                                           (1 mark)
Apply the principle of moments to determine the weight W1 of the 50g mass in water and hence determine the upthrust UW in water.
W1   =……………………………………………………………N                                             (2 marks)
UW   =………………………………………………….………... N                                            (1 mark)
Remove the 50g mass from the water and dry it using tissue paper.
3. Now balance the metre rule when the 50g mass is fully submerged in liquid L. Record value of the distance X.
X    = ………………………………………………….…..cm                                                   (1 mark)
Apply the principle of moments to determine the weight W2 of the 50g mass in the liquid L and hence determine the upthrust UL in the liquid
W2  =……………………………………….                                                                               (2 marks)
UL    = ………………………………….….                                                                                 (1 mark)
4. Determine the relative density RD of the liquid L given that:(2 marks)

PART B
You are provided with the following.

• One ammeter (0 – 2.5A) or (0 – 1A).
• One voltmeter (0 – 5V) or (0 – 3V).
• 6 connecting wires, each at least 50cm long and at least four crocodile clips.
• Nichrome wire AB fixed on a meter rule.
• One cell holder.
• One switch.
• One micrometer screw gauge (to be shared).
• Two new dry cells

Proceed as follows

1. Connect the circuit as shown below.

Close the switch and read the current (I) and the p.d across AB (V).
V = ………………………………V (1mark)
I = ………………………………A                                                                            (1 mark)
2. Calculate the resistance R of the wire using the values V and I.
R ……………………………………………                                                               (1 mark)
3. Measure the diameter of the wire using a micrometer screw gauge and calculate its cross sectional area.(A)
d…………………………………..mm              (1 mark)
d…………………………………..m                  (1 mark)
Area (in m2)             (2marks)
4. Determine the quantity φ given that;     (1 mark)

Question 2

You are provided with the following;

• Candle
• Plane mirror
• Metre rule
• Lens
• Lens holder
• A cardboard with cross-wire at its centre
• Screen
• A piece of Cellotape

Proceed as follows;

1. Attach the plane mirror to the lens using cellotape.
2. Set up the apparatus as shown below.

Ensure that the candle flame is at the same level as the cross wire.
3. Place the cardboard with cross wires at the 0cm mark
4. Move the lens along the metre rule until a sharp image of cross-wire is formed alongside the object cross wire. Measure the length d.
d = ………………………………cm                                                                                       (1 mark)
5. Now set up the apparatus as shown below.
6. Set u = 25cm and adjust the screen until a clear image of the cross wire appears on the screen. Measure the value of v and record in the table.
7. Repeat the procedure (e) above for other values of u and complete the table below. ( 7 marks)
 u (cm) 25 30 35 40 45 50 v (cm) m = v/u
8. Plot a graph of m against v. (5 marks)
Determine the slope of the graph.                                                                                                    (3 marks)
9. Given that the equation of the graph is given by (m = v/n-1) where n is a constant, determine the value of n. (2 marks)
10. What physical quality does n represent?     (1 mark)
11. Find the value of v when m = 0.      (1 mark)

## MARKING SCHEME

1. PART A
1. = 50.0 cm ± 0.1                                1 mark( must be to 1 d.p.)
2. X = 18.5cm ± 0.1                             1 mark( must be to 1 d.p.)
D = 40.0 cm ± 0.1                            1 mark( must be to 1 d.p.)
W1 = 0.463N                                      2 marks (must show working)
UW = 0.038N                                                1 marks
3. X = 17.3 cm ± 0.1                            1 mark( must be to 1 d.p.)
W2 = 0.433N                                      2 marks(must show working)
UL = 0.068N                                      1 mark
4. RD = UL = 0.068
UW = 0.03
= 1.79                                         2 marks

PART B
1. I = 0.20 ±05A 1 mk
2. V = 2.80 ±1V 1mk
3.
4. Diameter d = 0.28 ±02mm Substitution 1mk
D=0.00028m
Calculation of cross section
Evaluation 1mk
A = πr2= 3.142 x 0.14 x 0.14 x 10-6
= 6.1583 x 10-8 m2
5. Calculation of
φ=R x A                   substitution
= 14 x 6.1583 x 10-8  evaluation
= 8.622 x 10-6 m    (2 marks)
5. d = 15.0 ±1cm( 1d.p)
6. Table
 u (cm) 25 30 35 40 45 50 v (cm) 49 36.5 32 28 26 25 m = V/U 1.96 1.22 0.91 0.7 0.58 0.5

v – cm @ value 1mk max 6 marks (1 dp)
m – 1mk for all values (Correct to 4 s.f)
7. Graph
Axes labelled with units                         – 1mk
Scale                                                       – 1mk
Plotting                                       – 2mks
Straight line with positive gradient          – 1mk
8. Slope S = = 0.0595
Accuracy 0.052 – 0.058
9.
10.
11. n is the focal length of the lens

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