# KCSE 2012 Physics Paper 3 with Marking Scheme

1. You are provided with the following:
-Two biconvex lenses labeled A and B.
-A light source
-Screen I with a hole and cross wires at its centre.
-Screen II
-A metre rule
-A plane mirror
-A piece of cellotape
-Two lens holders
-A stand, boss and clamp.

Proceed as follows:
1. Mount lens A on the lens holder. Fix the plane mirror at the back of the lens using the cellotape provided. Use the stand to hold the light source in line with the cross wires on the screen I and lens A with the plane mirror as shown in figure 1.

2. Switch on the lamp. Adjust the position of the lens with the mirror until a sharp image of the crosswires is formed on screen I beside the crosswires. Measure the distance between l1 between the screen and lens A.

l1 = ……………………………..cm (1 mark)

3. Replace lens A with lens B. Fix the plane mirror at the back of lens B. Repeat the procedure in (b) above. Measure the distance l2 between the screen and lens B.

l2 = ………………………………cm (1 mark)

4. Remove the mirror from the lens holder.
Arrange the light source, Screen I (with crosswires), lens A , lens B and screen II in the line as shown in figure 2.

5. Set the distance between screen I and the lens A to be 40 cm. Ensure that this distance is maintained throughout the rest of the experiment. State the distance d between the lens A and B to be 65 cm. Adjust the position of the screen II to obtain a sharp image of the cross wires on it. Measure the distance v between lens B and screen II. Repeat for the other values of d shown in table 1 and complete the table.
 d(cm) 65 67 69 71 73 77 80 v(cm)
6. On the axes provided, plot the graph of v(y-axis) against d. (3 marks)

1. From the graph, at d = 70 cm, determine;
1. The value of v. (1 mark)
2. The slope S of the graph. (3 marks)

2. Given that
Determine the value of K. (2 marks)
3. Determine the value of K given that m = S/K (2 marks)
2. You are provided with the following.
-A voltmeter
-A diode with ends labeled B and C.
-A 1 kΩ resistor
-A 50 Ω potentiometer
-3 dry cells and a cell holder
- a switch
-8 connecting wires (at least four with crocodile clips)

Proceed as follows:

1. Set up the circuit as shown in figure 3.

2.
1. Connect the crocodile clip K to point A. Adjust the potentiometer by turning the knob until the voltmeter reading is maximum.
Maximum voltmeter reading = ………………………… volts. (1 mark)
2. Without adjusting the potentiometer, disconnect the photometer from the crocodile clip K from point A and connect to it to point B. Record the voltmeter reading.
3. Explain why the voltmeter reading in b(i) is different from that in b(ii). (2 marks)
3. Disconnect the crocodile clip K from point B and connect it to point A. Adjust the potentiometer so that the voltmeter reading VA is 1.0 V. Disconnect the crocodile clip K from A and connect it to point B. Record the voltmeter reading VB.
VB = ……………………………………volts. (1 mark)
4. Adjust the potentiometer to obtain other values of VA (when K is at A) shown in table 2, repeat the procedure in (c) to obtain the corresponding values of VB (when K is at B and complete the table.
(7 marks)
Table 2
 VA /V) VB /V) I = VA − VB (A)        1000 1.5 2.0 2.5 3.0 3.5 4.0
5. On the grid provided, plot a graph of I (y-axis) against VB. (5 marks)

6. Use the graph to determine the resistance of the diode when the current is 0.45mA. (3 marks)

## MARKING SCHEME

1.
1. f1 = 20cm ±2cm    (1)
2.
3. f2 = 15cm ±2cm     (1)
4.
5.
 d(cm) 65 67 69 71 73 77 80 v(cm) 37.5 33.8 31.1 29.1 27.5 25.2 24 ±2
6.

(6 marks)
Graph(6 correctly plotted points)
Labelling axes   (1)
Plot  ( 2 marks)
Curve/line on at least 4 correctly plotted points    (1 mark)
7.
1.
1. Value of V = 30 ± 1  (1mark)
2. Slope s =    35 − 20
81.25 − 63.75
= −0.86
≃  −0.9
No curve/line no slope     (3 marks)
2. K = −225   = −225 = −1                (2 marks)
(d−55)2     225
3. M = S = −0.9  =0.9  (2marks)
K      −1
2.
1.
2.
1. maximum Votmeter reading = 4.4volts   (1mark)
2. Voltmeter reading VB = 3.7 Volts   (1 mark)
3. In (i) p.d measured is across both.   (1 mark)
diode and resistor, while in (ii) p.d. is across diode only.   (1mark)
3. VB = 0.8volts. (1 mark)
4.
 VA/V VB/V I = VA − VB (A)        1000 1.5 1.2 0.3 × 10−3 2.0 1.7 0.3 × 10−3 2.5 2.1 0.4 × 10−3 3.0 2.5 0.5 × 10−3 3.5 2.9 0.6 × 10−3 4.0 3.4 0.6 × 10−3
Column I = 1mark
Value of VB = 5 marks
Total for table = 6 marks
5.

Axes labelled   1 mark
Scale( simple & uniform)   1 mark
Plotting       3 marks
Curve(line)   1 mark
(5 marks)
6. I = 0.45mA, VB = 2.3volts
∴R = VB      2.3
1      0.45 × 10−3
=5.1 × 103
=5.1kΩ                       (3 marks)

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