## PHYSICS PAPER 3 - 2020 KCSE PREDICTION SET 1 (QUESTIONS AND ANSWERS)

Question one

1. You are provided with the following apparatus
- A lens
- Lens holder
- Candle
- Screen
- A screen with a hole having cross-wire
- Metre rule

Proceed as follows

1. Set up the apparatus as in the figure below with distance S = 42cm Without changing the distance S move the lens slowly away from cross-wires until a sharp enlarged inverted image is formed on screen position L1. Measure the distance U1 from cross-wires to the lens and record this value in table 2. Keeping distance S, constant move the lens away from cross-wires to a new position L2 where a small sharp inverted image is formed on the screen. Measure the new object distance U2 and record in table 2. Determine the displacement d of the lens from L1 to L2 (i.e d = L2 – L1)
2. By setting the distance S to distances 44, 46, 48, 50 and 52cm as shown in table 2 repeat procedure (a). Measure and record the corresponding values of U1 and U2 in table 2
Table 2 (10mks)
 S (cm) 42 44 46 48 50 52 U1 (cm) U2 (cm) d (U2 – U1) (cm) d2 (cm2) S2 (cm2) S2 – d2 (cm2)
3.
1. Plot graph of S2 – d2 against S                                                                        (5mks)
2. Determine the slope of the graph                                                                    (3mks)
3. Given that S2 – d2 = 4fS, use your graph to determine the focal length of the lens   (2mks)

QUESTION TWO

1. You are provided with the following:
- a metre rule;
- a retort stand, a boss and clamp;
- 200m1 of a liquid in a 250ml beaker labelled W;
- 200m1 of a liquid in a 250m1 beaker labelled L;
- Two masses labelled m1 and m2.

Proceed as follows:

1. Suspend the metre rule so that it balances at its centre of gravity G and record its value G = .................................................................................cm                                        (½mk)
2. Position mass m2 at a distance x = 5 cm from the centre of gravity G and adjust the position of mso that the metre rule balance at G. Record the x1 of m1 from the point G in table 2.
3. While maintaining the distance x = 5cm, immerse m2 completely in water. Adjust the position of m1 until the metre rule balances again (see figure 2(b))
Record the new distance x2. 4. Still maintaining the same distance x = 5cm, remove the beaker, W with water and replace it with the beaker L with the liquid. Immerse m2 completely in the liquid. Adjust the position of ml until the metre rule balances again (see figure 2(c)). Record the new distance x3.
5. Remove mass m2 from the liquid and dry it with a tissue paper.
6. With the metre rule still suspended from its centre of gravity G, repeat the procedure in (b), (c), (d) and (e) for other values of given in table 2. Complete the table.
TABLE 2
 Distance x (cm) Distance x1 (cm) Distance x2 (cm) Distance x3 (cm) L0 = (x1 – x2)(cm) L1 = (x1-x3)(cm) 5 10 15 17 20
7. Plot a graph of L0(y-axis) against L1                                                                           (5mks)
8. Find the slope S of the graph.                                                                         (3mks)
9. Find the value of k given that L1 = 25/K      (2mks)

## MARKING SCHEME

1.
1.
 S (cm) 42 44 46 48 50 52 U1 (cm) 16 15 15 14 14 13 (3mks) U2 (cm) 25 28 31 33 36 38 (3mks) d = (U2 – U1) (cm) 9 13 16 19 22 25 (1mk) d2 (cm2) 81 169 256 361 484 625 (1mk) S2 (cm2) 1764 1936 2116 2304 2500 2704 (1mk) S2 – d2(cm2) 1683 1767 1860 1943 2016 2079 (1mk)(10mks)

2.
1. 2. Slope = (1945 – 1770)cm2 = 175cm = 43.75cm
(48 – 44)cm            4            (3mks)
3. S2 – d2 = 4fs
y = nx
4= slope
f = slope = 43.75 = 10.9cm
4            4            (2mks)
1.
1. G = 50.0 + 1cm √ ½MK  (49.0 – 51.0) ----MUST BE IN 1dp
 x (cm) x1 (cm) x2 (cm) x3 (cm) L0 (cm) L1 (cm) 5.0 10.0 8.0 8.5 2.0 1.5 10.0 21.0 18.0 19.0 3.0 2.0 15.0 31.0 27.0 28.0 4.0 3.0 17.0 35.5 31.0 32.0 4.5 3.5 20.0 42.0 36.0 37.5 6.0 4.5 √ ½ each √ ½ each √ ½ each √ ½ max of 1mk √ ½ max of 1 mk       Total 9½mks
2. 