INSTRUCTIONS TO CANDIDATES:
- This paper consists of TWO sections. Sections A and B
- Answer ALL the questions in section A and B
- All working MUST be clearly shown.
- Mathematical tables and Electronic calculators may be used.
- Take acceleration due to gravity, g = 10ms-2
SECTION A (25 MARKS)
Answer all questions in this section
- State one reason why in the construction of car head lamps parabolic reflectors are preferred to spherical reflectors. (1mk)
- It is common practice that once an accumulator is recharged the terminals are connected using a wire to assess its state of charge. How is this dangerous to the life of the accumulator? (1mk)
- Two identical spheres A and B each standing on an insulating base are in contact. A negatively charged rod is brought near sphere A as shown in figure 1
In what way will A differ from B if separated while the rod is near? Explain. (2mks) - The ammeter in the circuit in figure 2 has negligible internal resistance. The cell has an internal resistance of 0.5 and an electromotive force of 3.0v.
Determine the value of current the ammeter registers when switch S is closed. (2mks) - Figure 3 represents a step down transformer of ratio 10:1. The turns are wound uniformly on the core and the primary coil is connected to a 250va.c supply. The lengths KL and MN are as indicated.
Determine the p.d across LM. (4mks) - The diagram in figure 4 below shows a wire placed between the poles of two bar magnets.
Indicate with an arrow the force that acts on the section AB of the wire. (1mk) - An electric heater 480Ω is connected to a 240v main supply. Determine the energy dissipated in 4 minutes. (3mks)
- A pin at the bottom of a beaker containing glycerine appears to be 6.8cm below the surface of glycerine. Determine the height of the column of glycerine in the beaker. (take the refractive index of glycerine as 1.47) (3mks)
- A girl shouts and ears an echo after 0.6 seconds later from a cliff. If velocity of sound is 330m/s, calculate the distance between her and the cliff. (3mks)
- What do you understand by ‘doping’ as applied with semiconductors? (1mk)
- Arrange the following in order of decreasing wavelength Gamma radiation, Radio waves, Infrared and x-rays. (1mk)
- Explain why soft iron keepers are suitable for storing magnets (2mks)
- Figure 7shows a trace obtained on a cathode ray oscilloscope screen when an a.c is applied to the Y-plates and time base switched on.
On the same figure draw a waveform showing what would be observed if the time base is doubled. (1mk)
SECTION B ( 55 MARKS)
Answer ALL the questions in this section in the spaces provided
-
- What is meant by an open circuit? (1mk)
- The graph in figure 5 shows the terminal voltage, V, of a certain battery varies with the current, I, being drawn from the battery.
- Write an expression relating the e.m.f. E, terminal voltage, V, current, I and the internal resistance, r, of the battery for the circuit drawn in (i) above. (1mk)
- From the graph determine the; I internal resistance, r, of the battery. (2mks)
- When the device, X is connected in the circuit below, the voltage across it is 0.70 V.
Calculate the value of the resistance R. (3mks) - The cell in figure 10 has an e.m.f of 2.1 V and negligible internal resistance.
Determine the- Total resistance in the circuit (2 marks)
- Current in the circuit (1 mark)
- Reading of the voltmeter (2 marks)
-
- The figure 9 below shows magnetic relay circuit
Explain what will be observed when the switch is closed (4mks) - The figure 10 below shows a current carrying conductor
On the same diagram draw, the magnetic field pattern produced. (2mks) - State two possible rules that can be used to predict the field direction produced in the above diagram. (2mks)
- List two applications of magnetic effect of electric current. (2mks)
- The figure 9 below shows magnetic relay circuit
-
- The figure below represents a cathode ray oscilloscope (C.R.O)
- Name the parts labelled A and B. (2mks)
A
B - What are the functions of parts labelled C and D? (2mks)
C
D - Explain how electrons are produced . (1mk)
- Give a reason why the tube is evacuated. (1mk)
- The potential between the anode and the cathode of an X-ray tube is 80kv. Calculate;
- The energy of an electron accelerated in the tube. (Electronic charge e = 1.6 x 10-19 C) (3mks)
- The velocity of electrons in the tube. (Mass of an electron = 9.11 x 10-31kg ) (3mks)
- The figure below represents a cathode ray oscilloscope (C.R.O)
-
- The figure 12 below shows two rays starting from the top of an object OB incident on a converging lens of focal length 2cm.
Complete the diagram to show the image formed (3mks) - Add one or more incident ray from B and draw the corresponding refracted ray (1mk)
- Calculate the magnification produced by the lens (2mks)
- The figure 13 below shows an object placed at right angles to the principal axis of a thin converging lens.
- Calculate the position of image formed (3mks)
- Give an application for this arrangement of a lens. (1mk)
- Describe the nature of the image formed (2mks)
- The figure 12 below shows two rays starting from the top of an object OB incident on a converging lens of focal length 2cm.
-
- State Lenz’s law of electromagnetic induction (1mk)
- The figure 14 below shows a diagram of a simple electric generator
State three factors that would affect the value of the voltage output (3mks). - A transformer supplies a current of 13.5A at a voltage of 48v to a device from an AC main supply 240V. Given that the transformer is 80% efficient, calculate;
- Power supplied to the transformer (3mks)
- Current in the primary coil (2mks)
MARKING SCHEME
SECTION A
- Parabolic reflectors do not form a caustic curve;
The filaments can be arranged so as to have parallel beams to help see far and a set of others to enable see near / directed downwards ;
Any one (1 mrk) - This causes buckling of the plates as this is short circuiting;
- A will have positive charge while B negative; when charged rod is brought near A positive charges are attracted towards it while the negative charges are repelled to B
On separation while strip is near A, B goes with negative charge and A remains with the positive; - E = IR + Ir Formula of substitution give 1 mark
3.0 = I x 3.5 + I x 0.5;
I = 0.75A ; - Vs = Ns
Vp Np
Vs = 1 ;
250 10
Vs = 25V;
Now VLm = 4/5 x 25 ;
= 20 V;
NB force (arrow) must touch the wire / conductor- W – V 2 t;
R
= 2402 x 4 x 60 ;
480
= 28,800J;
n = Real Depth
Apparent Depth - 1.47= R.d
6.8cm
R.d = 6.8cm x 1.47
= 9.996cm - V= 25 , →S = Vt
t 2
S =330 x 0.6
2
= 99m - Introduction of controlled amounts of impurities into the lattice of a pure semi-conductor in order to Enhance its electrical conductivity;
- Radio waves; Infrared; x –rays and Gamma rays
Decreasing wavelength - Magnetism is easily induced in them; the dipoles of the Keepers form a closed loop with those in the magnets
Hence protecting the magnets from being demagnetized; -
SECTION B
-
- (1 mark)
Where no current is flowing through/ where there is voltage drop a cross the source is equal to e.m.f -
- E= V + Ir
- Gradient (R) 1.475 – 1.05 = 0.425 = 0.53125 Ώ
0.8- 0 0.8 (2 marks) - From the graph current flowing when pd is 0.70 is 60.MA
Pd across R = 6.0 – 0.7 = 5.3v
R = 5.3 V 36mA
= 147Ω (3 marks)
- Parallel circuit 1/30 + 1/20 = 5/60 or 60/50
R = 12 Ω
Total resistance = 10 + 12 = 22Ω (2 marks)- l = V/R = 2.1/22 = 0.095A (1 mark)
- Reading of the voltmeter
V = lR = 10 x 2.1
22 = 0.95 (2 marks)
- (1 mark)
-
- Lamp lights
When s is closed soft iron get magnetized it repels magnet that pushes contact and closes it.
Current flows lighting the bulb. -
-
- Right hand grip rule
Maxwell – cork screw rule -
- Electric bells
- Magnectic locks
- Loudspeakers
- Relays
- Motors and generators
-
- Lamp lights
-
-
- A – Grid✓1
B – Electron gun✓1 - C – Vertical deflection of beam of electrons✓1
D – Horizontal deflection of beam of electrons✓1 - By thermionic emission or heating the filament✓1
- To prevent ionization of electrons as they move to the anode✓1
- A – Grid✓1
-
- E = ev✓1
E = 1.6 x 10-19 x 80000✓1
= 1.28 x 10-14 J✓1 - ½ mv2 = 1.28 x 10-14✓1
v2 = √2 ×1.28 × 10-14
(9.11 ×10 - 31)
v = √2 ×1.28 ×10-14
9.11 ×10 - 31 ✓1
v = 2.23 x 108 ms✓1
- E = ev✓1
-
-
- GRAPH
-
- u=6cm
f=8cm
v= ?
1/f= 1/u- 1/v
1/v= 1/f- 1/u
=1/8- 1/6= 3-4
24
-1/24
v= -24cm - Hand lens / magnifying glass
-
- Image formed is
- Virtual
- Erect/upright
- Magnified
- u=6cm
- GRAPH
-
- The direction of the induced current is always as to oppose the change producing it.
- X carbon brush
Y split ring - Speed of rotation
Number of turn of the coil
Strength of the magnet - IS = 13.5
VS = 480
Vp = 240
Ƹ = 80%
P=IV
= 13.5 x 48 = 64.8 watts
80=648
100=x
⇒648 ×100=810 watts
80
IV=810
I ×240=810
I=810=3.375A
240
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