Download nakuru district sec. schools trial examination – 2014

NAKURU DISTRICT SEC. SCHOOLS TRIAL EXAMINATION –
2014
MARKING SCHEMES PHYSICS 232/2
PAPER TWO- SECTION A (25 MARKS)
1. Figure1 shows two plane mirrors inclined at an angle x from each other.
A viewer counts a total of seven images by looking directly from the object O.
Determine the value of x
Figure 1
(2mks)
Answer
3600
 1  7  1;
x
3600
8
x
8 x = 3600
 1;
2. Figure 2 shows two spherical materials one an insulator while the other a
conductor. Negative charges are introduced at point A by contact method in each
case.
Insulator
Conductor
Figure 2
A
1|Page
A
On the same figure indicate the final position of the charges. Explain your answer
(2mks)
Answer
A
A
The conductor allows charge to be distributed while an insulator does not
3. Two capacitors of capacitance 2µF and 1µF are connected in parallel. A potential
difference of 3v is applied across them. Find the energy stored in the combination
(3mks)
Capacitance = 2+1=3µF
E= ½ Cv2
= ½ x3x10-6 x 3x3
=13.5x10-6 joules
4. A girl shouts and hears an echo after 0.6 second later from a cliff. If velocity of
sound is 330m/s. Calculate the distance between her and the cliff
(3mks)
𝒗=
𝒔=
2|Page
𝟐𝒔
𝒕
𝒗𝒕 𝟑𝟑𝟎𝒙𝟎. 𝟔
=
= 𝟗𝟗𝒎
𝟐
𝟐
5. In the circuit diagram shown in figure 3, the lamps are identical and cells are also
identical.
State with reason, in which circuit the lamp will be lit for a longer period
(2mks)
Circuit A: current drawn from each cell is less than in B.
Or
Internal resistance in circuit A is less than in B.
6. Figure 4 shows how rays from a distant and near object are focused inside a
human eye with a certain defect.
Name the defect and state the cause of this defect
(2mk)
Defect: short sightedness
Cause: the eye ball being too long or short focal length
3|Page
7. Two pins are hanging from a magnet as shown in the figure 5
S
Figure 5
N
Explain why the pins do not hang vertically downwards
(1mk)
They acquire North poles at the ends thus they repel
8. State the necessary conditions for stationary waves to be generated
(2mks)
-two progressive waves moving in opposite direction and must have the
same speed
-have same frequency
-have the same Amplitude
or-two coherent waves moving in opposite direction interfere
9. The speed of yellow light in the prism is 1.88x108m/s. Determine the refractive
index of the glass prism material. (Speed of light in vacuum,
c= 3.0x108m/s)
(2mks)
aη g
=
𝑐
𝑣
=
3.0𝑥108
1.88𝑥108
=1.596
10. A 2.5 kW immersion heater is used to heat water. Calculate the operating
voltage at the heater if its resistance is 24Ω
(3mks)
Electrical power,𝑷 =
V2 = P x R
=2500 x 24
=60000
V= 245v
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𝑽𝟐
𝑹
11.
Arrange the following in order of increasing frequency
Visible light, x-rays, Infra-red radiation, UV light, Radio waves
(1mk)
Radio waves, infra-red radiations, visible light, UV light, x-rays
12.
Distinguish between a fuse and the earth wire
(2mks)
- Fuse protect a device from large currents
-Earth wire protect the person from electrical shock should the
appliance develop a fault.
SECTION B (55MARKS)
13.
(a) State Ohm’s law
(1mk)
The current flowing through a conductor is directly proportional to the p.d
across it provided that temperature remains constant.
(b) An electrician installed electric wiring in a house and connected the bulbs and
the switches as shown in figure 6.
L1
L2
S1
L3
S2
To mains
S3
Figure 6
State what happens when switch;
(i) S1 only is closed
(1mk)
L1 lights, L2 and L3 are off
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(ii)
S1 and S3 only are closed
(1mk)
L1 lights, L2and L3 off
(iii)
All the switches are closed. Explain your answer (2mks)
L1 light, L2 and L3 go off. The switch S1 create a short circuit to L2 and L3
(b) Figure 7 shows a circuit
4Ω
R1
6v
R3
R2
3Ω
6Ω
Figure 7
Calculate
(i)
The total resistance of the circuit
𝑹𝑻 = 𝑹𝟏 +
(2mk)
𝑹𝟐𝑹𝟑
𝑹𝟐 + 𝑹𝟑
=4+3x6/3+6
=6Ω
(ii)
The total current flowing in the circuit
𝑰=
(iii)
The voltage drop across R1
(2mk)
𝒗
𝟔
= = 𝟏𝑨
𝑹𝑻 𝟔
(2mk)
V=IR
=1x4
=4v
14. The following graph shows the variation of image distance, v, with
magnification, m, for a converging lens.
6|Page
Image
distance,
V(cm)
40
30
20
10
𝑉
Using the graph and the equation
(i)
𝑓
= 𝑀 + 1 to determine:
The object position when the image position is 45cm
2
1
For v=45cm, M= 3.5
3
4
(3 marks)
Magnification, M
But M=v/u
Therefore u=v/M=45/30=12.86cm
(ii)
(iii)
The focal length of the lens
The power of the lens.
𝟏
𝟏
𝒑𝒐𝒘𝒆𝒓 = 𝒇 = 𝟎.𝟏 = 𝟏𝟎𝑫
7|Page
(2 marks)
𝟏
=𝒎+𝟏
𝒇
V=Mf+f
Focal length= V=10cm when M=0
(that is, the v-intercept)
(2 marks)
15. (a)
(i)
State one cause of energy losses in a transformer and explain how it
can be minimized.
(2mks)
Causes
Eddy currents
Method of minimising
Laminations
Hysterises loss/Magnetic reversals
Heating effect (T2R)
Use of soft iron
Use of thick copper wires
Magnetic flux leakage
Winding secondary on
soft iron core
Any one, with correct method
primary/use of
(2mks)
(ii) Describe briefly the energy changes involved in the generation of electrical
energy at a hydropower station
(2mks)
Potential energy
kinetic energy
electrical energy;;
(iii)What are the advantages of transmitting power at?
(b)(i)
(I)
Very high voltages (1mk)
To minimize power loss;
(II)
Alternating voltage (1mk)
Can be stepped up or stepped down;
Explain how electrons are produced in a cathode ray oscilloscope (CRO)
(2mk)
The cathode is heated by filament and electrons are released from cathode ray
by thermionic emission
(ii)
State two functions of the anodes in a CRO (2mks)
-to accelerate the electrons
-to focus the electrons towards the screen
(iii)
At what part of the cathode ray oscilloscope would the time be connected
(1mk)
Across x-plate
(iv)
State why the tube is highly evacuated
(1mk)
To reduce collision/interaction/interferance of electrons, (hence ionization)
with air molecules in the tube
Or
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To maintain K.E or minimise loss of K.E
16. (a)(i) An x-ray tube is operating with an anode potential of 50kV. Calculate: the
maximum speed of an electron. (Charge of an electron is 1.6x10-19 C, mass of an
electron is 9.1x10-31kg)(3mks)
𝟏
𝒎 𝒗 = 𝒆𝑽
𝟐 𝒆
½ x9.1x10-34x 𝒗 =1.6x10-19x50x103
𝒗 = 𝟖. 𝟕𝟗𝟏𝟐𝒙𝟏𝟎𝟏𝟕 𝒎/𝒔
(ii) A faulty X-ray tube generates X-rays of higher intensity than required. State
what adjustment should be made to correct the defect. (1mk)
Adjust the rheostat connected to step down transformer Reduce the cathode
current
(iv)
Distinguish between hard and soft X-rays (2mk)
Hard x-rays have very short wavelength, thus high penetrating power while soft
x-rays have longer wavelength, hence less penetrating power
(v)
Explain the effect of exposing a patient to X-rays for too long (1mk)
-destroy the body cells or organ of the patient
or
-can kill the patient
(b)(i)What is meant by the term work function?
(2mk)
Work function of a metal is the minimum amount of energy required to dislodge
electrons from the surface of the metal
ii)
The work function of a certain material is 3.2eV. Determine the threshold
frequency for the material. (1 electron Volt (eV) = 1.6x10-19J and planks
constant= 6.62x10-34Js)
(3mks)
𝒘𝒐𝒓𝒌 𝒇𝒖𝒏𝒄𝒕𝒊𝒐𝒏, 𝒘𝒄=𝒉𝒇𝒐
𝒇𝒐=𝒘𝒄
𝒉
𝟑. 𝟐𝒙𝟏. 𝟔𝒙𝟏𝟎−𝟏𝟗
=
𝟔. 𝟔𝟐𝒙𝟏𝟎−𝟑𝟒
=7.734x1014Hz
9|Page
17.(a)
Study figure 8 and answer the following questions
A
Figure 8
X
B
Y
C
Radium source
i)
State the charge on plate X (1mk)
Positive
ii)
Identify the radiation A and B
A- beta particles
(1mk)
B-B- gamma rays
(1mk)
iii)
A nuclear reaction is represented by the following equation
𝑎
92𝑥
→
234
𝑏𝑌
Determine the values of a and b
+ 𝐴𝑙𝑝ℎ𝑎 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒
(2mks)
a=234+4=238
b=92-2=90
(vi)
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A radioactive source has an activity of 810 c/s and after 63hours the count
rate falls to 110c/s. If the background count is 10c/s, determine the half-life
of the source (3mks)
The actual counts=810-10=800c/s
Final counts
=110-10=100c/s
𝟏𝟎𝟎
𝟏
Fraction remaining=𝟖𝟎𝟎 = 𝟖
𝟏
𝟏
=
𝟖 𝟐𝒏
n=3
𝑻 = 𝒄𝒕𝟏
𝟐
800
½
𝑻 𝟔𝟑
𝒕𝟏 = =
= 𝟐𝟏𝒉𝒓𝒔
𝒏
𝟑
𝟐
Or
400
½
200
𝟑𝒕𝟏 = 𝟔𝟑𝒉𝒐𝒖𝒓𝒔
½
100
𝟐
𝒕𝟏 =
𝟐
𝟔𝟑
= 𝟐𝟏𝒉𝒐𝒖𝒓𝒔
𝟑
(b) (i)
Draw using appropriate symbols the circuit diagram of a junction diode in
reverse bias
(1mk)
ii)
Extrinsic semiconductors are made through a process called doping. Explain
how doping produces an n-type semi-conductor
(2mks)
-doping a pure semi-conductor with group 5 impurity
-a group 5 impurity has four of its electrons for bonding and the fifth
electron being free.
iii)
Distinguish between a semiconductor and a conductors
(2mks)
Semi-conductors have wide forbidden band while conductors do not
have
Semiconductors conduct by use of holes while conductors by use of
electrons
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