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YU CHUN KEUNG MEMORIAL COLLEGE NO.2
MOCK EXAMINATION (2005-2006)
F.7 PHYSICS (I)
Time allowed: 3 hrs.
Name: __________________ (
)
F.7__________
This paper consists of TWO sections, A and B. Answer ALL questions in BOTH
sections. Section A and B must be handed in separately at the end of the examination.
SECTION A (67 marks)
I
II
Answer ALL questions.
Write your answers in the spaces provided in this question paper. You should
show all the main steps of your calculations.
III Assume: velocity of light = 3x108 ms-1
Acceleration due to gravity = 10ms-2
Question No.
1
2
3
4
5
Marks
13
14
14
11
15
Essence 1 P123_4
1. Given that G = 6.67x10-11 Nm2kg-2, earth – to - moon distance = 3.82x108 m, the
mass of the earth = 5.98x1024 kg, the mass of the moon = 7.36x1022 kg, the radius
of the earth = 6.37x106 m, and the radius of the moon = 1.74x106 m.
(a) Find the gravitational potential at the
(i)
surface of the earth(V1), and
(2marks)
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(ii)
surface of the moon(V2).
(2 marks)
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(b) It is given that at a certain distance r0 from the center of the earth, the
gravitational potential is maximum and equals to –1.29x106 J kg-1 . What is
the physical significance for the gravitational field strength at this point?
(2 marks)
___________________________________________________________
___________________________________________________________
(c) Draw a graph of gravitational potential with positions along the line joining
the centers of the earth and the moon.
(2 marks)
2
(d) A spaceship of mass 2000 kg leaves the surface of the earth and travels to the
surface of the moon.
(i)
What is the minimum energy it required on take-off to arrive at the
moon?
(2 marks)
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(ii)
With the minimum energy at take-off, find the speed of the spacecraft
when it reached the moon’s surface.
(2 marks)
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(iii) In face, the spacecraft has to keep some fuel on board for landing on the
moon safely. Explain this.
(1 marks)
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3
Essence2 P168_1
2.
5 cm
A
B
Fig. 1
An ideal liquid of density 900 kg m-3 flows along a venturi tube with 2 sections A
and B of diameter 10 cm and 5 cm respectively as shown in figure 1. The
difference in manometer heights at A and B is 5 cm.
(a)
(i)
Find the difference in pressure between A and B on the same horizontal
level.
(2 marks)
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(ii) Find the velocity of the liquid at
(1) A, and
(2 marks)
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(2) B.
(2 marks)
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(iii) Find the rate of kinetic energy on the liquid at
(1) A, and
(2 marks)
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__________________________________________________________
(2) B.
(2 marks)
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(iv) What is the rate of work done on the liquid in moving from A to B?
(2 marks)
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(b) Comment on the statement : Bernoulli’s equation is not applicable when a
heavy oil flows through the venture tube.
(2 marks)
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Essence2 P192_2
3. A cylinder fitted with a piston contains 0.2 mole of an ideal gas. Initially the
volume and the pressure of the gas are 5x10-3 m3 and 105 Pa respectively.
Given that the molar gas constant = 8.31 J K-1 mole –1.
5
(a)
(i)
What is meant by the term ‘internal energy’ of the gas?
(1 marks)
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(ii) State one difference between the internal energy of an ideal gas with
that of a real gas.
(1 marks)
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(iii) Find the initial temperature of the gas
(2 marks)
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(b)
The gas is then
(1) heated at constant volume to 500 K, and then
(2) cooled at constant pressure to its initial temperature, and finally
(3) expanded isothermally to its initial volume.
(i)
Sketch the above changes on a P – V diagram, inserting all the
initial and final values for the pressure, volume and temperature for
each processes (1), (2) and (3).
(4 marks)
6
(ii) Consider process (2), find
(1) the work done by the gas,
(2 marks)
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(2) the change in internal energy of the gas
(2 marks)
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(3) the amount of energy released by the gas.
(2 marks)
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Essence 2_P105_3
4.
(a) Define the term ‘sound intensity level’.
(1 marks)
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(b) The minimum intensity for a sound to be heard is 10-12 W m-2. For a
loudspeaker producing sound waves at a power of 10 mW, find the
maximum distance for the loudspeaker that sound can be heard by an
audience. State the assumption in your calculation
(3 marks)
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(c)
A certain machine in a construction site produces a 95 dB intensity level
when operating.
(i) Find the maximum number of machines which can be operating at the
same time in the site if the noise level is not permitted to exceed 100 dB.
(3 marks)
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__________________________________________________________
(ii) Suppose the background produces a noise level of 90 dB. Would the
total noise level exceed the noise limit of 100 dB when the maximum
number of machines found in (a) (i) are operating in the construction
site?
(4 marks)
__________________________________________________________
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(d) Write down the approximate noise level found in the following situations.
Situation
Noise level / dB
(i) inside the hall when an examination is in progress
(ii) beside a swimming pool where the cheering team is
shouting
(iii) when a jet-plane lands on the runway
(e) Suggest one method to minimize sound pollution on flats built near highway.
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5.
(a)
eyepiece
objective lens
object
ho
Fo
hi
P
Q
Fo
FE
FE
final
image
Figure 2.1
A student uses two converging lenses to set up a compound microscope in
normal adjustment. Figure 2 shows two light rays, P and Q, form the top of
an object falling on the objective lens of the microscope. The foci of the
objective lens are denoted by FO and the foci of the eyepiece are denoted by FE.
(i)
On Figure 2, complete the ray paths for P and Q as they pass through the
microscope, showing how the final image is formed.
(2 marks)
(ii)
Indicate on Figure 2 the visual angle  subtended by the final image at
the eye of an observer using the microscope.
(1 mark)
(iii) Distinguish between linear magnification and angular magnification.
(2 marks)
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_____________________________________________________________
(iv) Find the angular magnification of the microscope in terms of the height of
the object, ho, and the height of the final image, hi. Show your working.
(Take the least distance of distinct vision to be D)
(2 marks)
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(b) Figure 3 shows four light rays from an object passing through a microscope in
normal adjustment. R and S come from the top of the object, T and U come
from the bottom. R and T pass through the top of the objective lens, S and U
pass through the bottom.
R
object
T
S
U
objective
lens
eyepiece
X
Figure 2.2
3
10
(i)
On Figure 3, X is the best position for the eye to view the image. With
reference to the ray diagram, briefly explain the advantage(s) of choosing
X as the viewing position.
(3 marks)
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(ii)
Why should the diameter of the beam at X be no wider than about 2 mm?
(1 mark)
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Name: __________________ (
)
F.7__________
SECTION B (53 marks)
I
II
Answer ALL questions.
Write your answers in the spaces provided in this question paper. You should
show all the main steps of your calculations.
III Assume: velocity of light = 3x108 ms-1
Acceleration due to gravity = 10ms-2
Question No.
6
7
8
9
Marks
14
11
12
16
6.
(Given: Planck constant h = 6.63  10-34 Js
charge of electron e = 1.60  10-19 C
speed of light in vacuum c = 3.00  108 m/s)
G
C
A
4.9 V
Ia
4.9 V
Electrons
Q
Glass tube
A
Variable
d.c. supply
V1
Figure 11.1
4
R
V2 =1V
P
V1 /V
Figure 11.2
5
Figure 4 shows the schematic diagram of the apparatus used by Franck and Hertz.
The electrons emitted at the cathode C are accelerated to the grid G by a potential
difference of V1. The electrode A has a voltage V2 of 1 V negative with respect to
the grid G. Inside the glass tube there is mercury vapour at a pressure of about
100 N/m2. A graph of anode current Ia against V1 is shown in Figure 5.
12
(a) Account for the shape of the graph when V1 is
(i)
less than P,
_____________________________________________________________
_____________________________________________________________
(ii)
between P and Q,
_____________________________________________________________
_____________________________________________________________
(iii) between Q and R.
(5 marks)
_____________________________________________________________
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(b) From the graph in Figure 5, estimate the lowest excitation energy for mercury.
(1 mark)
_____________________________________________________________
_____________________________________________________________
(c) What is the wavelength of the radiation emitted by the mercury atoms as they
return to their ground state? Could such a radiation be detected? Explain
your reasoning.
(4 marks)
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13
(d)
Energy in eV
0.0
-1.6
Diagram NOT to scale
-3.7
-5.5
-10.4
(i)
Ground state
The energy level diagram shows that mercury has another excitation
potential at 6.7 V. Why is this not shown by the graph in Figure 5 ?
(2 marks)
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_____________________________________________________________
(ii)
What would your expect to happen to a photon of energy 9 eV when it
collides with a mercury atom?
Explain your answer briefly. (2 marks)
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14
7.
+6 V
20 k
30 k
4k
10 k
Y
OA
+
X
50 k
_
R
Vo
to air-conditioner
switching circuit
LED
T
-6 V
6
Figure 12
The temperature of an air-conditioned room can be controlled by means of a
thermostat circuit which consists of a thermistor bridge circuit and an ideal
operational amplifier OA shown in figure 6. T is a thermistor. An indicator LED
light is to be switched on if the room temperature is higher than the preset value
and the air conditioner is then switched on automatically.
(a) (i)
With the variable resistor R set at its midpoint position, the LED lights up
when the room temperature is 25 ºC. If now R is decreased slightly the
LED turns off automatically. Explain why this happens. (You may
assume that the LED lights when Vo > 0).
(3 marks)
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_____________________________________________________________
_____________________________________________________________
(ii)
Briefly explain how the circuit controls the switching on and the
switching off of the air-conditioner.
(3 marks)
15
_____________________________________________________________
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_____________________________________________________________
(iii) What is the use of the variable resistor R in the bridge circuit?
(2 mark)
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(b) For a room temperature of 25 ºC, and with R set at its mid-position (R = 25 k),
calculate the potential at point X with reference to earth.
(3 marks)
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8.
V
Figure 67
16
(a) In an experiment with an illuminated photocell using caesium as the cathode, a
small current is detected by the microammeter even when the anode is made
slightly negative with respect to the cathode, using the circuit of Figure 7.
Briefly account for this.
(2 marks)
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
(b) The current falls to zero only when the reverse p.d. across the tube reaches a
value Vs, which varies with the frequency f of the radiation used to illuminate
the cathode. Figure 8 shows the relationship between Vs and f.
1.5
1.0
V /V
s
0.5
0
4
5
6
7
8
f /1014 Hz
Figure 78
(i)
What is the relationship between Vs and f as predicted by Einstein’s
photoelectric theory?
(2 marks)
_____________________________________________________________
_____________________________________________________________
17
(ii)
What is the value of the threshold frequency for caesium?
(1 mark)
_____________________________________________________________
(iii) If the electronic charge is 1.6  10-19 C, estimate a value for the Planck
constant.
(2 marks)
_____________________________________________________________
_____________________________________________________________
(iv) Calculate the work function for caesium in electron-volts.
(3 marks)
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
(v)
Sketch on Figure 8 the corresponding variation between Vs and f for a
photocell whose cathode has a larger work function than caesium.
(2 marks)
essence 1 P275_3
9. (a)
Figure 9
18
Figure 9 shows a circuit consisting of an air – cored solenoid L, a light bulb B and
an a.c. source of constant voltage. Explain what happens to the brightness of B
when each of the following changes occurs.
(i) An iron rod is inserted into the coil.
(2 marks)
_____________________________________________________________
_____________________________________________________________
(ii) The frequency of the a.c. source decreases.
(2 marks)
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_____________________________________________________________
(b)
V
R
L
C
Figure 10
Figure 10 shows a resistor R, an inductor L and a capacitor C connected to an
a.c. source of r.m.s. voltage V and frequency 2 kHz. The p.d. across R, L and
C are 5V, 4V and 9V respectively. The r.m.s. current is 2A.
(i) Find the value of R, L and C.
(6 marks)
_____________________________________________________________
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19
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
(ii) Find the value of V
(2 marks)
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_____________________________________________________________
(iii) With the aid of a phasor diagram, find the power factor
(2 marks)
(iv) Find the power of the circuit.
(2 marks)
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
***** End of Paper *****
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