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96 AL Physics/Essay/P.1
HONG KONG ADVANCED LEVEL EXAMINATION
AL PHYSICS
1996 Essay Type Question
1. (a) The centripetal acceleration, a, of a body undergoing a uniform circular
v2
motion of radius r can be expressed as either (1) a 
or (2) a = 2r, where
r
v and  are linear and angular speeds of the body respectively. For a uniform
circular motion of constant period, student A thinks that a decreases with r
according to equation (1), while student B argues that a increases with r
according to equation (2). Comment on their arguments.
(2 marks)
(b) To study a circular motion, a small rubber bung of mass m is attached to one
end of a piece of string passing through a thin glass tube, which has a weight
W hanging at its other end. The rubber bung is set into a horizontal circular
motion by a student holding the glass tube.
L
A
glass
tube
paper
marker
rubber
bung
W
(i) Draw a diagram to show the forces acting on the rubber bung and explain
why the string is not horizontal but dips at a small angle .
(ii) Show that the weight W equals m2L in theory, where  is the angular
speed and L is the length of the string beyond the upper opening of the
glass tube. Give TWO reasons to explain why there are discrepancies
between the experimental and theoretical results.
(iii) Suggest TWO ways to increase the rate of revolution of the rubber bung.
(iv) When the rubber bung is at position A, the string suddenly breaks.
Describe and explain its subsequent motion.
(7 marks)
(c) With the aid of a diagram, describe and explain the action of a centrifuge.
Give a practical use of a centrifuge.
(5 marks)
(d) A satellite revolves round the earth whereas an electron revolves round a
proton inside a hydrogen atom. State TWO similarities and TWO differences
between the two systems.
(2 marks)
96 AL Physics/Essay/P.2
2. (a) (i) For a sound wave of constant amplitude and frequency travelling through
air, sketch a graph to show the time variation of the displacement of the
vibrating air particles at a point in the path of the wave. Using the same
time axis, sketch also the variation of air pressure with time at the same
point.
(ii) What is the phase relationship between the displacement and the air
pressure in (a)(i)?
(2 marks)
(b)
X
loudspeakers
O
Y
Two identical loudspeakers connected to the same signal generator are placed
inside a room as shown. All the surfaces of the room are covered with
sound-absorbing materials. Point O is equidistant from the loudspeakers are
line XOY is parallel to the line joining the loudspeakers. The variation of
sound intensity along XOY is shown below:
sound
intensity
position
X
O
Y
(i) Explain why
(I) the graph shows alternate maximum and minimum;
(II) the sound intensity at a minimum point is non-zero.
(ii) State clearly the difference between sound intensity and sound intensity
level. Explain why the sound intensity level at O wound decrease by 6
dB if one loudspeaker is turned off.
(iii) Explain the change(s), if any, in the variation of sound intensity along
XOY if the frequency of the signal generator is increased.
(8 marks)
(c) With the aid of a diagram, describe an experiment to find the speed of sound in
air by using Kundt’s tube together with a loudspeaker. State any precautions
96 AL Physics/Essay/P.3
which should be taken in the experiment and explain what would be observed.
(6 marks)
3. Models are frequently used by physicists to illustrate abstract concepts.
question deals with three of them.
This
(a) (i) In an ideal gas model, gas consists of numerous molecules of negligible
volume. These molecules move randomly without intermolecular forces
acting between them except during collisions, which are all elastic.
Describe qualitatively how this model could provide a microscopic
interpretation of the macroscopic quantities such as the pressure and
temperature of a gas.
(ii) What is an ideal gas from the macroscopic point of view?
conditions would a real gas behave like an ideal gas?
Under what
(4 marks)
(b) Briefly describe a simple model for electron conduction in a metal. Explain
how this model can account for the heating effect of an electric current in a
metal.
(6 marks)
(c) (i) One of the early models of an atom was introduced by Rutherford. Give
a brief description of this atomic model.
(ii) How could this model explain the experimental results obtained in the particle scattering experiment?
(iii) Name ONE phenomenon of an atom that Rutherford’s model failed to
account for.
(6 marks)
4.
Source of
energy
Generator
Transformers
Consumers
The above block diagram shows how electric power is supplied to consumers.
(a) (i) Name ONE major source of energy that is currently used in electric power
stations in Hong Kong. Briefly describe how the energy released from
the source can be used to drive a generator in the power station.
(ii) Draw a labelled diagram of a simple a.c. generator. State the conditions
for the output to be sinusoidal a.c. of frequency 50 Hz.
(6 marks)
96 AL Physics/Essay/P.4
(b) For steady power transmission, show that the power loss in the cables is
inversely proportional to the square of the output voltage from the power
station.
(2 marks)
(c) (i) Electric power companies usually use overhead power cables for
transmitting electricity over large distance, however the general public
often prefer cables to be underground. Give TWO supporting reasons
for each party.
(ii) An overhead power cable usually consists of a central core of several steel
wires, which is surrounded by many strands of aluminium wire. Give
ONE advantage and ONE disadvantage of using aluminium instead of
copper. Why is it necessary to include several steel wires in the cable?
(6 marks)
(d) Low voltage steady d.c. is required for charging a battery inside a Walkman.
Draw a circuit diagram to show how the mains supply of 220 V a.c. can be
stepped down by a transformer, rectified by a bridge rectifier consisting of four
diodes, smoothed by a circuit consisting of two capacitors and an inductor, and
finally connected to the battery.
(2 marks)
5. (a) Draw a labelled diagram of the basic structure of a cathode-ray tube (CRT)
including the electron gun, the deflecting system, the display system and a
potentiometer circuit for the E.H.T. supply applied to various parts of the
CRT. (Detailed electronic circuits NOT required.)
(4 marks)
(b) (i) Describe briefly how electrons are produced in the electron gun.
(ii) Explain how the sharpness and brightness of the trace on the screen of a
CRT can be adjusted.
(iii) Explain why the inside of a CRT is coated with graphite and why it is
earthed.
(6 marks)
(c) (i) With the aid of a graph, describe the function of the time base circuit in a
CRO.
(ii) At the input select of a CRO, the input terminal for a.c., compared with
that for d.c. has an extra built-in component. Name that component and
state its function.
(4 marks)
(d) A CRO can be used to measure voltages. Give TWO advantages and TWO
disadvantages of using a CRO as a voltmeter as compared with a moving-coil
meter.
(2 marks)
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