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05 AL Physics/Essay/P.1
HONG KONG ADVANCED LEVEL EXAMINATION
AL PHYSICS
2005 Essay Type Question
1. (a) State the necessary condition(s) for an oscillation to be simple harmonic. State
whether there is a relationship between the period and amplitude of a simple
harmonic oscillator.
(2 marks)
(b) A small pendulum bob of mass m is suspended by an inextensible light string
of length l. The simple pendulum is now set into oscillation in a vertical plane.
(Neglect air resistance.)

l
m
(i) Show that, for small amplitudes of oscillation, the motion is simple
harmonic. Derive an expression for the period of oscillation in terms of l
and the acceleration due to gravity g.
(ii) Describe an experiment to measure the acceleration due to gravity g using
a simple pendulum. You should include THREE precautions that enable
you to obtain a reliable result.
(11 marks)
(c) Describe and explain the effect(s), if any, on the oscillation of a simple
pendulum if it were set up
(i) on the moon’s surface;
(ii) in a spacecraft orbiting around the earth.
(3 marks)
2. (a) (i) Explain, by referring to its nature, why light can be plane-polarized. State
the reason why sound cannot be plane-polarized.
(ii) Describe how you would demonstrate the following in a school laboratory.
(I) Light from a filament lamp is not plane-polarized
(II) Polarization by scattering of light
05 AL Physics/Essay/P.2
(iii) Explain why polaroid sunglasses can effectively reduce the ‘glare’
reflected from the surface of water.
(11 marks)
(b) Describe and explain TWO practical applications involving ‘interference’.
(5 marks)
3. (a)
A
B
R
(i) By considering a unit charge moving round the above circuit, distinguish
between the electromotive force (e.m.f.) of the battery and the potential
difference (p.d.) across its terminals A and B. Explain why, in general, the
p.d. across the terminals of the battery is lower than its e.m.f.
(ii) The resistor R is about 10 k. Draw a circuit diagram to show how a
moving-coil milliammeter and a voltmeter should be connected in the
circuit in order to measure the resistance of R. Explain whether the value
of R calculated from the readings of the milliammeter and voltmeter would
be greater or smaller than its actual value.
(6 marks)
(b) Consider a fixed mass of an ideal gas contained in a cubical box. Show that,
from first principles, the pressure p of the gas is proportional to the mean
square speed c 2 of the gas molecules. State the major assumption(s) you have
made.
(10 marks)
4. (a) (i) State Faraday’s law of electromagnetic induction.
(ii) Explain how eddy currents are formed and describe their application in
moving-coil meters.
(5 marks)
(b) (i) With the aid of a labelled diagram, describe the structure of a transformer
suitable for supplying 12 V a.c. from a 220 V 50 Hz mains supply. The
transformer may be considered to be ideal.
(ii) For an ideal transformer, explain
(I) why it will not operate with a steady d.c. input;
05 AL Physics/Essay/P.3
(II) how a sinusoidal input voltage will produce an output e.m.f. of the
same frequency.
(6 marks)
(c) Describe an experiment to show that the magnetic flux density B at the centre
of a flat, circular coil is directly proportional to the current I in the coil.
(5 marks)
5. (a) A group of students carried out a series of experiments investigating the
emission of electrons from metals that are illuminated by electromagnetic
radiation. They found that
(1) more electrons are ejected from the metal when a higher intensity of
incident radiation at a constant frequency is used to illuminate the
metal;
(2) the maximum kinetic energy of the ejected electrons is independent of
the intensity of the incident radiation;
(3) no electrons are emitted from the metal if the frequency of the
incident radiation is below a certain value.
(i) Describe briefly how you would demonstrate the observation (2).
(ii) According to the wave theory of electromagnetic radiation, energy is
delivered continuously and it is solely determined by the intensity of the
radiation.
(I) Describe clearly which two observations are contrary to the
predictions of wave theory.
(II) Einstein suggested a theory to explain the two observations in (I).
Briefly explain Einstein’s theory and use it to account for these two
observations.
(11 marks)
(b) When compared with ordinary light, laser light is monochromatic, coherent and
in general more intense. Explain these characteristics by referring to the
principle of how laser light is produced.
(5 marks)
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