Download 07essay

07 AL Physics/Essay/P.1
HONG KONG ADVANCED LEVEL EXAMINATION
AL PHYSICS
2007 Essay Type Question
1. (a) (i) Describe an experiment using an air track to verify that momentum is
conserved for a completely inelastic collision in which a body collides
with another body initially at rest.
(ii) Show that the law of conservation of momentum for two bodies colliding
in one dimension can be deduced from Newton’s laws of motion.
(7 marks)
(b) A bullet is fired horizontally towards a wooden sphere suspended from point
O by a light rigid rod. The rod is free to rotate about O. On striking the
sphere, the bullet is embedded in it and the sphere then swings upwards.
Discuss and explain whether the law of conservation of momentum and the
law of conservation of mechanical energy can be applied to the system (bullet
+ sphere) in the following processes:
Process 1: From the time when the bullet just strikes the sphere until they start
to move together, assuming this time interval is short so that the position of
the sphere remains almost unchanged.
Process 2: From the time when the system (bullet + sphere) starts to swing
upwards until it reaches its highest position.
Neglect air resistance in your discussion.
(5 marks)
(c) The figure shows an elastic oblique collision between two spheres in two
dimensions. Sphere B is initially at rest. Spheres A and B move away at 90
after collision. Draw a diagram to illustrate the relation among the momentum
vectors before and after collision. Hence show that the two spheres must be of
equal mass.
(4 marks)
07 AL Physics/Essay/P.2
2. (a) State ONE feature to distinguish longitudinal and transverse waves. Give
ONE example for each kind of wave.
(2 marks)
(b) Describe how a sound wave is produced and how it passes through air to cause
our sensation of sound. Draw a graph to represent the spatial variation of the
displacement of the air particles along the path of a travelling sound wave of a
certain wavelength and indicate the locations corresponding to compressions
(C) and rarefactions (R).
(6 marks)
(c) A loudspeaker L, a microphone M, a signal generator and a dual trace
oscilloscope are connected as shown below.
(i) With the aid of a diagram, describe how you would perform an
experiment in the school laboratory to show that the phase of the
vibration of the air particles changes for different points along the path of
a travelling sound wave.
(ii) State and explain how to find the wavelength and speed of the sound in
air. Given that the speed of sound in air is of the order of 102 ms-1,
suggest an appropriate separation between the loudspeaker and the
microphone, and state the frequency of the signal generator to be used.
Explain briefly.
(iii) Explain why the loudspeaker and the microphone should be placed close
to the bench.
(8 marks)
3. (a) (i) The figure shows a circuit consisting of a cell of e.m.f.  and internal
resistance r, and two uniform resistance wires BC and CD. The two wires
are equal in length and are made of the same material, but the cross-
07 AL Physics/Essay/P.3
sectional area of BC is double that of CD. The total resistance of the two
wires is R. All other connecting wires are of negligible resistance.
Sketch a graph showing the readings of the voltmeter versus the path
length along the circuit when its flying lead is connected to various points
along the path ABCDE. Indicate on your graph the voltmeter readings at
various points in terms of , r and R. Explain the conversion of electrical
potential energy of a charge +q passing through each section of the
complete circuit.
(ii) Using a simple model of electron conduction, describe how heat is
generated in a resistance wire by a current.
(9 marks)
(b) (i) The figure shows a metallic conductor of length L carrying a current I.
The number of conduction electrons per unit volume is n and the charge
of an electron is e. The conductor is placed perpendicular to a uniform
magnetic field of flux density B. It is known that the magnetic force
acting on a conduction electron due to its average drift velocity v is Bev.
Hence, derive the magnetic force acting on the whole length of the
conductor in terms of current I.
(ii) With no current flowing in the conductor, it is being moved with uniform
velocity u inside the uniform magnetic field B pointing into the paper as
shown, a potential difference  is developed across its ends XY.
Referring to the force acting on a conduction electron, briefly explain
how  is produced and find an expression for its magnitude.
(7 marks)
07 AL Physics/Essay/P.4
4. Kinetic theory relates the macroscopic behaviour of an ideal gas with the
microscopic properties of its molecules.
(a) The gas pressure is due to the collisions of the gas molecules with the walls of
the container.
(i) Despite the fact that the incident angle and the incoming speed of the gas
molecules vary greatly among the molecules, the pressure on the
container wall is steady if the volume and the temperature of the gas are
kept constant. Explain.
(ii) The collisions are assumed to be elastic.
What would happen
macroscopically if the collisions were inelastic? Explain how you can
deduce from observation that the collisions are in fact elastic.
(5 marks)
(b) (i) In terms of the microscopic interpretation of pressure, explain
qualitatively why the pressure of a gas can be kept constant when its
volume increases with the temperature.
(ii) Describe an experiment to find the relationship between the volume and
the temperature of a gas when its pressure is kept constant. Sketch and
label the set-up of the experiment. How can the absolute zero be
estimated from the experiment?
(9 marks)
(c) A gas at low temperature and under high pressure would deviate from ideal
gas behaviour. Explain briefly with reference to the assumption(s) of the
kinetic theory.
(2 marks)
5. (a) What is meant by the binding energy of a nucleus? Sketch the graph of
binding energy per nucleon against atomic number, and use the graph to
explain
(i) which of the following reactions can produce more energy per unit mass
of reactant:
(I) fission of uranium in a nuclear reactor
(II) fusion of hydrogen into helium-4 in the sun’s nucleus
(ii) why iron ( 56
26 Fe ) is one of the most stable nuclei.
(5 marks)
(b) The energy levels of the electron in a hydrogen atom is given by
En =
 13.6
eV
n2
07 AL Physics/Essay/P.5
(i) Explain the meaning of ground state, excitation energy and ionization
energy of a hydrogen atom with this expression.
(ii) Explain the production of emission line spectra in terms of light quanta
and energy levels.
(5 marks)
(c) (i) Explain what Fraunhöfer lines in the solar spectrum are and how they are
produced. Describe how scientists can obtain information about the
composition of the sun from these lines.
(ii) Describe briefly how the velocity of a distant star relative to the earth
along the line of sight can be estimated from the light spectrum of the
star.
(6 marks)
END OF PAPER