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03 AL Physics/Essay/P.1
HONG KONG ADVANCED LEVEL EXAMINATION
AL PHYSICS
2003 Essay Type Question
1. (a) (i) A block moving with a certain initial speed is acted upon by a constant
resultant force F along its direction of motion. Show that the work done
by F is equal to the change in kinetic energy of the block.
(ii) An electron is projected with an initial velocity v into a region with a
uniform magnetic field perpendicular to v. Discuss the work done by the
magnetic force acting on the electron and its change in kinetic energy.
(Neglect the effects of gravity.)
(5 marks)
(b) An object is thrown vertically upward from the earth’s surface with a certain
initial speed. It rises to a maximum height and then falls back to the starting
point.
(i) What is the work done by the gravitational force in the process? Explain.
(ii) Discuss the effect(s), if any, of the air resistance and upthrust on the final
speed of the object when it returns to the starting point. Referring to this
example, explain the principle of conservation of mechanical energy and
state the necessary condition for it to be valid. (Note: When an object is
immersed in a fluid, a constant upward force or upthrust acts on it by the
fluid.)
(6 marks)
(c) Based on the kinetic theory model of an ideal gas, we have the equation
1
pV  Nmc 2 .
3
(i) State TWO assumptions of the kinetic theory model of an ideal gas.
Besides pressure p and volume V of an ideal gas, what do the rest of the
symbols in the equation represent?
(ii) Use the given equation to show how the average kinetic energy of the
molecules of an ideal gas is related to its absolute temperature. Explain
briefly.
(5 marks)
2. (a) State THREE major differences between sound waves and light waves in terms
of their physical nature and properties.
(3 marks)
(b) (i) Describe how Huygens’ construction method can be used to show that
(I) light travels in straight lines, and
03 AL Physics/Essay/P.2
(II) light refracts when it goes from one medium to another.
(ii) Account for the dispersion of white light into different colours by a prism.
(Label only the red and violet light rays if you choose to draw a diagram.)
(7 marks)
(c) Describe an experiment to determine the speed of sound in air. Give the theory
and show how the speed of sound is calculated from the measurements. State
the source(s) of error of the experiment.
(6 marks)
3. (a) (i) A circuit consists of a battery and a resistor connected by conducting wires.
Explain why the potential difference across the terminals of the battery is
smaller than its e.m.f.
(ii) Consider an accumulator being charged by a voltage source, explain the
relation between the potential difference across the accumulator and its
e.m.f. Briefly describe the energy conversion inside the accumulator
during charging.
(4 marks)
(b) (i) Explain why the order of magnitude of the drift velocity of the free
electrons in a circuit is much smaller than their random speeds (~105 ms-1).
(ii) Explain why an electric current starts to flow at every point in a circuit
almost at the same instant when the switch is closed.
(5 marks)
(c) (i) Based on the torque acting on a rectangular current-carrying coil in a
magnetic field, describe and explain the design feature(s) of a moving-coil
galvanometer that give a linear scale. (Mathematical derivation is
expected.)
(ii) Discuss TWO factors that determine the current sensitivity of the
galvanometer.
(7 marks)
4. (a) Explain why sparking would occur between the switch contacts if the current in
a coil is interrupted when the circuit breaks but not when it is closed. Explain
the energy change when the circuit breaks and how sparking can be prevented
by adding a capacitor to the circuit.
(5 marks)
(b) A source of sinusoidal a.c. voltage V = V0 sin (2 f t) is applied across a coil of
resistance R and inductance L.
(i) Sketch the graphs of current and voltage of the coil against time for 2 f L
>> R and explain their phase relationship.
03 AL Physics/Essay/P.3
(ii) Illustrate the difference between resistance and reactance by stating their
physical meaning. Explain why the size of the current depends on the
frequency f of the source.
(5 marks)
(c) The figure shows the circuit for a power pack.
L
A
220 V
a.c.
D
B
C1
C2
R
output
C
(i) Briefly explain how the rectification unit ABCD works. Sketch and
explain the output voltage if capacitor C2 and inductor L are absent.
(ii) Sketch the voltage across L as well as the output voltage. Account for
their shapes by explaining the respective functions of C2 and L.
(6 marks)
5. (a) (i) Briefly describe how a hydrogen spectrum can be produced and observed
in the laboratory.
(ii) Explain why the existence of spectral series of hydrogen supports the
theory of discrete energy levels in atoms.
(5 marks)
(b)
Account for the dark lines in the sun’s spectrum. Explain how the elements in
the sun’s atmosphere can be identified by studying these dark lines. (4 marks)
(c)
In an X-ray tube, electrons are accelerated through a large potential difference
so that X-rays are produced when the electrons strike a tungsten target.
Describe and explain the characteristics of the X-ray spectrum produced.
(There is no need to describe the structure of the X-ray tube.)
(7 marks)
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