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AL-Essay-E&M / p.1
1.
(89-IIA-4)
(a) Give the theory of the production of an a.c. voltage by a plane coil rotating in a uniform magnetic
field, identifying the maximum and zero voltages with the positions of the coil.
(5 marks)
(b) Draw a diagram of the circuit of a d.c. power pack used for generating a variable d.c. voltage from
the a.c. mains, giving rough values for any inductors, capacitors and resistors used.
(3 marks)
(c) With reference to your circuit in (b), briefly explain the physical principles involved in
(i) the transformer (working under ideal conditions),
(ii) the full-wave rectification process, and
(iii) the filter.
For (iii) explain your choice of the values of any inductances, capacitances and resistances.
(8 marks)
2.
(90-IIA-4)
(a) Two long parallel wires, each of length l and separated by a distance d, carry a current I in the
same direction. Draw a diagram of this arrangement and on it show
(i) the magnetic field at each wire due to the other, and
(ii) the corresponding forces,
giving their magnitudes.
(3 marks)
(b) How may a similar arrangement be used to define the ampere unit of current?
(1 mark)
(c) Using suitable diagrams, explain the working of a simple d.c. motor, where the magnetic field is
produced by a permanent magnet.
(4 marks)
(d) For this motor explain, in detail, the observed sequence of physical effects of
(i) switching it on with no mechanical load and,
(ii) after a while, applying such a load.
(8 marks)
3.
(91-IIA-4a)
(a)An a.c. voltage supply is connected across a coil of many turns, this coil being placed over the
vertical iron rod of a retort stand and resting on the base. Explain clearly your expected
observations, and the physical principles involved when
(i) a small aluminium ring is dropped over and slides down the vertical rod of the retort stand,
(ii) the ring of (i) is replaced by a similar ring, but broken by a vertical slot and
(iii) the ring of (i) is fastened down on top of the coil.
(6 marks)
4.
(92-IIA-5b,c)
By considering the force on free electrons of charge –e moving with a constant velocity v in a path,
perpendicular to a uniform field B,
(a) explain the production of an electric field E when an electric current is passed through an n-type
semiconductor crystal in a direction perpendicular to a magnetic field B.
(5 marks)
(b) further, explain the production of another electric field E  in a linear conductor moved
perpendicular to the magnetic field B.
(4 marks)
(c) describe an experiment to measure the charge/mass (e/m) ratio for electrons using an electric
field and magnetic field perpendicular to each other. (No theory of the production of the magnetic
field is expected.) Briefly indicate main difficulties.
(7 marks)
AL-Essay-E&M / p.2
5.
(c)
6.
(93-IIA-3c)
Draw a labelled diagram of a moving-coil meter. Briefly describe its working principles and
explain how a linear scale may be achieved.
(8 marks)
(94-IIB-4)
(i) Magnetic fields are usually described in terms of magnetic field lines. Use this concept to
explain the term ‘magnetic flux density’.
(ii) State the factor(s) which determine(s) the total flux linkage for a plane coil placed in a
uniform magnetic field.
(3 marks)
(b)(i) With a suitable diagram, explain qualitatively the working principles of a Hall probe.
(ii) Briefly describe an experiment to investigate the variation of magnetic flux density along the
axis of a solenoid by using a Hall probe. State any precaution(s) needed in the experiment.
(9 marks)
(c) Explain, by means of the laws of electromagnetic induction, how a search coil can be used to
investigate the flux density of an alternating magnetic field. (No mathematical derivation is
required)
(4 marks)
(a)
7.
(96-IIB-4)
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)
(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 distances, 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)
8.
(97-IIB-4)
(a)You are given two bar magnets, a long copper wire and a light beam galvanometer. Describe how
you would use the apparatus to investigate qualitatively the factors affecting the e.m.f. induced in
a coil by electromagnetic induction.
(5 marks)
AL-Essay-E&M / p.3
(b)
(i)
Consider a rectangular coil of N turns rotating uniformly in a uniform magnetic field about
an axis perpendicular to the field. Derive an expression for the e.m.f. produced.
(ii)
With the aid of a labelled diagram, describe the construction of a generator to provide a d.c.
to a light bulb using the method in (i) (smoothing is not required). Show that the current
generated is always flowing in one direction through the bulb.
(iii) Explain carefully why a greater driving torque is needed to maintain the coil of the
generator rotating at the original speed when an identical light bulb is connected in parallel
with the first one. Also explain how this change agrees with the principle of conservation of
energy.
(11 marks)
9.
(98-IIB-3)
(a)
(i)
The figure shows a rectangular metallic loop ABCD being pulled with constant speed out of
a magnetic field, which points into the paper. State Lenz’s law and use it together with the
idea of changing flux to find the direction of the induced current flowing in the loop. Briefly
explain your answer.
(ii) What do you understand by the principle of conservation of energy? Referring to the above
example, justify whether Lenz’s law satisfies the principle of conservation of energy.
(6 marks)
(b) (i) Explain why electrical energy is classified as ‘high grade’ while thermal energy is classified
as ‘low grade’.
(ii) If energy is conserved, why is there an energy crisis?
(iii) Discuss THREE advantages and THREE disadvantages of the domestic use of solar energy.
(6 marks)
(c) (i)
Give TWO reasons why the energy in coal or oil is usually converted into electrical energy
first instead of being used directly.
(ii) Describe the major energy conversions in a coal-fired power station. State a typical
conversion efficiency for this kind of power station.
(4 marks)
10
(98-IIB-4)
(a)(i) Draw a labelled diagram of a simple d.c. motor using permanent magnets. Indicate the
direction of rotation of the motor and state the function of the commutator.
(ii) Describe and explain THREE modifications for improving the turning effect in practical d.c.
motors.
(iii) The magnetic fields of some electric motors are provided by electromagnets. Give the
advantages of using electromagnets over permanent magnets.
(7 marks)
AL-Essay-E&M / p.4
(b)
The above figure shows a simple motor circuit, in which a constant voltage V is applied to a motor
M with coil resistance r while the current flowing is I .
(i) Explain why a back e.m.f. ( Eb ) is generated when the motor rotates and express Eb in
terms of I , V and r . Hence explain carefully why the coil of a motor is liable to be
damaged when a motor is first switched on or when a running motor is suddenly jammed.
(ii) Discuss the variation of current in the above circuit and describe the corresponding changes
in the motor’s rotation speed and driving torque when:
(I) the switch S is closed and the motor does not carry any mechanical load;
(II) a mechanical load is connected to a steadily running motor.
(9 marks)
11.
(99-IIB-5)
(a)
(i)
(b)
12.
Using a solenoid connected to an a.c. source of variable frequency, describe a
simple experiment to investigate how the induced e.m.f. in coil depends on the rate
of change of the magnetic flux linkage through it. List the apparatus used and
describe the observation.
(ii)
State THREE factors that would affect the magnetic flux linkage through a coil.
(9 marks)
Referring to appropriate physical laws, explain the following:
(i)
Sparks occur when opening the switch of a circuit with an electromagnet in it.
(ii)
There is potential difference developed between the wing-tips of an aircraft when it
is flying horizontally in air.
(iii)
The pointer of a moving-coil meter stops at the final steady value as soon as it
reaches that value and shows no further oscillation.
(7 marks)
(00-IIB-3)
(a)
(i)
(ii)
(iii)
(b)
Give TWO necessary conditions under which the relation Vs : Vp  Ns : Np holds for a
transformer.
State and explain TWO designs for a transformer to achieve high efficiency.
The figure below shows a practical transformer with a light bulb connected across the
secondary coil.
If an identical light bulb is connected in parallel with the first one, describe what would happen to the
secondary current, the secondary voltage, the back e.m.f. in the primary coil and the primary current.
(6 marks)
Using a simple current balance, describe an experiment to investigate how the magnetic force depends on
the length of the current-carrying conductor in the magnetic field. State the precautions of this experiment.
(5 marks)
AL-Essay-E&M / p.5
(c)
13.
(i)
Consider a moving-coil meter with a certain current flowing in it. Explain
(I) why the magnitude of the deflecting torque due to the current remains the same when the coil
rotates, and
(II)
how equilibrium is achieved at the steady state.
(ii) Do you agree that a moving-coil meter would be more accurate when it works in a vacuum ? Explain
briefly.
(iii) Describe and explain what would happen when a d.c. moving-coil meter is used to measure the a.c.
mains.
(5 marks)
(01-IIB-4)
(a)
(i)
(ii)
Explain how the SI unit of magnetic field strength, the tesla, is defined.
Explain why two infinitely long straight wires carrying currents in the same direction exert
forces on each other.
(3 marks)
(b) Describe how you would produce a uniform magnetic field using a current-carrying conductor.
Briefly describe an experiment using a Hall probe to show that the field is uniform.
(6 marks)
(c) A metal rod PQ of length l is moved with constant with constant velocity v across a uniform magnetic field of
flux density B as shown. A potential difference of magnitude Blv is developed across PQ.
(i)
By considering the force(s) acting on an electron in the rod, explain how the potential difference is
developed and why it remains constant.
(ii)
Show that the result is consistent with the law of electromagnetic induction.
(iii) What would the situation be if the rod is moved with acceleration? Explain briefly.
(7 marks)
14.
(02-IIB-4b,c)
(b) A rectangular coil of N turns, each of area A, is placed with its plane perpendicular to a uniform magnetic
field of flux density B as shown. At time t=0s, the coils is rotated in a clockwise direction with a uniform
angular speed w.
(i)
Derive an expression for the variation of the induced e.m.f. in the coil with time. Sketch a graph to
show its variation with in one cycle and indicate when the plane of the coil is parallel to the
magnetic field.
(ii) In an actual a.c. generator the coil is wound around a soft iron cylinder, which is laminated. State
and explain TWO advantages of such a design.
15.
(03-IIB-3c)
(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.
16.
(03-II-4a)
(a) Explain why sparking would occur between the switch contacts if the current in a coil in 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.
AL-Essay-E&M / p.6
17.
(04-IIB-4)
(a) Give the meaning of magnetic flux density B in terms of the magnetic force on a current-carrying wire in a
uniform magnetic field. (2 marks)
(b) (i) What is the relation between magnetic flux  and magnetic flux density B?
(ii)
With the aid of an example, explain the meaning of the equation
  N
d
for electromagnetic
dt
induction.
(iii)
Consider a bar magnet being moved towards a coil connected to a galvanometer as shown, illustrate
Lenz’s law and how energy is transferred in this process.
(7 marks)
(b) Referring to a simple d.c. motor, explain the meaning of back e.m.f. An unloaded simple d.c. motor is
connected to a constant voltage s
(c) ource. Describe and explain how the back e.m.f. and the current in the coil of the motor change
(i) before the motor attains its maximum steady speed, and
(ii) when a mechanical load is added to the motor, which then runs steadily to raise the load.
(7 marks)