Download Essay 90-4cd DC motor

C5b Induction Essay
Essay 83-2
Hall effect and Search coil
Describe briefly the methods adopted in the measurement of
(a) a steady magnetic field using a Hall probe, and
(b) an alternating magnetic field using a search coil.
Compare the different physical mechanisms in these tow methods and derive any necessary
mathematical expressions.
Comment briefly on the main factors that might affect the accuracy of the measurements.
[15]
Essay 84-6a
(a)
Inductor
inductance
L
current
I
resistance
R
Key K
e.m.f. E
Figure 1
Write down a differential equation for the current I in the circuit in Figure 1 after the key K has
been closed at a time t = 0, explaining clearly each term in the equation.
[3]
Essay 84-6b
(b)
Generator
rotation axis
perpendicular to paper

magnetic field
of uniform
flux density B
starting position of coil ( t = 0)
Figure 2
A coil of N turns, each of area A, is situated initially (at time t = 0) with the plane of the coil
perpendicular to a magnetic filed of uniform flux density B. The coil is then rotated at a uniform
angular velocity  about an axis perpendicular to the magnetic field and passing through the centre
of the coil, as shown in Figure 2. Derive an expression for the e.m.f. induced in the coil at any
subsequent time t.
[3]
Essay 86-4a
DC motor
Using the rotating coil of a simple d.c. motor as an example, describe, with the aid of a diagram, the
force effects of a magnetic field on an electrical current. (Details of the commutator are not required.)
[3]
Essay 90-4cd
DC motor
c) Using suitable diagrams, explain the working of a simple d.c. motor, where the magnetic field is
produced by a permanent magnet.
[4]
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.
[4]
Essay 98-4a
DC motor
i) Draw a labeled 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.
motor.
iii) The magnetic fields of some electric motors are provided by electromagnets. Give the advantages
of using electromagnets over permanent magnets.
[7]
Essay 96-4
Transformer
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 of 50 Hz.
[6]
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]
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]
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]
Essay 97-4
Generator
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]
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.
With the aid of a labelled diagram, describe the construction of a generator to provide a d.c. to a light
bulb using the methods in (I) [ smoothing is not required ]. Show that the current generated is always
flowing in one direction through the bulb.
ii) 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]
Essay 98-3a
Induced emf
a) i) The figure below shows a rectangular metallic loop ABCD being pulled with constant speed
out of 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]
Essay 98-4
Motor
4. a) i) Draw a labeled 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]
b) The figure below 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]
Essay 99-5
Induced emf
5. a) i) 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 a 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]
b) 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]
Essay 00-3a
Transformer
3. a) i) Give TWO necessary conditions under which the relation Vs : V p  N s : N p holds for a
transformer.
ii) State and explain TWO designs for a transformer to achieve high efficiency.
iii) 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]
Essay 01-4c
Induced emf
c) A metal rod PQ of length l is moved 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]
Essay 02-4b
Generator
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 = 0 s, the coil is rotated in a clockwise
direction with a uniform angular speed .
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 within 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.
[7]
Essay 03-4a
Self induction
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)