Electromagnetic Induction Summary and Review
Summary
1. The basic principle of electromagnetic induction, discovered by Faraday, states: "Whenever the magnetic field in the
region of a conductor is changing in magnitude or direction, electrons are induced to flow through the conductor."
2. The magnitude of the electric potential difference induced in a coil depends on:
(a) the number of turns on the induction coil,
(b) the rate of change of the inducing magnetic field, (c) the strength of the inducing magnetic field.
3. The direction of an induced current is given by Lenz's Law, which states: "The electrons in an induced current flow in
such a direction that the magnetic field they create opposes the action of the inducing field."
4. Mechanical energy may be converted into electrical energy by means of a generator. A coil of wire is rotated in a
uniform magnetic field in such a way that an electric current is continuously being induced in the coil.
5. The magnitude of the electric potential produced by a generator depends on:
(a) the speed of rotation of the coil, (b) the strength of the field magnet, (c) the number of turns on the coil.
The direction of the current induced in the coil at any point in the rotation is determined by Lenz's Law. The direction
of the current in the coil is reversed every half-cycle, when the axis of the coil is parallel to the field lines.
6. If connection is made to the rotating coil by two slip rings and brushes, the current leaving the generator changes
direction every half-cycle, and it is called alternating current. If connection is made to the rotating coil by a single split
ring (commutator) and two brushes, the current leaves the generator in the same direction throughout the entire cycle,
and it is called direct current.
7. Alternating current forms the basis of modern large-scale electrical energy transmission and distribution networks.
Electrical energy is transmitted with much greater efficiency at high voltages, and these high voltages are achieved
by the use of transformers, which operate on AC.
8. A transformer can increase or decrease the electric potential, depending on the relative number of turns on its
primary and secondary coils. For an ideal transformer:
V1 N1
=
V2 N2
V1 I2
=
V2 I1
9. For efficient transmission of electrical energy, the potential is increased at the generator to a very high value, and
then progressively decreased after transmission. Transformers are used at the various stages of the distribution
network until the potential is reduced to a value of 240 V and 120 V, ready for use in the home.
Review
1. (a) Explain what is meant by electromagnetic induction, and describe how an electric current or potential difference
may be induced in a conductor.
(b) State three factors that affect the magnitude of the potential difference induced in a coil.
(c) Where does the electrical energy of the induced current and potential difference come from? What other form of
energy is used up?
2. Two identical coils are placed side by side, as shown on the next page, and one is connected to a battery through a
knife switch, while the other is connected to a galvanometer.
(a) Describe the effect on the other coil of opening and closing the knife switch.
(b) Would the same effect occur if one of the coils had been rotated by 90°, putting it at right angles to the other
coil?
(c) How would the insertion of a bar of iron into the two coils, as shown by the dotted line, change the effect of
opening and closing the switch?
3. (a) State Lenz's Law for the direction of an induced current.
(b) In this diagram, a straight conductor is lifted through the magnetic field shown. Determine the direction of the
induced current in the wire.
(c) Determine the direction of the current induced in the coil illustrated, by the moving magnet.
4. (a) What is the major structural difference between an AC generator and a DC generator?
(b) How does the current flowing through the armature coil of a DC generator differ from the current flowing through
the armature coil of an AC generator?
(c) In comparison with the number of separate armature coils in a DC generator, how many segments should the
generator's commutator have?
5. (a) Sketch a simple AC generator, labelling four key parts and describing the function of each.
(b) Show how Lenz's Law and the motor principle may be used , together to determine the direction of the induced
current at any point in the rotation.
(c) What features in the design of a practical form of AC generator have an effect on the potential difference the
generator will produce?
(d) Draw a graph of induced potential difference versus time for a model AC generator, rotating three times per
second and producing a peak potential difference of 2.5 V. Label the time axis at each point where the graph
crosses it.
6. What are the advantages of using high-voltage AC when transmitting electric power over long distances?
7. Draw a sketch of a transformer, labelling three main parts. State the function of each part, and show which laws of
electromagnetism and electromagnetic induction govern its operation.
8. List careers where knowledge of electromagnetism and electromagnetic induction are necessary.
9. A bar magnet inserted completely into a coil of 500 turns produces an induced potential difference of 1.5 V.
Determine the potential difference induced when each of the following changes is made, considered separately.
(a) A 250-turn coil is used.
(b) The bar magnet is moved twice as quickly.
(c) Three identical magnets of the same polarity and strength are inserted at once, side by side.
(d) All three of the above changes are made together.
10. A step-up transformer is designed to operate from a 12 V AC supply and to deliver energy at 240 V. If the
secondary winding is connected to a 60 W, 240 V lamp, determine
(a) the turns ratio of the transformer
(b) the primary current.
11. The transformer used to operate a model electric train has 1125 turns on its primary coil and 75 turns on its
secondary. If it is plugged into a 120 V circuit, what potential difference does the model train receive?
12. A magnet induces a current of 8.0 mA in a 500-turn coil connected to a galvanometer. The total resistance of the
coil and galvanometer is 100 Q. The coil is then replaced by a 1500-turn coil, making the total resistance in the
circuit 150 Q. What current would then be induced by the same magnet moving at the same speed?
13. A generating station develops 15,000 kW of electric power. Assuming 100 per cent efficiency of transmission,
calculate the current flowing through a 500,000 V transmission line leading from the generating station.
Answers!
9. (a) 0.75 V
(b) 3.0V
(c) 4.5 V
(d) 4.5 V
10. (a) 1:20
(b) 5.0 A
11. 8.0 V
12. 16 mA
13. 30 A