Electromagnetic Induction – SS3 Physics Lesson Note (Second Term)

Lesson note at a glance

Subject: Physics
Class: SS3
Term: Second Term
Topic: Electromagnetic Induction
Order: Topic 2 of 6 this term
Reading time: 11 min

Electromagnetic induction is the production of an e.m.f. or current in a conductor whenever there is relative motion between the conductor and a magnet or magnetic field. The size of the induced e.m.f. is given by Faraday’s law and its direction by Lenz’s law. The principle is used in induction coils, generators and transformers.

Behavioural objectives

By the end of the lesson, students should be able to:

explain electromagnetic induction and list the factors that affect the induced e.m.f.
state Faraday’s law and Lenz’s law
describe the parts and uses of an induction coil
describe the parts of an A.C. generator and explain how a commutator makes a D.C. generator
explain how a transformer works, name its energy losses and solve turns-ratio problems
explain why power is transmitted at high voltage and low current

Topics in this lesson

Induced current
Laws of electromagnetic induction
Induction coil
A.C. and D.C. generators
Transformer and power transmission

What is electromagnetic induction?

Electromagnetic induction is the production of electric current or voltage in a conductor whenever there is a relative motion between the conductor and a magnetic field or a magnet.

What does the induced e.m.f. depend on?

The induced e.m.f. or current depends on:

the speed of motion of the magnet. The faster the speed of motion, the larger the induced current
the number of turns of the coil
the presence of a soft-iron core inside the coil

The direction of the induced current reverses when the direction of motion of the magnet or coil is reversed.

How is an induced current produced?

The coils A and C are wound on a steel ring R. When the switch S is closed, a deflection is obtained on the galvanometer G, and when S is opened, G shows a deflection in the opposite direction.

Current flows in the galvanometer whenever there is relative motion between the coil and the magnet. There is such a relative motion when the magnet moves towards the coil or away from the coil, and when the magnet is stationary but the coil is moved. Such a current is called an induced current and the phenomenon is called electromagnetic induction.

What are the laws of electromagnetic induction?

There are two laws of electromagnetic induction:

Faraday’s law
Lenz’s law

Faraday’s law

Faraday’s law of electromagnetic induction states that whenever there is a change in the magnetic flux linking a circuit, an e.m.f. is induced, the size of which is proportional to the rate of change of the flux linked with the circuit.

The magnetic flux or field lines linking a coil depend on:

the magnetic field strength
the number of turns of the coil
the area of each turn

To obtain a large induced e.m.f., move the conductor at a high speed across a strong magnetic field. Faraday’s law gives the magnitude of the induced e.m.f.

Lenz’s law

Lenz’s law of electromagnetic induction states that the induced e.m.f. is in such a direction as to oppose the motion or change producing it.

Lenz’s law gives the direction of the induced e.m.f. or induced current.

What is an induction coil?

An induction coil is an electrical device that can produce a very high intermittent e.m.f. by electromagnetic induction from a low-voltage d.c. source, for example a battery.

It consists of:

A primary coil, made up of a few turns of thick copper wire, wound on a laminated soft-iron core. The lamination of the core reduces the loss of energy due to eddy currents.
A secondary coil, which overlaps the primary coil. It is made up of a large number of turns of insulated thin copper wire. The ends of this coil are connected to an adjustable spark gap formed by two adjustable brass rods.
A make-and-break device in front of the soft-iron core of the primary coil. It consists of a soft-iron armature carried on a light brass spring, to which is attached a platinum contact with an adjustable screw. The primary circuit is completed from this contact through a key and the battery which provides the current.

The induction coil is commonly used in motor car ignition systems and in the operation of X-ray tubes.

How do A.C. and D.C. generators work?

A machine that converts mechanical energy into electrical energy or electrical energy into mechanical energy is called a dynamo. When it changes mechanical energy into electrical energy, it is called a generator. When it changes electrical energy into mechanical energy, it is called a motor.

There are two classes of generators: the alternating current (A.C.) generator and the direct current (D.C.) generator.

What are the parts of an A.C. generator?

An armature: a rectangular coil consisting of a large number of turns of insulated wire, wound on a laminated soft-iron core.
A magnetic field created by the curved poles of a horse-shoe magnet or an electromagnet.
Two copper slip rings to which the ends of the rectangular coil are connected and which rotate with the armature.
Two stationary carbon brushes which are made to press lightly against the slip rings.

How does a D.C. generator differ from an A.C. generator?

An A.C. generator can be made to produce a d.c. by replacing the two slip rings with a single split ring or commutator. A split-ring commutator is a slip ring that has been split into two segments which are insulated from each other. The ends of the coil are connected one to each commutator segment.

The commutator is a current reverser. When the armature coil is rotated, the commutator automatically switches each end of the coil from one brush to the other each time the coil completes one-half of a revolution. As the current reverses in the coil after each half revolution, the connection between the coil and the brushes is reversed through the action of the commutator.

What is a transformer?

A transformer is an electrical device for changing the size of an a.c. voltage. It acts to increase or decrease the e.m.f. of an alternating current. It consists of two separate sets of coils, the primary coil and the secondary coil. The primary coil is the input winding of turns of wire and the secondary coil is the output winding.

The coils are wound around a soft-iron core. The soft-iron core acts to increase and concentrate the magnetic flux within the core. It is also laminated, that is, it consists of sheets of soft iron insulated from each other instead of a solid block of iron. This lamination reduces the loss of energy in the form of heat due to eddy currents induced in the core.

How does a transformer work?

When an alternating e.m.f. or a.c. voltage (E_p) is applied at the terminals of the primary coil (p), an alternating magnetic flux is produced in the iron core. This flux links or threads the secondary coil (s). An alternating e.m.f. (E_s) of the same frequency as E_p is induced in the secondary coil by mutual inductance.

Mutual inductance is the flow of induced current or voltage in a coil due to an alternating or varying current in a neighbouring coil.

The total flux linking the two coils is proportional to their number of turns. The induced e.m.f. in the secondary coil (E_s) depends on the e.m.f. in the primary coil and on the ratio of the number of turns in each:

E_s / E_p = N_s / N_p

In an ideal transformer with 100% efficiency, the power developed in the secondary coil is equal to the power developed in the primary coil.

To use a transformer to increase an applied voltage, that is, to make E_s greater than E_p, N_s must be greater than N_p. Such a transformer, which increases or steps up the applied or primary voltage, is called a step-up transformer. In a step-up transformer, the primary current is greater than the secondary current but the primary voltage is less than the secondary voltage.

What energy losses occur in a practical transformer?

There are energy losses in practical transformers due to:

Eddy currents
Hysteresis loss
Heat loss
Leakage of magnetic flux

Eddy currents reduce efficiency because they consume power, and this causes energy to be lost in the form of heat. Such loss can be reduced by laminating the core.

Hysteresis loss is wasted energy due to reversing the magnetisation of the core. It is reduced by the use of special alloys in the core of the primary coil.

Heat loss: the primary and secondary coils have resistance, so some energy is lost in the form of heat (I²R) in the coils. This can be reduced by using thick wires or low-resistance coils.

Leakage of magnetic flux: some energy is lost because not all the lines of induction due to current in the primary coil pass entirely through the iron core. This loss is reduced by efficient core design.

Examples

Find the turns ratio in a transformer which delivers a voltage of 120 V in the secondary coil from a primary voltage of 60 V.
A transformer has 500 turns in the primary coil and 300 turns in the secondary coil. If the primary coil is connected to 220 V mains, what voltage will be obtained from the secondary coil? What type of transformer is this?
A transformer supplies 15 V from 220 V mains. If the transformer takes 0.7 A from the mains when used to light three lamps connected in parallel, each rated 15 V, 40 W, calculate:
1. the efficiency of the transformer
2. the cost of using it for 24 hours at 30k per kWh.

How is power transmitted over long distances?

Power generated at power stations is distributed over large distances to consumers through metal cables. Power can be transmitted either at low current and high voltage or at high current and low voltage. The metal cables have a certain amount of electrical resistance, so transmitting power at high current leads to loss of energy in the form of heat. To avoid this, power is transmitted at high voltage and low current. This is known as high-tension transmission.

Low currents lead to low energy loss. They also require thinner cables, and the cost of cable materials is considerably reduced if power is transmitted with low current and high voltage.

Step-down transformers are used to reduce the high transmitted voltages to the lower voltages required in homes and factories.

Evaluation