PHYSICS

FORM 4
Chapter 7
THERMAL EXPANSION

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Compiled By: Stephan Kanyenda

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OBJECTIVES
By the end of this chapter, the student should be able to:
1. describe magnetisation and demagnetization
2. describe electromagnetism
3. explain uses of electromagnetism

CONTENTS
• magnetisation and demagnetization
o domains
o experimentations to illustrate magnetisation and
demagnetization
• electromagnetism
o investigation of electromagnetism
o field patterns of electromagnets
o magnetic fields of current-carrying conductors
o force on current-carrying conductor in a magnetic
field(descriptive, no equations)
o Fleming’s left-hand rule
• uses of electromagnetism
o simple experimentations to illustrate electromagnetic induction.
o factors affecting magnitude and direction of the induced emf
o Faraday’s and Lenz’s laws of electromagnetic induction
(descriptive, no equations)
o ac and dc generators
o dc motor
• Transformers and power transmission
• Power loss in transformers and transmission line
• Environmental impact of power generation and transmission
• Solving mathematical problems involving transformers
• Making and testing a simple transformer and an electric motor (project)

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A magnet is any ferrous material that can exert a force of attraction on
other metallic objects Magnets have the following properties:
(i) They attract materials made of iron, steel, nickel and cobalt.
(ii) When a magnet is free to swing it will always point north-south
when it comes to rest.
Observation Experiment
• Suspend a magnet using a string and stirrup on a bench as
demonstrated below

• Let the magnet swing and allow it to come to rest freely

Observation: It points north-south.
Conclusion: Magnets are used as compass needles because when a magnet
is free to swing it will always point north-south when it comes to rest.
Magnetic Field: This is the region around the magnet in which magnetic
materials experience magnetic force. This field consists of imaginary lines
with arrows called magnetic field lines or magnetic lines of force.
How to detect field pattern of a magnet.
- Place a bar magnet on a flat surface.
- Lay a stiff paper over it. Sprinkle iron fillings on the paper.
- Tap the card gently.
- The iron fillings set along the field lines and produce a pattern that
appears as shown below.

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How materials get attracted to a magnet
• It is by magnetic induction. Moving a magnet close to iron makes the
magnetic particles rearrange so that the side close to the magnet
acquires a pole different to that of the magnet. The two unlike poles
attract.

Three ways of making magnets

I. By Induction: Bringing a magnet close to a magnetic material.

II. By Stroking: A magnetic material is rubbed with a magnet in single

stroking. In double stroking the magnetic material is rubbed with two
magnets.

III. Electrical Method: This is done by placing an iron core (a magnetic

material) in a solenoid which has been connected to direct current. A
Solenoid is copper wire coiled into a helical shape. When electric
current flows through the solenoid the bar becomes magnetised. It
becomes an electromagnet.

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How to determine the poles of a solenoid as an electromagnet –
Right Hand grip rule Imagine your right hand gripping the coil such that
the fingers point the same way as the conventional current direction. Then
the thumb points towards the northpole.

PERMANENT MAGNETS AND TEMPORARY MAGNETS

• Permanent magnets retain their magnetism even after the magnetic
material that induced the magnetism in them is removed.
• Temporary magnets lose the magnetism once the magnetic material
inducing the magnetism in them is removed e.g. electromagnet.
How to demagnetise a magnet
i. Stopping the flow of current in an iron electromagnet
ii. Placing the iron core in a coil carrying alternating current
iii. Heating a magnet
iv. Hammering or dropping.
Electric current has a magnetic effect
A wire carrying current sets up a magnetic field (flux). If a compass is
placed close to the wire the compass needle is deflected towards the wire.
The current direction and direction of magnetic flux is given by Maxwell’s
corkscrew rule.
Maxwell’s corkscrew rule
Imagine a corkscrew being screwed along the wire in the direction of
current, the direction of rotation of the screw gives the direction of the

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magnetic flux. The direction (movement) of the screw is the direction of
the current.

ELECTROMAGNETS
These are temporary magnets made by placing a ferrous material (soft iron
core) in a solenoid of direct current. When electricity passes through a
solenoid it acts as a magnet. The solenoid demagnetises by stopping the
flow of current or by allowing a.c. flow through it.
Uses of electromagnets
• Making electric bells
• Separating ferrous materials from non-ferrous ones
• Removing steel splinters from a patient’s eye in hospitals
• Lifting iron and steel loads
• Used as telephone relays
• Switching on different circuits
• Used in generators
• Used in transformers
The electric bell.

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How the electric bell works
• The bell push switch is closed.
• When electric current flows in the solenoid the solenoid is
magnetized.
• A soft iron armature is pulled to one end of the solenoid by its
magnetic attraction.
• The iron core strengthens this attraction.
• A hammer is also held into the steel spring that carries the soft iron
armature. As the armature is pulled the hammer is pulled as well.
• The hammer strikes the gong and rings the bell.
• The circuit breaks at the copper strip. Current stops flowing and the
solenoid is demagnetised.
• The steel spring flies back to its original position.
• The circuit is again complete and the action is repeated.
Advantages of electromagnets
1) It is easily demagnetised by switching off current (or allowing a.c
pass through it)
2) It can easily be controlled unlike a permanent magnet.
3) Magnetism can easily be increased. HOW?
• By using soft iron core
• By increasing the turns in the solenoid
• By increasing the current
• By making poles closer.
Electricity in a magnetic field produces movement

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A wire carrying current in a magnetic field experiences a force. The force
acts at right angles to both the current and the field
Explanation: There are two field patterns. One due to the magnetic field
and the other one due to the wire carrying current. The resultant field
produces movement.
Fleming’s Left Hand Rule
Hold the thumb and the first two fingers of the left hand at right angles to
each other. The first finger indicates the direction of the magnetic field.
The second finger points in the direction of the current. The thumb points
in the direction of the thrust (force). This applied in an electric motor.
How an electric motor works
An electric motor consists of a coil of wire in a magnetic field. When
electricity flows through the wire movement is produced.
Input: Electric energy and magnetic field
Energy changes: electrical energy to kinetic energy.

• The commutator is a half split ring of copper. The brushes are carbon
blocks. They are connected to an electrical supply.
• As electricity flows XZ will experience an upward force. WY will
experience a downward force.

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 • The coil rotates in an anticlockwise direction until it is vertical. In a
vertical position the brushes are in line with the gaps in the
commutator and the current stops.
• The coil overshoots the vertical because of inertia. The commutator
halves change contact from one brush to the other. Then the
direction of current reverses and the direction of forces reverses as
well.
Galvanometer (microammeter)
▪ It is a very sensitive ammeter.
▪ It measures very small currents.
▪ Therefore it is a microammeter.
▪ INPUT: Electric current and magnetic field

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The coil is suspended on a vertical wire about which it rotates. The rotation
causes the connected pointer to move across a graduated scale. Rotation
of the fine wire makes the coil twist.
Disadvantages of the galvanometer
▪ It cannot measure alternating current
▪ It can easily break the suspension wire with a large current
▪ It has to be on a level surface before use

ELECTROMAGNETIC INDUCTION
Remember that a wire carrying current sets up a magnetic flux. And
electricity in a magnetic field produces movement.
Now hypothesise what can happen if there is movement in a magnetic
field???
✓ Movement in a magnetic field produces electricity
✓ Whenever a conductor cuts magnetic lines of force electromotive
force is induced.

✓ Electromagnetic induction is the effect of producing electricity when

magnetic field lines cut a coil of wire.
✓ The electric current produced by magnetic field lines cutting a
conductor is called Induced Current.
Ways of increasing induced e.m.f

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 ✓ Moving the coil (or magnet) faster
✓ Increasing the number of turns in the coil
✓ Using a stronger magnet
Faraday’s law
▪ It states that the electromotive force induced in a conductor is
directly proportional to the rate of change of the magnetic flux linked
to the conductor.
▪ The size of the induced current in a wire is directly proportional to
the rate at which the conductor cuts the magnetic field lines.
Lenz’s law
▪ It states that the direction of the induced current is such that it
opposes the change producing it.
Fleming’s right hand rule
Hold the thumb and the first two fingers of the right hand at right angles to
each other. The thumb points in the direction of motion. The first finger
points in the direction of the magnetic flux and the second finger points in
the direction of current. This is applied in a dynamo/generator.
Generator (dynamo)
A dynamo consists of a coil of wire in a magnetic field. The coil of wire is
rotated mechanically. As it rotates it cuts magnetic field lines and produces
induced.
E.M.F. A dynamo produces a.c.
Input: Kinetic energy and magnetic field
Energy changes: Kinetic to Electrical
If slip rings are replaced by a commutator a.c. changes to d.c.

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As the coil rotates it gives the highest induced current when it is horizontal
because it cuts the magnetic field lines at the greatest rate. When the coil
is vertical no magnetic field lines are cut and induced E.M.F is zero.
Comparing and contrasting a dynamo and an electric motor.
Both an electric motor and a dynamo consist of a coil of wire in a magnetic
field. The differences are:
• The input in an electric motor is electrical energy while in a dynamo
the input is kinetic energy.
• The electric motor circuit has a battery.
• Dynamos have slip rings while electric motors have commutators.
NOTE: An a.c. generator becomes a d.c. generator if the slip rings are
replaced by a commutator.
A bicycle dynamo
A bicycle dynamo consists of a coil of wire and a magnet. Movement of the
bicycle wheel turns the dynamo. The magnet inside turns as well. As the
magnet moves the fixed coil cuts the lines of force producing electric
current in the coil. The current lights the rear lamp and the head lamp.
Mutual induction – faraday’s iron ring experiment

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A primary coil P and a secondary coil S are wound on opposite sides of an
iron ring. Coil P is connected to a battery and a tapping key. Coil S is
connected to a galvanometer.
Observation: On pressing the key the galvanometer gives a momentary
deflection. When the circuit is being broken current is induced in the
opposite direction.
Explanation: Pressing the key builds up a magnetic flux through the iron
ring. The secondary coil S cuts the magnetic field lines to produce induced
current. Breaking the circuit makes the magnetic flux to collapse and the
secondary coil re-cuts the lines of force producing induced current.
✓ The magnetic flux must grow out and collapse to produce induced
E.M.F. A stationary magnetic flux cannot produce induced E.M.F.
✓ A soft iron core is used to increase the strength of magnetism. More
lines of force cut the secondary coil to increase the induced E.M.F.
Transformers
A transformer is a device which steps up or steps down voltage.
How a transformer works

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 ▪ A transformer has a primary coil and secondary coil which are wound
on opposite sides of an iron ring.
▪ When alternating current flows through the primary coil a magnetic
field builds up which grows out and collapses i.e. moves to and fro.
▪ The secondary coil cuts and recuts the field lines as they grow out
and collapse thereby producing induced EMF in the secondary circuit.
Types of transformers
a. Step up transformer – has more turns in the secondary coil than in the
primary coil
b. Step down transformer – has more turns in the primary coil than in the
secondary coil.
Transformer equations
INPUT POWER = OUTPUT POWER (The assumption is that it is a an ideal
transformer with no power losses)
𝑉𝑝 𝐼𝑠
I. 𝑉𝑝𝐼𝑝 = 𝑉𝑠𝐼𝑠 =
𝑉𝑠 𝐼𝑝

𝑁𝑝 𝐼𝑠
II. 𝑁𝑝𝐼𝑝 = 𝑁𝑠𝐼𝑠 =
𝑁𝑠 𝐼𝑝

𝑁𝑠 𝑉𝑠
III. 𝑁𝑠𝑉𝑝 = 𝑁𝑝𝑉𝑠 =
𝑁𝑝 𝑉𝑝

Example 1
A step down transformer has 1200 turns in the primary coil and 50 turns
in the secondary coil. Calculate the voltage in the secondary coil if the
voltage in the primary coil is 240 V. (Maneb 2010).
Solution:
Ns Vs
=
Np Vp

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 𝑉𝑝
Vs = 𝑁𝑠 ×
𝑁𝑝

50
𝑉𝑠 = × 240
1200

𝑉𝑠 = 10𝑉 𝐴𝑛𝑠𝑤𝑒𝑟

Example 2
A step up transformer has 100 turns in the primary coil. The input power is
6kW and current in the primary coil is 30A. Work out the number of turns
in the secondary coil if the output voltage is 1200V.
Solution:

The Equation Input Power = Output power makes the assumption that
there is no power losses in a transformer. This is just ideal because in
reality there is power losses.
Causes of power losses in transformers
i. Leakage of field lines – This happens when the secondary coil
does not cut all the field lines due to poor designing of the
transformer
Solution: Proper designing

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 ii. Eddy currents: The soft iron core is a conductor. The moving
magnetic field in the primary coil induces eddy currents in the soft
iron core. The eddy currents have a heating effect
Solutions: Using oil and laminating the iron core.
iii. Resistance of the windings. The coil of wire is not a perfect
conductor. It has resistance of its own and heats up as electric
current flows through it.
Solution: Using thick copper wire.
iv. Hysteresis Losses: the magnetisation and demagnetisation of the
core by a magnetic field requires energy. This energy heats up the
core and is lost as heat energy.
Solution: use soft iron core because it is easy ta magnetise.
Efficiency of a transformer
Remember a transformer is not 100% efficient because there are power
losses in it.
𝑂𝑈𝑇𝑃𝑈𝑇𝑃𝑂𝑊𝐸𝑅
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝑋 100
𝐼𝑁𝑃𝑈𝑇𝑃𝑂𝑊𝐸𝑅

Example
Calculate the efficiency of a transformer that steps down voltage from
240V to 20V if current in the primary coil is 1A and current flowing through
the secondary coil is 10A.
Input power = 240V × 1A = 240V
Output power = 20V × 10A = 200W
𝑂𝑢𝑡𝑝𝑢𝑡𝑃𝑜𝑤𝑒𝑟
Efficiency = × 100
𝐼𝑛𝑝𝑢𝑡𝑃𝑜𝑤𝑒𝑟
200𝑊
Efficiency = × 100
240𝑉

Therefore, the efficiency = 83.33% Answer

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RECAPITULATORY EXERCISE
1. State and in each case explain with relevant diagrams three ways of
a) Magnetisation
b Demagnetisation
2. Explain how you would identify the north and south poles of an
unmarked magnet.
3. Describe briefly how you would prove that powers of a magnet are
concentrated at the poles.
4. Give the basic law of magnet.
5. Explain magnetization and demagnetization in terms of a domain theory
of magnetism.
6. What is the name of the smallest particle of a magnetism?
7. State three applications of an electromagnet.
8. With the aid of a diagram describe and explain the working of a simple
electric bell show how you should connect so that it can be rung from two
different points.
9. Draw a diagram of a current-carrying solenoid showing clearly the
direction of the current in and out of the solenoid. Add the magnetic field
pattern associated with the solenoid.
10. State two ways of increasing the strength of an electromagnet.
11. Define the following:
a) Mutual induction
b) Electromagnetic induction
12. Explain two types of the transformer.

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13. A transformer has 400 turns in the primary winding and 10 turns in the
secondary winding. The primary electromotive force is 250 and the primary
current is 2.0A. calculate:
a) The secondary voltage
b) The secondary current assuming 100% efficient
14. Describe the two features in a transformer design which help to
achieve the efficiency.

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