CHP 3 ELECTROMAGNETISM

Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

CHAPTER 3: ELECTROMAGNETISM

3.1 MAGNETIC EFFECT OF A CURRENT-CARRYING CONDUCTOR

3.1.1 Electromagnets
1. Electric and magnetic fields occur naturally wherever there is electricity.
2. Electric fields are produced by electrically charged objects.
3. A bar magnet produces the magnetic fields around it. The magnetic fields of a magnet cannot
be “switched off”.
4. Magnetic fields also can be produce by an electric current in a wire
5. The experiment above shows that a magnetic field is produced when current flows through a
wire.
6. When the direction of the current is reversed, the compass needle deflects in the opposite
direction.
7. This is called electromagnet. Electromagnet is a temporary magnet.

3.1.2 The Magnetic Field Pattern of a Current-carrying Straight Wire

1. When the switch is on, the resulting magnetic field lines are series concentric circles around
the wire.

2. The Right-hand Grip rule can be used to determine the direction of the magnetic field.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.1.3 Right-hand Grip Rule for Current-carrying a Straight Wire

1. Right-hand Grip Rule states that the thumb of the right hand points to the direction of current
flow and the other four curled fingers will curl the same way as the direction of the magnetic
field.

2. As the direction of the current is reversed, the direction of the magnetic field will also be
reversed.
Magnetic field patterns from the top views

Means that a wire carrying current
into the plane of paper.

Means that a wire carrying current
out of the plane of paper.

3. The strength of the magnetic field represented by field lines that show the shape of the field.
4. Magnetic field lines which are close together represent a strong field.
5. The strength of the magnetic field is greater when it is closer to the wire.
6. The strength of the magnetic field also depends on the magnitude of the current flowing through

the wire.
7. The larger the magnitude of the current, the greater the strength of the magnetic field.
*Maxwell’s Screw Rule also can use to determine the direction of
the magnetic field around a wire.

3.1.4 Right-hand Grip Rule for Current-carrying in a Coil
1. The magnetic field is stronger at the centre of the coil.
2. This means that more magnetic field lines per unit area lie
on the inside region of the coil.
3. Note that the field lines at the centre are straight and
perpendicular to the plane of the coil.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

4. The strength of the magnetic field can increase by increasing the current.
5. We can also increase the number of turns of the flat coil.
3.1.5 Right-hand Grip Rule for Current-carrying in a Solenoid

1. We can use a small compass to find the direction of the magnetic field.
2. The magnetic field of a solenoid has the same shape as the magnetic field around a bar magnet.
3. Hence, the solenoid can be said to have

poles.
4. If the current is flowing in a clockwise

direction, it shows the end of the coil is the
South Pole.
5. If the current is flowing in an
anticlockwise direction, it shows the end
of the coil is the North Pole.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

6. The strength of the magnetic field can be increased by:
a. Increasing the current,
b. Increasing the number of turns per unit length of the solenoid,
c. Placing an iron core within the solenoid.

7. However, the strength of the magnetic field decreases with distance from a current-carrying
source.

3.1.6 Applications of Electromagnet
1. Electromagnet is a temporary magnet; it can be switch “on” and “off”. This makes them very
useful.
2. Electromagnets are used to make electric bells,
electromagnetic relays and many others.
3. Iron is used to make electromagnets because
unlike steel, it can be easily magnetised and
demagnetised.

3.1.6.1 Electric Bell
1. When the button is depressed, current flows through the coil.
2. The electromagnet (soft iron and coil of wire) attracts the soft iron armature.
3. The hammer hits the gong.
4. When the hammer moves towards the
gong, the contact opens.
5. The circuit is broken at S and the
current stops flowing.
6. The iron core in the electromagnet
loses its magnetism and it no longer
attracts the armature.
7. The metal strip (spring) returns to its original position, completing the circuit again.
8. Current flows in the circuit and the process is then repeated.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.1.6.2 Electromagnetic Relay

1. A relay is a switch that is operated by an electromagnet.
2. There are two separate circuits.
3. The first circuit is for the electromagnet and the second circuit is for the electric motor.

4. A small current in the electromagnet circuit is used to
switch on a large amount of current in the motor circuit.

5. When S is switched ON, the current flows in the coil and
the coil becomes magnetised, attracting the soft iron
armature.

6. The movement of the iron armature closes the contacts
and the electric motor is switched on.

7. A relay is used to switch on a large amount of current
through the use of a smaller current. This is useful for two reasons:
a. First circuit may contain a component such as a light detecting resistor (LDR), which only
uses small currents,
b. Only the circuit with a large current need to be connected with thick wire.

8. Examples of these are start the car engine, automatic doors and computer.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.1.6.3 Telephone Earpiece

1. A varying current received from the caller in
telephone line.

2. The varying current passes through solenoid
and magnetized the soft-iron core.

3. The electromagnet varies in magnetic
strength according to the verifying current.

4. The alloy diaphragm will attract to
electromagnet by varying force.

5. Sound produced as compression and
rarefaction of air particles.

3.1.6.4 Circuit Breaker

1. It operates as automatic switch to breaks circuit
to open when current becomes too large.

2. When the current becomes high (eg: short circuit)
the electromagnet strength increase suddenly.

3. The iron catch is pulled toward electromagnet.
4. The spring pulling apart the contacts.
5. The circuit will break and the current flows stop
immediately.
6. The system can be reset when the reset button is
pressed. Spring pulls the soft iron armature back to its
original position and the contacts close again.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.2 THE FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

We have seen that an electric current produces a magnetic field around it. So it is not surprising to find
that when a wire carries an electric current through another magnetic field, the two magnetic fields
interact and produce a force on the wire.

1. The figure shows a copper rod, which is placed on a pair of copper rails between the poles of the
two magnets.

2. When the switch is on, we can observe that the copper rod moves to the right.
3. We can deduce that the current-carrying conductor experiences a force when placed in

magnetic field.
4. There is an interaction between the magnetic field of the current-carrying copper rod and the

magnetic field of the permanent magnet.
5. This will lead to a force being experienced by the conductor.

6. When the direction of the current is reversed, the direction of the force exerted on the copper
rod is also reversed.

7. The force is at right angles to the other two directions:
a. Direction of the current.
b. Direction of the magnetic field.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.2.1 Fleming’s Left-hand Rule

1. We can deduce the direction of the force on the current-carrying wire by means of Fleming’s
Left-hand Rule.

2. The forefinger represents the direction of the
magnetic field (N to S pole).

3. The second finger represents the direction of the
current (+ve to –ve).

4. The thumb represents the direction of the motion of
the wire (force).

5. If the forefinger points in the direction of
the magnetic field and the second finger the
direction of the current in the wire, then the thumb
will point in the direction of the force on the
conductor.

6. To obtain a maximum motion (maximum
force) the direction of the current must be
perpendicular to the direction of the magnetic
field.

7. If the direction of the current and direction of the
magnetic field are parallel, there will be no force
exerted on the wire.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.2.2 Catapult Field

1. Fleming’s Left-hand Rule only helps us predict the direction of force exerted on the wire.
2. We can explain the origin of the force by

examining the combined magnetic field of
the wire and the magnet.
3. Figure A shows the magnetic field of the
magnet and the magnetic field of the current-
carrying wire separately.
4. Figure B shows the combined magnetic field
when the wire is placed between the poles of
the magnet.
5. When the two fields are combined, a complex
field pattern is produced.
6. If free to move, it will be catapulted from the stronger field towards the weaker field or a
neutral point.

7. We can see that there is a stronger field on one side of the wire at A.
8. At B, the magnetic field is weaker.
9. A force will then act on the wire in the direction as shown in Figure B.
10. This type of unbalance magnetic field is called a catapult field.

11. When the direction of current is
reversed, the direction of the force is also
reversed.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.2.2.1 Forces between Two Parallel Current-Carrying Wires
1. If currents pass along two parallel wires, each wire will set up a magnetic field and the fields will
interact with each other.

2. If the two parallel wires have current flowing in the same direction, they will attract each other.

3. On the other hand, if they have current flowing in opposite directions, they will repel each other.
3.2.2.2 Forces on a Current-Carrying Rectangular Coil in a Magnetic Field

1. When a current passes through the loop, we will find that there is a turning force on the wire loop.
2. The loop will turn about the axis PQ.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3. The wire AB will have force acting on one side while the
force on wire CD is acting on the other side.

4. These two forces produce a turning effect, which turns
the loop clockwise.

5. If the magnet or the battery is reversed, the directions of
the forces are also reversed.

6. This turning effect on a loop of current-carrying wire
has a very important application → the motors.

3.2.2.3 The Direct Current Motor (D. C. Motor)
1. The ends of the wire are connected to split rings X
and Y, the commutator.
2. The commutator rotates with the loop.
3. Two carbon brushes are made to press lightly against the commutator.
4. The rectangular loop ABCD of wire is mounted on an axle that will allow it to
rotate about the axis PQ.
5. When the current flows through the loop ABCD, a downward force would
act on the left-hand side AB, and an upward force on the right-hand side CD.
6. The loop would then rotate about axis PQ until it reaches the vertical position.
7. The current is now cut off but the momentum and inertia of the loop carries it past the vertical
position.This reverses the current in the wire arm CD and now a downward force acts on it.
8. An upward force now acts on the other wire arm AB.
9. Hence, the loop continues to rotate in the same direction.
10. The commutator is there to reverse the direction of the current in
the loop whenever the commutator changes contact from one
carbon brush to the other.
11. This ensures that the loop will always be turning in one direction.
12. When the terminal of the battery is reversed the direction of the rotation of the loop will be
reversed.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.2.2.5 Factors that affect the speed of rotation of an electric motor
To increase the turning effect or rotation speed of the wire loop, we can
a. Increase the number of turns of the wire in the rotating coil,
b. Increase the current,
c. Use a stronger magnet to strengthen the magnetic field,
d. Place a soft-iron core within the magnetic field lines to strengthen the magnetic
field.

3.2.2.6 Moving Coil Ammeter

The angle of deflection The hair spring
is directly proportional will restore the
to the current flows in pointer back to
the coil its original
position.
The force acting causes
the coil to rotate and
lead the pointer to the
deflection

When current flows in
moving coil, magnetic
field of radial magnet
will interacts with
magnetic field produce
by the coil

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.3 ELECTROMAGNETIC INDUCTION

1. When the metal rod move upwards through the strong magnetic field, we can see there is
deflection of the needle of galvanometer.

2. The deflection of the needle shows the direction of the current flows in the wire.
3. That current calls Induced Current.
4. Induced Electromotive Forces (Induced e.m.f.)

are induced on a coil whenever the magnetic flux
through the coil changes.
5. Induced current can be produced in a wire simply by
moving a magnet in or out of the coil.
6. No battery or other voltage source is needed.
7. This relative movement between the conductor and the magnetic field changes the magnetic flux
and induces a current and a voltage on the conductor. This phenomenon is called
electromagnetic induction.
8. The magnitude of the induced e.m.f. and direction of the induced current can be determined by
application of the laws of electromagnetic induction:

a. Faraday’s Law
- To determine the magnitude of the induced e.m.f.

b. Lenz’s Law
- To determine the direction of the induced current.

3.3.1 Faraday’s Law of Electromagnetic Induction
1. The magnitude of the induced electromotive force (e.m.f.) is directly proportional to the rate of
change of magnetic flux (which the conductor cuts the magnetic fields lines).
2. Hence, the magnitude of the current increases with:
a. The speed of motion.
b. The number of turns on the coil.
c. The strength of the magnet.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.3.2 Lenz’s Law
1. The direction of the induced e.m.f. (electromotive force) is such that its magnetic effect always
opposes the change which produces it.

2. The direction of the induced current opposes the change that produces it.
3. The strength of the magnetic field at the solenoid increases as the magnet moves towards it,

and it is seen that the flowing current makes end A the North Pole.
4. If the magnet moves away from the solenoid, current will flow in the opposite direction and end A

now becomes the South Pole. It opposes the motion of the magnet.

5. The direction of induced current can be determined using Fleming’s Right-hand Rule (Dynamo
Rule).

3.3.3 Applications of Electromagnetic Induction
1. A generator is essentially the opposite of a motor which
converts mechanical energy into electrical energy.
2. A simple generator consists of a rectangular coil
rotating in a stationary magnetic field.
3. The generator produces electrical energy in the loop
when it is rotated in the magnetic field.
4. As the speed of rotation of the coil increases, the
induced voltage also increases.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.3.3.1 Direct Current Generator (D.C. Generator)
1. At a vertical position, the coil moves in parallel with the
magnetic field.
2. When there is no conductor cutting the magnetic lines of
force, the induced current is zero.
3. When the coil rotates from a vertical to a horizontal

position (90o), the current induced in the coil
increases from zero to maximum.
4. The direction of the current induced can be
determined by Fleming’s Right-hand Rule.
5. When the coil rotates to a vertical position again
(180o), the current induced will decreases from
maximum to zero.
6. At a horizontal position (270o), the current
induced in the coil will flow in the reverse
direction.
7. The position of the commutator changes, therefore the current flowing in the external circuit is
always in the same direction.

8. The magnitude of the induced e.m.f. depends on the position of the rotating coil.
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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.3.3.2 Alternating Current Generator (A.C. Generator)
1. An alternating current (a.c.) generator can be
modified from a d.c. generator by replacing the
commutator with two copper slip-rings.
2. The magnetic flux through the coil changes when
the coil rotates and electromotive force (e.m.f.) is
induced on the coil.

3. As the loop rotates, there is a change in the number of magnetic lines of force it encloses.
4. The number varies from a maximum to a minimum and back to a maximum again, depending on

the position of the loop.

5. If we double the number of turns on
the coil without changing the frequency of
rotation of the coil, the output voltage V
waveforms will be as shown in the diagram.

6. If we double the frequency of rotation of the coil
without changing the number of turns on the coil,
the maximum output voltage will also double as
shown in the diagram.

7. The induced e.m.f. of an a.c. generator can also be
increases by:
a. Using stronger magnetic fields
b. Winding the coil on a soft-iron core to concentrate the magnetic lines of force through the
coil.
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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

Alternating Current Direct Current

1. An alternating current (a.c.) is an electrical 1. Direct current (d.c.) is the continuous flow of
current. electric current through a circuit from the
higher potential to the lower potential.
2. The magnitude and direction of the alternating
current varies cyclically. 2. In a direct current circuit, electric charges
always flow in the same direction.
3. Our houses are normally supplied with
alternating current by an a.c. generator. 3. Direct current (d.c.) is the type of electricity
stored in batteries or solar electric
4. The alternating current reverses direction very devices.
rapidly, for example, at 50 times a second
(50Hz) for our household electrical supply. 4. D.C. is commonly found in many low-
voltage electrical appliances, especially for
5. The usual waveform of an a.c. power circuit is a those powered by batteries.
sine wave.

3.4 TRANSFORMER

A step-up A step-down
transformer in a transformer near
a housing estate
power station

1. A transformer usually consists of a soft-iron 4. The flux linkage of the secondary coil is
core which is wound with two separate coils of constantly changing.
insulated wire.
5. So, an alternating voltage is induced across it.
2. One coil is called the primary coil and the 6. For a step-up transformer, the number of
other is the secondary coil.
turns on the primary coil is less than that on
3. An alternating current that flows in the primary the secondary coil.
coil produces an alternating magnetic field in 7. For a step-down transformer, the number of
the soft-iron core. turns on the primary coil is more than that on
the secondary coil.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

In the case of an ideal transformer where no energy is loss:

3.4.1 Ways to Reduce Power Loss in Transformers

Practically, the efficiency of a transformer can never be 100%. This is due to the power lost during

transmission of a.c. supply from the primary circuit to the secondary circuit.

Causes of Power loss Way to Increases efficiency of a Transformer

• The heating effect of the current in the • Use thicker copper wires which have lower

copper wires of the coils. [Power loss = I2 R] resistance.

• Eddy currents* induced in the iron-core will • Use a core made of thin sheets which are

move in circles and heat up the core. laminated and insulated.

• The loss of energy in the process of changing • Use a soft-iron core so that it can be

the magnitude and direction of magnetic field magnetised and demagnetised easily.

in the core.

• Flux leakage because the magnetic field lines • Wind the insulated coils one on top of the

produced by the primary coil are not other.

completely linked to the secondary coil. • The soft-iron core must always form a closed

loop of iron.

*Eddy currents, which are loop currents, flow radically and will dampen the flow of current.

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Mr Ng Han Guan Form 5 Chapter 3: Electromagnetism Date:

3.5 GENERATION AND TRANSMISSION OF ELECTRICITY
3.5.1 National Grid Network

1. National Grid Network is a network of underground cables or pylons which connect all the power
stations and substations for the whole country.

2. This network starts at electrical power plant like Chenderoh Lake Power Station which is
hydroelectric power station, and end at our houses.

3. Electrical energy is transmitted from the power station to the consumer using long transmission
cables. This will bring to power loss as heat energy.

4. The power loss can reduce by:

i. Reducing the resistance of the cables
ii. Reducing the current

iii. Increasing the voltage in the cable.
5. Power loss can be calculate as follow:

Pheat = I 2R I = current flows in the cable
R = resistance of the cable

Renewable energy
1. Energy plays a very important role in economic development but the reserves of fossil fuels such

as oil and gas are very limited.
2. Hence, there is modern trend of the nations that is to harness the renewable energy.

3. Renewable energy sources are continually replenished naturally means they are sustainable.
4. The example of renewable energy:

Hydroelectric Solar Biomass Wind

Wave Tidal Geothermal

1. The example of non-renewable energy: Fossil fuel such as oil, gas and coal.
2. The benefits of using the renewable energy to our nation:

a. Avoid depletion of fossil fuels c. Avoid harming flora and fauna
b. Cleaner sources for little pollution d. Avoid the disruption of ecological balance

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