CHAPTER 3 ELECTROMAGNETISM FULL NOTES

PHYSICS

FORM 5

Chapter 8

Electromagnetism

Chapter 8

Electromagnetism

Concept Map

Electromagnetism

Magnetic effect Force on a current Electromagnetic Transformer
of a current carrying conductor Induction
Step-up and step-
Magnetic field Catapult field Induced e.m.f. down transformer
pattern due to a and current
Fleming’s Left- Transformer
current Hand Rule Faraday’s Law Equations

Solenoid Turning Force Len’z Fleming’s Energy losses
Law Right-
Electromagnet D.C Motor Generation &
Hand Rule Transmission of
Applications of
Electromagnets Generators Electricity

A.C. & D.C Current

8.1 Magnetic Effect of a Current-carrying Conductor

When a current flows
through the coil, it
produces a magnetic field.

Temporary magnet The soft iron core
which can be made by becomes
sending an electric temporarily
current through a coil magnetized when
of wire wound the current is
around an iron switched on.
core
When the current is
Used in motors, Electromagnet switched off, it loses
generators, relays, its magnetism.
loudspeakers, hard
disks, MRI machine.

Soft Iron Core
Wire

3

What is a magnetic field?

✓ The magnetic field is a region around a magnet or a current-carrying conductor in which a
magnetic force will act on a magnetic material.

✓ The magnetic field consisting of magnet lines or magnetic flux.
✓ Magnetic field lines which are close together represent a strong field.
✓ The field direction is defined as the direction indicated by a compass needle placed in the

magnetic field.

The Right-Hand Grip Rule

✓ Grip the wire using the right hand, with your thumb pointing in the direction of the current. Your
other fingers now point round the wire in the direction of the magnetic field.

✓ When the direction of the current is reversed, the magnetic field direction also is reversed.

Thumb points Magnetic field
in the direction lines
of the current
Current
flow

Other fingers Right Hand
give the
direction of the Straight wire
field carrying a
current
4

The magnetic field pattern and the direction of the magnetic field due to a current in :
1. Straight Wire

Current into the paper Current out of the paper

5

2. Coil
3. Solenoid

6

The thumbs is Note:
pointing to the N pole. The right hand grip rule for a

N s solenoid:
The thumb points towards north
Fingers curl in the pole of the magnetic field while
direction of current the other fingers indicate the
direction of the current in the
solenoid.

current Number of
turns in
flowing in the
solenoid solenoid

Factors that affect
the strength of the

electromagnet.

Distance Use of soft
iron core
between the
turns

7

Application of Electromagnet

Electric Bell Contacts

1. When the bell push is
pressed, a current flows in
the coils of the
electromagnet, causing it
to be magnetized.

2. The magnetized
electromagnet attracts the
soft-iron armature, causing
the hammer to strike the
gong.

3. The movement of the
armature breaks the
contact and causes the
electromagnet to lose it
magnetism.

4. The light spring pulls the armature back, remaking the contact and completing the circuit
again.

5. The cycle is repeated so long as the bell push is pressed and continuous ringing occurs.

How Electric Bell works? 8

Electromagnetic Relay

1. Electromagnetic relay is used
as a switch that uses the
small current small to switch
on or off one another circuit
that uses a large current.

2. Circuit 1 requires only a small
current.

3. When the switch is closed,
current flows in the coil,
causing the soft-iron core to
be magnetized and attracts
the armarture.

4. The movement of the iron armature closes the contacts in the second circuit. Circuit 2 is now

switched on.

5. Circuit 2 may have a large current flowing through it to operate powerful motors or very bright

lights

6. The advantage of using a relay is

a) that a small current (circuit 1) can be used to switch on and off a circuit with a large

current (circuit 2).

b) circuit 1 may contain a component such as a light detecting resistor (LDR), which uses

small currents.

c) Only the circuit with a large current needs to be connected with thick wire. 9

Telephone Ear-piece iron core
current
1. The varying current from the microphone
flows through the coils of an electromagnet in coil
the earpiece.

2. This pulls the iron diaphragm towards the
electromagnet by a distance which depends
on the current.

3. As a result, the diaphragm moves in and out
and produces sound waves that are replicas
of those that entered the microphone.

Circuit Breaker

1. Acts as an automatic switch that Reset button Soft iron armature
breaks open a circuit when the pulled towards
current becomes too large. electromagnet

2. Current may become excessive Spring pulls
when there is a short circuit or open the
an overload. contacts

3. The strength of the magnetic

field of the electromagnet From mains Electromagnet
increases suddenly. The soft iron supply
armature is pulled towards the To household
electromagnet. circuit

Contacts

4. This results in the spring pulling apart the contacts. The circuit is broken and the current flow

stops immediately.

5. After repairs have been made, the reset button is pushed to switch on the supply again. 10

8.2 Force on a current-carrying conductor in a magnetic field

What happens to a current-carrying conductor in a magnetic field ?
conductor is
pushed upwards

Switch closed

When a conductor carries an electric current through another magnetic field, a magnetic
force is exerted on the wire.

The combination of the magnetic field due to the current in the conductor and the external
magnetic field produce a resultant magnetic field. This resultant magnetic field produce a
magnetic force which later act on the current carrying conductor.

Force direction

The direction of the magnetic Magnetic field

force, F, acting on the conductor can direction
be determined by using Fleming’s
Current
left-hand rule.
direction
11

Investigation on the force on a current-carrying conductor in a magnetic field

Magnadur Thick copper
magnets wire

U-shaped +
steel yoke
- d.c. power

supply

Short copper
wire

Show the direction of the force using the Fleming’s left-hand rule.

F F 12

Diagram 1 Diagram 2

FF

Diagram 3 Diagram 4

The direction of magnetic field is parallel to the
direction of current.

The short wire stays at rest.

Diagram 5

1. When a current-carrying conductor is in a magnetic field of a permanent magnet, the

interaction between the two magnetic fields produce a force on the conductor.

2. The direction of the magnetic field, the current and the force acting on a conductor is

perpendicular to each other.

13

How a current-carrying conductor in a magnetic field experiences a force ?

1. The magnadur magnets produce a uniform, parallel magnetic field.
2. The current-carrying vertical wire produces a circular magnetic field around itself.

3. The two fields interact to produce a resultant magnetic field known as a catapult field.

4. Upward, the two fields are in the same direction and they produce a stronger combined field.
5. Downward, the two fields act in opposite directions and the combined field is weaker.
6. The wire carrying a current thus experiences a resultant force in the direction from the stronger

to the weaker field, i.e from upward to downward.

Magnetic Field of Magnetic Field of
Permanent Magnet Current carrying conductor

NS

NS

14

Catapult Field Region of stronger
magnetic field

NS

Region of weaker
magnetic field Force
on wire

Increase current in Greater force

the conductor The magnitude of the
force on a current-
Use stronger
permanent magnet carrying conductor in a
magnetic field

Greater force

15

Resultant magnetic field or catapult field for a current-carrying coil in a magnetic field.

Magnetic Field of Magnetic Field of
Permanent Magnet Current-carrying Coil

NS

F 16

NS

F
Catapult Field for a Current-carrying Coil

Show the direction of the resultant force, F.
F

Current F What will happen to the coil?
The coil will rotate clockwise.
How a direct current motor works?
Commutator:
Permanent Coil of wire Reverse the direction of current in the
Magnet coil every half rotation so that the coil
continues to turn in same direction
Spring Commutator
Carbon Brush:
Carbon To contact with the commutator so
brush the current from the battery enters the
coil.

Spring:
Push the brush so it will always contact
with the commutator.

17

The working principle of the direct current motor

1 • When current flows through the horizontal

B C coil, magnetic field is produced around it.

NF FS • The interaction between the magnetic field
of the current and the magnetic field of the

AD permanent magnet produces a _c_a_t_a_p_u_l_t _
field and 2 turning force.

• The direction of the force is determined by

Fleming’s left-hand rule.

• The two forces _r_o_t_a_te________the coil

_a_n_ti_c_lo__c_k_w_i_se___________________.

2 ND C

• When the coil gets to the upright position, S
the contact between the carbon brushes
with the commutator is broken. B

• There is __n__o____turning force on it A
because no current flows in the coil.

• But the coil continues to rotate because of
its __in_e_r_ti_a_________.

18

3 C B

• When the coil in a horizontal position NF FS
again, the sides of the coil changes
position. DA

• The commutator___r_e_v_e_rs_e_s________
the direction of the current in the coil to
ensure that the forces on the coil turn
the coil in one direction only.

• So the coil is still rotating in the same
_d_i_re_c_t_i_o_n_______.

4B • The above processes are repeated and
the motor continues to rotate.

N S
A
C

D

19

current Number of
turns in coil
flowing in the
coil

Factors which
affect the speed of

rotation of the
motor

Strength of Area of the
the magnetic
coil
field

“ Life is like riding a bicycle, in order to keep
”your balance, you must keep moving.
Albert Einstein

(1879 - 1955) 20

Exercise 8.1

a) On diagram 1.1, sketch the magnetic field between the two magnadur magnets.

NS

b) On diagram 1.2, sketch the magnetic field produced by a wire carrying a current into the paper.

21

c) On diagram 1.3, sketch the magnetic field produced by a wire carrying a current into the paper
between the two magnadur magnets. Hence, draw the direction of the force acting on the wire.

NS

Force
d) What is the name of the field that you drawn in diagram 1.3 ?

Catapult field
e) State two factors that affect the force acting on the wire.

Magnitude of current in the wire
Strength of permanent magnet

22

8.3 Electromagnetic induction

Electromagnetic induction is the production of an electric current by a changing
magnetic field as a result of relative motion between the conductor and the
magnetic field.

How induced current is produced?
• The induced current is produced only when there is relative motion between the

conductor / coil and the magnetic field lines.

• The relative motion of a conductor across a magnetic field can be produced by:

1. moving a straight wire quickly 2. Moving a permanent magnet towards
across a magnetic field between two one end of a solenoid or vice versa.
flat magnets or vice versa.

23

Each time the straight wire cuts across the magnetic field, or the permanent magnet moves towards
the solenoid, a current is induced in the coil and a deflection is observed in the sensitive
galvanometer.

This current is called induced current. The electromotive force that is produced is called the
induced e.m.f.

A. Electromagnetic Induction in a Galvanometer
straight wire.
Connecting
Observation : wires with
crocodile clips
1. If a galvanometer shows a
deflection, it means there is an Copper rod
induced current produced with bare

2. Current is induced in a straight ends
conductor when it moves and
cuts the magnetic field lines Magnadur
(1 & 2 only). magnet

3. The motion of the copper rod
must be perpendicular to the
direction of the magnetic field
lines ( 1 & 2 only ) so that an
induced current will be
produced.

24

B. Electromagnetic Induction in a solenoid Galvanometer
Galvanometer

Galvanometer

Solenoid Solenoid Solenoid
Magnet Magnet Magnet

1. Push the bar magnet 2. Hold the bar magnet 3. Pull the bar magnet out
into the solenoid. stationary in the of the solenoid.
Observe the deflection solenoid. Note the Observe the deflection
of the galvanometer. reading of the of the galvanometer.
galvanometer.

Observation :
1. The galvanometer showed a positive reading when the bar magnet and solenoid were
coming closer to each other. This shows that a current is produced in the solenoid in certain
direction.

25

2. The galvanometer showed a negative reading when the magnet and solenoid were moving
further away from each other. This shows that a current is produced in the solenoid in the
opposite directions.

3. A current is induced in a solenoid when there is relative motion between the solenoid and a
magnet.

Conclusion

1. Current is induced in a straight conductor when it moves and cuts the magnetic field
lines.

2. Current is induced in a solenoid when there is relative motion between the solenoid and
a magnet.

1 How to indicate the direction of the induced current in a straight wire ?

Magnetic field Force/ motion Fleming’s right-hand rule
direction direction

Current If the thumb and the first two fingers on the right
direction hand are held at right angles to each other with
the first finger pointing in the direction of the
magnetic field and the thumb in the direction of
the motion, then the second finger points in the
direction of the induced current.

26

Q

Magnetic field lines Direction of
motion

A wire PQ is moved vertically downwards in a
magnetic field.

The induced current will flow from P to Q.
[Apply Fleming’s right-hand rule]

P

2 How to indicate the direction of the induced current in a solenoid ?

Lenz’s Law Relative motion between Polarity at the end of the
magnet and solenoid solenoid facing the magnet
The resulting induced
current always flows Toward each other Same polarity as the magnet
in the direction
opposing the change Away from each other Opposite polarity as the
in magnetic flux that magnet
produced it.

27

Magnet is moved towards the solenoid

Induced current produces a North pole to
oppose the motion of the magnet coming
towards the solenoid

Repulsion

Magnet is pushed
towards solenoid

Magnet is moved away from the solenoid

Induced current produces a South pole to attract
the magnet moving away from it

Attraction

Magnet is pushed
away from solenoid

28

Faraday’s Law:

The magnitude of the induced e.m.f is directly proportional to the rate at which
the conductor cuts through the magnetic field lines.

Speed of Movement of The magnitude of Greater induced
induced current in current
the magnet or the
solenoid solenoid Greater induced
current
Number of turns on
Greater induced
the solenoid current

Increase strength of the
magnetic field through the

use of a stronger
magnet

29

Show the correct direction of the induced current when the magnet is moved in the direction
shown.

Galvanometer Galvanometer

Direction of Direction of N
the movement the movement

S NN S N SS

Galvanometer Galvanometer

Direction of Direction of S
the movement the movement 30

S NS N N SN

Applications of electromagnetic induction

Current Generator
1. Current generator functions by converting mechanical energy to electrical energy.
2. Current generator works based on electromagnetic induction and uses the Fleming’s Right

hand rule.
3. Current generator is divided into direct current generator and alternate current generator.

Direct Current Generator

Permanent Coil of wire
Magnet

Spring Commutator
Carbon brush
reverses the connections of the coil
with the external circuit after every half
cycle, so that the current in the outside
circuit always flows in the same
direction.

31

How does a direct current generator work?
Show the direction of movement of the coil AB and CD.
Mark the direction of the induced current in the coil and the galvanometer

12 F C
BC
BS
N F FS ND
F
AD

A

1. Coil AB moves __d_o_w__n_w_a_r_d_s___, coil CD 1. The sides AB and CD are moving
moves___u__p_w_a_r_d_s____. _p_a_r_a_ll_e_l ________ to the magnetic field
and thus do not _c_u_t________ the
2. When the coil rotates, its sides cut magnetic field lines.
across the magnetic field lines and
induced current flows in the coil from 2. No _In_d_u__c_e_d_ _c_u_r_r_e_n_t_______is
_B__to__A__a_n_d_D__t_o_C__. produced at the instant.

3. The galvanometer will deflect to 3. The galvanometer returns to _z_e_r_o__mark.
_ri_g_h_t________________.
32

3 4B
F
CB

N F FS NA DS

DA F

C

1. As the coil continues to rotate, current 1. The sides AB and CD are moving
will again be induced in the coil but its _p__a_ra_l_le_l__________ to the magnetic
direction in now opposite to that in figure field and thus do not ____c_u_t________
1 which is from ____C___ to ___D____ the magnetic field lines.
and from ___A____ to ___B____.
2. No _i_n_d_u_c_e_d____ __c_u_r_re_n__t ____is
2. However, the direction of the current produced at the instant.
through the external circuit remains the
same, so the galvanometer deflects to 3. The galvanometer returns to __z_e_r_o__
the _r_ig__h_t__. mark.

4. The process ___r_e_p_e_a_ts________.

33

The induced current varies from a maximum to zero, it flows in one direction only.
Hence, the induced current is called a direct current.

Induced Current

1 cycle

Angle

0° 90° 180° C 270° 360°B

B CB D BC A
A DA B AD C

C A D
D
34

Alternating Current Generator

a.c. generator Magnet Coil
invented by Nikola Magnet

Tesla

Bulb

Slip rings

Carbon brush

1. The two ends of the coil are connected to two slip rings which rotate with the coil.
2. Each slip ring is always in contact with the same carbon brush.

35

How does an alternating current generator work?
Show the direction of movement of the coil AB and CD.
Mark the direction of the induced current in the coil and the galvanometer.

12 F C
BC
ND BS
N F FS

A D PF

P A

QQ

1. Coil AB moves ___d_o_w__n_w_a_r_d_s__, coil CD 1. The sides AB and CD are moving
moves ____u_p_w__a_rd_s_______. __p_a_r_a_ll_e_l _______ to the magnetic field and
thus do not ____c_u_t________ the magnetic
2. When the coil rotates, its sides cut across field lines.
the magnetic field lines and induced
current flows in the coil from ___D___to 2. No ___i_n_d_u_c_e_d___ ___c_u_r_r_e_n_t______ is
___C____ and from ___B___ to __A____ produced at the instant.
(using Fleming’s right hand rule)
3. The galvanometer returns to __z_e_r_o____
3. In the external circuits, current flows from mark.
__P_____ to ___Q____
36

B

3 4F
CB
NA
N F FS CS

DA PF

P D

QQ

1. After the vertical position, the current 1. The sides AB and CD are moving
increases until it attains the maximum value _p__a_ra_l_le_l________to the magnetic field
when the coil is in a horizontal position. and thus do not __c_u__t ___the magnetic
field lines.
2. Coil CD moves _d_o_w__n_w_a_r_d_s____, coil AB
moves ___u__p_w_a_r_d_s_______ 2. No ____in_d_u_c_e_d___ ____c_u_rr_e_n_t______is
produced at the instant.
3. The direction of the induced current is from
____C___ to ___D____ and from ___A____ to 3. The galvanometer returns to __z_e_ro___
____B_____ mark.

4. The direction of the current through the 4. The process ____r_e_p_e_a_ts_________.
external circuit is from the brush ___Q____ to
____P______which is reversed. 37

Induced Current

1 cycle

Angle

0° 90° 180° 270° 360°

B C B

A C C BD B C A
D
B AP D P C
Q Q
P D D AP
Q P QA
Q

1. The output current that generated a.c. generator is an alternating current because the current

changes direction in the external circuit each time the coil passes the vertical position.

2. Assume the current flows from P to Q is positive and the current flows from Q to P is negative.

3. The current changes the magnitude and direction after every half rotation. 38

direct current alternating current

Magnitude of current may be: Magnitude of current may be:
(a) constant (a) constant
(b) changes with time but (b) changes with time and
constant direction direction changes periodically

I (A) Constant direction & I (A) Constant magnitude but

magnitude different direction

t (s) t (s)

I (A) Constant direction but I (A) Varying magnitude and

varying magnitude direction

t (s) t (s)

39

current that magnitude current which flows
flows in one may be: to and fro in two
direction only
in a circuit (a) constant opposite directions in
(b) changes with a circuit. It changes
its direction
time periodically.

direct current d.c. and a.c. alternating
(d.c.) current
(a.c.)
can flow
cannot flow through a can flow
through a resistor/ bulb through a
capacitor capacitor

capacitor bulb capacitor bulb
resistor resistor

40

I (A) t (s) ➢ The current increases from zero
Io A 2T to a maximum value of +Io (at A),
and back to zero (at B). It then
BD t (s) reverses direction and increases
OT 2T to -Io (at C) and back to zero
-Io C again.

1 cycle Io = peak current,
alternating current Vo = peak voltage

V (V) ➢ The time taken for a complete
Vo A cycle from O to D is called
period, T.
BD
OT ➢ Frequency of the current, f = 1/T
-Vo C
➢ In Malaysia, the frequency of the
1 cycle a.c supply is 50 Hz. Hence, the
alternating voltage period of the a.c is :

T = 1/50 = 0.02 seconds.

41

Exercise 8.1

1. Diagram 2 shows the structure of a simple electric generator.

Diagram 2.1
a) What type of generator is shown in Diagram 2?

alternating current generator

b) Name the part labelled
(i) P : __s_li_p_r_i_n_g___________________________________
(ii) Q : _c_a_r_b_o_n__b_r_u_s_h_______________________________

c) Name the physics phenomenon that is related to principle of a generator ?
Electromagnetic induction

42

d) Using the axes below, sketch a graph of the voltage (V) across the the resistor against
time (t).
Voltage (V)

Time (t)

e) State two ways that can increase the voltage produced by a generator.
Increase the number of turns in the coil
Increase the speed of rotation of the coil

f) Draw the direction of the current in the coil at the instant shown in Diagram 2.1.

43

2. Figure shows an alternating current with a magnitude that changes with time.
I/A
5

0.08 t / s
-5

a) What is the peak current?
b) What is the period of the a.c. current?
c) What is the frequency of the a.c current?

Ans :
a) 5 V
b) 0.04 s
c) 25 Hz

44

8.4 Transformers

A transformer is an electrical device which increases or decreases an alternating voltage
based on the principle of electromagnetic induction.

Structure of a simple transformer

Laminated soft-iron core

VP VS

Primary coil Secondary coil

Structure of transformer Symbol of transformer
➢ The purpose of the common iron core is
➢ A transformer consists of two coils of wire
wound round separately on a laminated to provide a magnetic field linkage in the
soft-iron core. secondary coil.

➢ The coil connected to the input voltage is 45
called the primary coil. The coil
connected to the output voltage is called
the secondary coil.

Operating principle of a simple transformer ✓ The changing magnetic flux is cut by the

✓ The principle of a transformer is involves secondary coil, producing induced e.m.f.
electromagnetism and the principle of
electromagnetic induction. in the secondary coil.
✓ Induced e.m.f. which produced in the
✓ When alternating current with an input
voltage, VP flowing through the primary secondary coil causes an alternating
coil, a varying magnetic field produced
around the primary coil. current with a output voltage, VS flows
through a load which connected across
✓ The soft iron core is magnetized and
transfer the varying magnetic field to the secondary coil.
secondary coil. ✓ The magnitude of the output voltage, VS

depends on the ratio of the primary coil

and the secondary coil.

Why does the transformer not work with a d.c power supply?

✓ A current is induced in the secondary coil ✓ A constant direct current whose
only when there is a changing magnetic magnitude and direction is constant does
flux due to a changing primary current. not create a changing magnetic flux in
(changes direction and magnitude) the secondary coil.

✓ A d.c. power supply gives a constant ✓ Therefore electromagnetic induction does
current in the primary circuit not take place.

46

Relationship between number of turns in coils with voltage in a transformer

✓ According to Faraday’s law:

Voltage ∝ number of turns in coils VP NP NS VS

V αN VP = VS Primary coil Secondary coil
V = kN NP NS
V =k
N Vp = input (primary) voltage
Vs = output (secondary) voltage
Np = number of turns in the primary coils
Ns = number of turns in the secondary coils

Step-up transformer X Step-down transformer

Step-up transformer Step-down transformer

If Ns is greater than Np, then Vs is greater than Vp If Ns is lower than Np, then Vs is lower than Vp

VP NP NS VS VP VS VP NP NS VS VP VS

Ns > Np , Vs > Vp Ns < Np , Vs < Vp 47

Relationship between output power and input power of an ideal transformer

➢ A transformer transfers electrical power ➢ In an ideal transformer, there is no energy
from the primary circuit to the secondary loss during the process of transforming the
circuit. voltage.

➢ The primary circuit of a transformer Power supplied to = Power used in the
receives power at a certain voltage from the
a.c power supply. The transformer delivers the primary coil secondary coil
this power at another voltage to an
electrical device connected to the Input power = Output power
secondary circuit.

VpIp = VsIs

A.C. power Output power Electrical
supply Input power
device

Primary circuit Secondary circuit

Energy loses in a transformer

➢ In a real transformer, some energy is lost in the transformer especially in the form of heat.
➢ The output power is less than the input power.
➢ Therefore the efficiency of the transformer is less than 100%.

48

Efficiency of a transformer The power loss is due to
Efficiency = Output Power 100%
Input Power (a) heating of coils
(b) eddy currents in the iron core
(c) magnetization and demagnetization of core
(d) leakage of magnetic flux.

Input power Useful output
from a.c. power power supplied
supply to an electrical
device
Causes of energy loss in transformers
Power loss
Resistance of the coils
➢ all coils will have resistance Ways to improve the efficiency of a
➢ heat is produced when current flows transformer
Use thick copper wires to make the coils
through them The resistance will reduce as the wire is thicker.

49

Causes of energy loss in transformers Ways to improve the efficiency of a
transformer
Eddy currents in the core
➢ The changing magnetic field will also Use laminated cores to reduce eddy currents

induces current in the iron core.
➢ This induced current is called eddy current.
➢ This current cause heat to be produced in

the iron cores.

Primary Secondary
coil coil

Laminated
core

Magnetization and demagnetization of the Use cores made from soft-iron as soft-iron core
can be easily magnetized and demagnetized.
core
➢ the alternating current flowing through the 50

transformer continually magnetizes and

demagnetizes the core
➢ work has to be done to change the

magnitude and direction of the magnetic

field in the core