The document discusses electromagnets and their magnetic field patterns. It defines an electromagnet as a magnetic field created by electric current through a coil. It describes how the magnetic field patterns of a straight wire, coil, and solenoid are determined using the right-hand grip rule. The strength of the magnetic field is affected by current, number of wire turns, and use of an iron core. Applications of electromagnets discussed include electric bells, relays, loudspeakers, and telephone earpieces.

3. 3.1 Magnetic effect of a current-carrying conductor
Learning outcomes
Student should be able to :
1. State what an electromagnet is
2. Draw the magnetic field pattern due to a current in a
straight wire, a coil and a solenoid
3. Plan and conduct experiments to study factors that
affect the strength of the magnetic field of an
electromagnet
4. Describe applications of electromagnets

5. WHAT IS
ELECTROMAGNET?

6. ELECTROMAGNET
The magnetic effect cause by a
current-carrying conductor

7. When a current through the coil, it produces a magnetic field. The soft
iron core becomes temporarily magnetized when the current is
switched on. It loses its magnetism when the current is switched off.

10. Magnetic field pattern
1. A magnetic field can be presented by field lines that show the shape of
the field.
2. Magnetic field lines which are close together represent a strong field.
3. The field direction is defined as the direction indicated by a compass
needle placed in the magnetic field.
4. “Right-hand grip rule” can be used to determine the direction of the
magnetic field around a current-carrying wire. (either a straight wire
or a coil)

12. MAGNETIC FIELD PATTERN DUE
TO A CURRENT IN A:
1) STRAIGHT WIRE

13. A wire carrying a current is passed through the centre of a manila
card. Iron filling are then sprinkled on the card and formed a pattern
of circles around the wire.
A small compass is placed on the pattern to indicate the direction of
the filed.
By using the Right-hand Grip Rule, grip the wire using the right
hand, with your thumb pointing the direction of the current.
The other fingers point around the wire in the direction of the
magnetic field.

15. The strength of the magnetic field produced by the wire is increased if:
the current flowing through the wire is increased
the number of wires carrying
the current is increased

16. 2) COIL

18. The field is identical to the fields of two straight wires, one carrying current
upwards and the other downwards.
In the centre of the coils, the field pattern is a straight line, while both
sides are curves.
The direction of the magnetic field at any point can be determined using
the right-hand grip rule.

19. The strength of the magnetic field of a circular coil is increased by

20. 3) SOLENOID3) SOLENOID
A solenoid is a long coil made up of several turns of wire like a
cylinder.
When a current flows through the solenoid, the pattern of magnetic
field is identical to that of a bar magnet.

21. 1. The polarity of the solenoid
can be worked out by
using right-hand rule for
a solenoid with the thumb
points to the N-pole if the
fingers of the right hand
grip the solenoid in the
direction of the current.

24. Experiment
Aim : To study the factors that affect
the strength of the magnetic field
of an electromagnet.
Procedure: As shown in the diagram
Study the relationship between :
a. the size of current carried by the coil
b. the number of turns on the coil
c. the use of a soft iron core
and magnetic field strength.
Submit the report within one week
A

26. WORKSHEET

27. 1. Draw the magnetic field
a)
SOLENOID

28. b) c)
COIL
STRAIGHT WIRE

29. APPLICATIONS OF THE
ELECTROMAGNETS

31. The soft ion armature is attracted to the electromagnet
The clapper hits the gong
The electromagnet loses its magnetism
The armature spring back to its original position
The current is restored in the circuit and the whole process is
repeated.
The armature breaks the circuit when it is pull away from the
contact point.
The current cut off
Switch is
pressed
Bell ring as
long as
switch is
pressed
• Current flow through the solenoid
• The iron cores become an electromagnet

34. 2.Arrange these statement in correct order:
i) Soft iron coil lost the magnetism and
the angker return to the original
position
ii) The clapper hits the gong
iii) The current cut off
iv) While the switch is pressed, the circuit
is completed. The soft iron coil is
magnetism.

35. Current flow through the solenoid
• Iron core becomes an
electromagnet
Soft iron armature attracted to
the electromagnet
• Switch Q is closed
• The high circuit is completed
Current flows in the solenoid being cut off
• Iron core losses its magetism
• Switch Q is open and current in high
voltage circuit is cut off
Switch P is Closed
Switch P is open
Relay

36. An Electromagnetic Relay
A device that uses a small current (Circuit 1) to switch on another circuit that carries a bigger current
(Circuit 2). It thus acts like a switch.
When the switch is closed, the soft-iron core becomes electromagnet.
The iron rocker arm is attracted to the electromagnet on the central pivot and pushes two contacts
together. This switches on Circuit 2.
When the switch is opened, no current flows through the solenoid and the iron rocker arm loses all its
magnetism.
The iron rocker arm is pulled by a spring back to its original position.
Hence, the contact is broken and no current flows in the Circuit 2. Circuit 2 is now switched off.
The advantages of using a relay are :
The Circuit 1 may contain light detecting resistor (LDR) only uses small currents
Only the Circuit 2 with a large current needs to be connected with thick wire.

38. When one speaks through a microphone change the mechanical energy (sound) into
electrical energy.
The varying current of the electrical energy passes through the solenoid then, the
solenoid become an electromagnet of varying magnetic strength.
The varying magnetic strength causes the diafragm to vibrate
The vibration of the diafragm causes the air column in front to be compressed an
stretched – sound produced

39. A Telephone Earpiece
Inside a telephone earpiece, two electromagnets are fixed on a permanent
magnet.
When a varying electric current which carries the sound signal from the mouth-
piece passes through the solenoids, the strength of the electromagnets
changes.
The soft-iron diaphragm is pulled by electromagnets with a changing force.
Hence the diaphragm vibrates at a same frequency of the sound signal.
The vibration of the diaphragm causes air molecules to vibrate and hence a
sound wave is transmitted to the earpiece.

40. Evaluation
• Answer all the Formative Quiz
• Complete The Topical Excercises