This document provides information about magnetism and electromagnetism. It discusses the poles of magnets, magnetic materials like iron, induced magnetism through magnetic induction, and how electromagnets can be created using coils of wire and electric current. Examples of applications that use electromagnets are also described, such as electric bells and circuit breakers. Magnetic fields and field lines are explained, including how they are produced by currents and plotted using compasses.

1. Magnetism and
Electromagnetism
GRADE 9, PHYSICS, ANWAR MOHAMED.

2. Poles of Magnets
 The ends of magnets are called poles. Most of the
magnetism is concentrated in the poles
 Magnets behave differently depending on which
poles you bring together
 If you hang a bar magnet from a thread it will come
to rest in a direction facing North-South
 Bring a North-seeking pole near to a South-seeking
pole. The magnets are attracted
 Bring a South-seeking pole near to a South-seeking
pole. The magnets repel each other

3. The Law of the magnet is
Like poles repel, Unlike poles
attract.

4. Magnetic Materials
 Material attracted to a magnet.
 Eg: Iron
 Due to the charge on the electrons, the movements of
these electrons will give rise to magnetic effects. These
magnetic effects can be seen as tiny atomic magnets.
The tiny magnetic effects occurs in all substances. Then,
why aren’t all substances magnetic? This is due to their
atomic structures. In those materials, the electrons are
arranged in configurations that result in the magnetic
effects cancelling out one another.

5.  Once those tiny atomic magnets are aligned properly, it will give rise to a
strong combined magnetic effect. At this point, the substance is
considered to be magnetised and is a proper magnet.
 Lodestone is the only natural substance that behaves as a magnet.
Magnetic materials like steel and iron can be made into magnets.
 Magnets attract magnetic materials such as iron, steel, cobalt and nickel.
 The stronger a magnet, the larger will be the attractive or repulsive force
between other magnets.
 The closer together the two magnets are, the greater is the magnetic force
between them.

 Only magnets can be made to repel each other. Otherwise, the magnets
will attract all other magnetic materials.

6. Induced Magnetism & Electrical
Method Of Magnetisation
 Magnetic Induction is one of the ways making magnetic materials like steel
and iron into magnets. In other words, magnetic induction is a process of
inducing magnetism in an ordinary piece of magnetic material.
 This method involves simply placing the magnetic material (soft iron) close to a
strong magnet without touching.
 The soft iron bar becomes an induced magnet with the end nearer the magnet
having opposite polarity to that of the magnet.
 Hence, the soft iron bar is attracted and attached to the permanent magnet.
Magnetic induction process reveals how magnetic materials can be attracted
to magnets.
 Induced magnetism is a temporary process. If the permanent magnet is
removed, the magnetic material will usually lose its induced magnetism.

7. Electrical method for magnetisation
 For magnetization, a direct current flowing into a solenoid (a long
insulated wire coiled into a cylinder) produces a magnetic field that, inside
the coil, is uniform in strength and direction.
 The solenoid becomes a magnet.
 A steel bar placed inside the coil for a short while becomes magnetised
due to magnetic induction from the solenoid.
 The polarities of the magnet depend on the direction of current flow.
 Magnetisation by electric current method creates more powerful magnets
than other magnetization methods such as stroking.

8. Iron as a temporary magnet:
 Iron can be easily magnetised or demagnetised (soft magnetic
material. It can even be magnetized by a weak magnetic field. it is
therefore suitable to be used in temporary magnets.
 When mixed with other metals (e.g. Ni, Cu, Mn, Si), powerful
temporary magnets can be made.
 These temporary magnets are used to make temporary
electromagnets. Electromagnets lose its magnetism when it is removed
from magnetising fields. Electromagnets are very useful because they
can be turned on and off and their strengths can be varied.
 In order to shield or contain any magnetic effects, soft permeable iron
is also used as effective magnetic shields. (magnetic keepers)

9. Steel as a permanent magnet
 Compared to iron, steel cannot be easily magnetised or
demagnetised (hard magnetic material). It can only be magnetized
by a strong magnetic field. But, steel has the ability to retain its
magnetism once it is magnetized. This trait allows steel to be
suitable to be used in permanent magnets.
 Steel is typically mixed with other magnetic material to ensure
structural stability. In this way, strong permanent magnets are made.
 E.g. Permanent magnets are used in compasses, magnetic door
catches, moving coil galvanometers, d.c. motors, a.c. generators,
loudspeakers, and for many other purposes.

10. Magnetic Field And Magnetic Field
Lines
 Magnetic Field is the region around a magnet where other magnetic
material will experience a force.
 A magnetic field can be graphically represented by magnetic field lines
which indicates its strength and direction.
 Note: Magnetic field is a vector quantity! (It has both magnitude AND
direction!)
 When the field lines are close together at a point, the point is said to have
a strong magnetic field.
 Arrows in the field lines outside the magnets show the direction in which a
free north pole would move (from north pole to south pole).
 Field lines NEVER cross over.

11.  Compass is used to find the direction and pattern of magnetic field. It has
a permanent magnet needle which is free to rotate in a horizontal plane.
The north pole of compass magnet (arrow head) will align and point along
the magnetic field line direction.
 Magnetic field strength can be measured using a teslameter.

12. Plotting of magnetic field lines with a
compass
 Apparatus Needed: Bar magnet, plotting paper and plotting compass.

Procedure:
 Place the bar magnet at the centre of the piece of paper so that its north
pole is aligned as shown.
 Place the compass near one pole of the magnet, and mark the positions of
the ends N and S, of the compass needle by pencil dots. Then, move the
compass until the end of the compass is over the second dot, and mark
the new position of the other with a third dot.
 Repeat the above until reaching the other pole. Join the series of dots and
this will give a field line of the magnetic field. Use this method to plot
other field lines on both sides of this magnet.

14.  Cassette Tape Head Arrangement
 The basic tape head action involves an oscillating current in a coil. The magnetic field
produced in a ring of ferromagneticmaterial fringes out to the tape material at the gap.
For stereo cassette tape heads, there are two such mechanisms to record and playback
from parallel tracks on the tape.

15. Magnetic Shielding
 Materials that allow magnetic lines of force to pass
through them are called nonpermeable because
magnetic fields do not form within them.
 Materials that gather magnetic lines of force are said
to be permeable, because they support the
formation of magnetic fields within those materials.
Only magnetic materials are permeable.

16. Magnetic Field of Current
 The magnetic field lines around a long wire which carries an electric
current form concentric circles around the wire. The direction of the
magnetic field is perpendicular to the wire and is in the direction the
fingers of your right hand would curl if you wrapped them around the
wire with your thumb in the direction of the current

17. Right-Hand rule
 Right-hand rule can be used to find the direction of the magnetic field
produced due to current flow.
 Right-hand rule: Grasp the wire with right hand so that the thumb points
in the direction of the conventional current, then the wrapped fingers will
encircle the wire in the direction of the magnetic field.
 The magnetic field is strong in the region around the wire and weakens
with increasing distance, i.e., the field lines near the wire are drawn closer
to another. With increasing distances, concentric circles are further apart.
 The larger the current, the stronger is the magnetic field.

19. Magnetic field due to current in a
solenoid
 Solenoid consists of a length of insulated wire coiled into a cylinder shape.

20.  Current in solenoid produces a stronger magnetic field inside the solenoid
than outside. The field lines in this region are parallel and closely spaced
showing the field is highly uniform in strength and direction.
 Field lines outside the solenoid are similar to that of a bar magnet, and it
behaves in a similar way – as if it had a north pole at one end and south
pole at the other end. Strength of the field diminishes with distance from
the solenoid.

21.  Strength of the magnetic field can be increased by:
 1. increasing the current in the coil
 2. increasing the number of coils in the solenoid; and
 3. using a soft iron core within the solenoid.
 Reversing the direction of the current reverses the direction of the
magnetic field.
 Right-hand rule can be used to find the direction of the magnetic field. In
this case, point the wrapped fingers (along the coil) in the direction of the
conventional current. Then, the thumb will point to the direction of
magnetic field within the solenoid.

23. application of electromagnet: Electric
Bell

24.  When the ‘push’ switch is depressed, the circuit is closed. Current passes through
the electromagnet windings and the core becomes magnetised.
 The magnetised core attracts the iron armature which makes the striker hits the
gong.
 However, the movement of the armature opens the ‘make and break’ switch which
switches the electromagnet off. The iron armature springs back to its original
position, closing the ‘make and break’ switch and start the cycle again.
 Notes:
 Soft iron is used to make electromagnets as it gains and loses magnetism quickly
depending on existence of magnetic fields. The armature is also made of soft iron
which can induce magnetism rapidly.
 No matter what direction is the current flow, the bell rings continuously as long as
the ‘push’ switch is closed because any pole induces the armature.

25. Circuit breaker

26.  An excess current circuit breaker is a ‘trip’ switch opened by an electromagnet in
the same circuit when the current through the windings exceeds a certain value.
 Unlike a ‘make and break’ switch, a ‘trip’ is designed to stay open after it has been
opened by the electromagnet. The trip switch is reset manually after the cause of
the excessive current has been removed.

27. Force on current-carrying conductor
 When current-carrying conductor is placed in a magnetic field, it will
experience a force when the magnetic field direction is not parallel to the
current direction. The magnitude of the force is maximum when the
magnetic field and current directions are mutually perpendicular to each
other. The force decreases when the angle between the magnetic field and
current directions is smaller than 90∘

28.  Factors that affect the strength of the force:
 Angle between the magnetic field and current directions (More about this
below)
 Magnetic field strength (Stronger magnetic field → stronger force)
 Amount of current in conductor (Higher current → stronger force)
 Length of conductor within magnetic field (Longer conductor → stronger
force)
 If the current direction is PARALLEL to the magnetic field, there will NO
force on the conductor by the magnetic field. The magnitude of the force
is MAXIMUM when the angle between the magnetic field and current
direction is 90∘.

29.  This is commonly exploited to produce a turning effect in a current-
carrying coil to produce an electric motor.
 It does not have to be a current carrying conductor to experience a force
due to the magnetic field. The magnetic field actually interacts with the
moving electrons in the conductor to produce the force. Hence, electrons
that are moving in the direction perpendicular to the magnetic field will
experience the force as well. This means that if you pass an electron beam
through a magnetic field, it will be deflected. (provided it is perpendicular)

30. Fleming’s Left Hand Rule

31.  When a conductor carrying a current is placed in a magnetic field, the
conductor experiences a magnetic force.
 The direction of this force is always right angles to the plane containing
both the conductor and the magnetic field, and is predicted by Fleming’s
Left-Hand Rule.
 F is Force, B is Magnetic field, I is current.
 From the name of the rule, use your left hand.
 E.g. If current flowing towards to right and the magnetic field is pointing
into the paper, the direction of the force is predicted by the Fleming’s left
hand rule to be upwards.

32. Workings Of D.C. Motor
 In order to understand how a D.C. motor works, we will need to learn how
a current-carrying coil generates turning effect in a magnetic field.
 Turning effect of a current-carrying coil in a magnetic field

33.  The coil is placed horizontally between two magnets as shown in the figure
above. The magnetic field points from the N to S. Using Fleming’s left
hand rule, the force on the left-side of the coil is upwards (magnetic field
points left, current into the page), while the force on the right-side of the
coil is downwards (magnetic field points left, current out of the page). The
magnitudes of the forces on the left-side and right-side of the coil are
equal to each other.
 The moment of force can be calculated by:
 Moment=F×d
 , where
 F is the magnitude of the force on one side of the coil
 d is the horizontal distance between the two side. (the length of the wire
connecting the left-side and right-side of the coil)

34.  Note: The wire segments connecting the two side of the coil do not
experience any force as the current and magnetic field are in the same
direction. Hence, according to Fleming’s left hand rule, that will generate
no force.
 Factors affecting the strength of the moment of force:
 Number of turns in the coil
 Current in coil
 Strength of magnetic field

35. D.C. Motor

36.  The figure above shows a d.c. motor in action. The coil is connected to a
split-ring commutator (circular ring) via carbon brushes (the brown blocks
in the figure). The split-ring commutator is vital to the operation of the d.c.
motor. There is a gap in the split ring commutator which causes current
flow to stop when the coil is vertical (Reference position 90∘ and 270∘).
 Steps in the operation:
 Coil starts in reference posiiton 0∘: Upward force on left-side, downward
force on right-side. Coil rotates clockwise to position 90∘
 Coil in reference position 90∘: The split-ring commutator cuts off the
current to the coil. No electromagnetic force is acting on the coil. The
momentum of the coil from the previous turning motion causes it to rotate
slightly beyond this vertical position.

37.  Coil in reference position 90∘ + a slight tilt: Current passes through the coil
again. Due to the turning, the originally labelled left-side of the coil is now
right side and vice versa. This causes the current direction in the two sides
to swap. The previously-labelled left-side of the coil (now: right-side) now
have current coming out of the paper and experiences a downward force.
The opposite is true for the other side. The coil will now rotate clockwise to
position 180∘.
 Coil in reference position 180∘: This is the same as the starting position
and the whole cycle repeats itself.
 The split-ring commutator reverses the current direction in the coil every
half a turn and allows the coil to always turn in the clockwise direction.

38. Factors affecting the speed of rotation
of the d.c. motor: (larger turning effect
= higher speed)
 Same as the factors affecting the strength of turning effect
 Inclusion of a soft iron cylinder: Soft iron is highly permeable to magnetic
field. This allows the magnetic field to be concentrated at the coil which
increases the magnetic field strength experienced by the coil → larger
turning effect → higher speed.

39. Principles Of Electromagnetic
Induction
 Faraday’s Law of Electromagnetic induction is the process in which an
electromotive force (emf) is induced in a closed circuit due to changes in
the magnetic field around the circuit.
 Lenz’s law states that the direction of the induced e.m.f. and hence the
induced current in a closed circuit is always such as to oppose the change
in magnetic flux producing it.

40. Faraday’s experiments

41.  No. 1: The north pole of a magnet is moved towards the coil. By Lenz’ law,
the coil will generate an e.m.f. such that a north pole is induced on the
right side of the coil to oppose the change. (Why north pole? To “repel”
away the incoming north pole) From the right hand grip rule, the current
flow is as shown in the diagram.
 No. 2: The north pole is moved away from the coil. By Lenz’ law, the coil
will generate an e.m.f. such that a south pole is induced on the right side
of the coil to oppose the change.
 No. 3: The south pole is moved away from the coil. (Line of reasoning
similar to above. Drop a comment if you have problems.)
 No. 4: The south pole is moved towards the coil. (Line of reasoning similar
to above. Drop a comment if you have problems.)

42.  From this, we can conclude that emf is induced whenever the magnetic
field lines are “cut” by the coil. (A more proper terminology will be the
change of magnetic flux in the coil induces an emf in the coil)
 You might say that there are electrical energy generated from thin air as
shown in the experiments. But there are no free lunches in the world
(universe in this case). Mechanical energy (from pushing/pulling of the
magnet) is converted into electrical energy. This is how cycling a bike with
a dynamo converts your mechanical energy into electrical energy.

43. Fleming’s right hand rule
 Using Fleming’s right hand rule, you can predict the direction of the
induced current with the knowledge of direction of magnetic field and
force.

44. A.C. Generator

45.  An alternating current (A.C.) generator is an important application of
electromagnetic induction. A.C. generator is an electromagnetic device
which transforms mechanical energy into electrical energy. It consists of a
rectangular coil of wire which can be rotated about an axis. The coil is
located between the poles of two permanent magnets. As the coil rotates,
the magnetic field through the coil changes, which induces an
electromotive force (e.m.f.) between the ends of the coil.
 Note: The induced current does not flow UNLESS the generator is
electrically connected to an external circuit with an electrical load, such as
a light bulb as shown in the above figure.

46. Purpose of slip rings:
 The slip rings allow the transfer of
alternating e.m.f. induced in the
rotating coil to the external circuit. Each
ring is connected to one end of the coil
wire and is electrically connected to the
external circuit via the conductive
carbon brushes.
 Note the difference between A.C.
generator and D.C. motor. D.C.
motor uses split-ring commutator,
which reverses the current direction in
the coil every half a turn and allows the
coil to always turn in the clockwise
direction

47.  Using the figure above, we will investigate the workings of an a.c. generator. Note
that the coil is being turned in a clockwise manner and the magnetic field is
pointing towards the right.
 Steps in the operation:
 Coil starts in reference position 0∘: The plane of the coil is perpendicular to the
magnetic field lines. This means that the sides of the coil are moving parallel to
the magnetic field lines and not “cutting” through any magnetic field lines. Hence,
no e.m.f. is induced.
 Coil gets turned to reference position 90∘: The plane of the coil is parallel to the
magnetic field lines. The sides of the coil are moving perpendicularly to the
magnetic field lines and will be “cutting” through the magnetic field lines at the
greatest rate. Hence, the induced e.m.f. is the maximum at this position. Using
Fleming’s right hand rule, the direction of force at A is upwards (due to clockwise
motion), while the magnetic field lines are pointing rightwards. This will give an
induced current pointing into the screen. You can do the same analysis for B,
which will be carrying an induced current pointing out of the screen.

48.  Coil gets turned to reference position 180∘ and 360∘: Same as the analysis
in reference position 0∘.
 Coil gets turned to reference position 270∘: Same analysis as in reference
position 90∘ BUT the e.m.f. is in the opposite direction. This is due to the
position of A and B switching places and by the Fleming’s right hand rule,
the inwards current will be carried by B and outwards current will be
carried by A.

 The frequency of rotation is related to the period T by:
 f=1T


49. Ways to increase emf in a.c. generator:
 Decrease distance between magnet and coil. (To increase magnetic field
strength experienced by coil)
 Use a stronger magnet.
 Increase frequency of rotation of the coil. (Double freq. = double max.
e.m.f. and halving T)
 Increase number of turns in the coil. (Double no. of turns = double max
e.m.f.

50. Turning the magnets instead of the
coil
 For the generation of large currents, it
is more practical and advantageous to
keep the coil fixed and to rotate the
magnetic field around the coil. In this
case, the magnetic field cuts the coil to
produce the induced e.m.f., instead of
the coil cutting the magnetic field. Note
that the slip rings and carbon brushes
(incapable of carrying large currents)
are absent in this design.

51. Workings of a Transformer
 A transformer is a device that changes a high alternating voltage at low
current to a low alternating voltage at high current or vice versa.

52. Structure of transformer:
 Primary coil: Connected to primary input voltage (Vp) with turns (Np)
 Secondary coil: connected to load with output voltage (Vs) with turns (Ns)
 Soft-iron core: The coils are wound around a laminated soft-iron core, which
consists of thin sheets of soft-iron insulated from one another. The lamination of
the soft-iron core reduces the heat loss due to induced currents that could be
formed in an otherwise unlaminated core.
 A step-up transformer is one where the e.m.f. in the secondary coil is greater than
the e.m.f. in the primary coil. A step-down transformer is one where the e.m.f. in
the secondary coil is less than the e.m.f. in the primary coil.

53. Equation relating the voltage and
number of turns:
 Vs/Vp=Ns/Np
 Turns ratio: Ns/Np
 Power transfer in transformer: Power in primary coil = power in secondary
coil
 Ip x Vp=Is x Vs

54. Problems and Improvements to
transformer:
 Heat can be generated in the wires causing inefficiencies in transfer of
electricity. Soln: Make the wires thick as this will reduce resistance →
reduce heat generated
 Magnetic field lines can leak away so not all lines pass through the
secondary coil. Soln: Use circular core
 Current induced in core can circle around heating up the core. Soln: Make
core from thin laminated sheets. The lamination is to prevent loss of
energy via heat that would otherwise be generated from the induced
currents of an unlaminated core as currents cannot pass between the
lamination.

55. Transmission of electrical power
 During the transmission and distribution of electrical energy from the power station,
there is power loss due to Joule heating, The power loss is given by:
 PL=I2R
 In order to reduce the power loss, we have to minimise I and R.
 Very thick cables can be used to reduce the resistance R. However, the thick cables are
heavy and expensive. Hence, the better option is to reduce I.
 In order to reduce I, we can employ step-up transformers to increase the transmission
voltage and reduce the current. This will reduce the energy loss due to Joule heating.