Magnetism
Properties of Magnets.
A magnet has the property of attracting iron and steel
(ferromagnetic materials).The attraction is greatest at the
ends of the magnet. When the magnet is suspended so that
it can rotate freely it always settles pointing approximately
geometrically north and south.
The Earth's Magnetic Poles
The fact that a compass needle always aligns itself in a
particular direction, regardless of its location on earth,
indicates that the earth is a huge natural magnet. The
distribution of the magnetic force about the earth is the same
as that which might be produced by a giant bar magnet
running through the center of the earth.The magnetic axis of
the earth is located about 15° from its geographical axis
thereby locating the magnetic poles some distance from the
geographical poles.
The ability of the north pole of the compass needle to point
toward the north geographical pole is due to the presence of
the magnetic pole nearby. This magnetic pole is named the
magnetic North Pole. However, in actuality, it must have the
polarity of a south magnetic pole since it attracts the north
pole of a compass needle. The reason for this conflict in
terminology can be traced to the early users of the compass.
Knowing little about magnetic effects, they called the end of
the compass needle that pointed towards the north
geographical pole, the north pole of a compass.
With our present knowledge of magnetism, we know the
north pole of a compass needle (a small bar magnet) can be
attracted only by an unlike magnetic pole, that is, a pole of
south magnetic polarity.
Experiments show that unlike poles attract and like poles
repel (similar to electric charges)
Poles cannot exist independently (unlike electric charges).
Every magnet has a north and south pole. If you break a
magnet in half each half will still behave like a complete
magnet with both north and south poles, no matter how
many times you break it in half there will never be a single
pole, even when your piece is one atom thick there are two
poles.
Magnetic Fields.
The space around a magnet in which it exerts a force is
known as its magnetic field. The field reduces with
distance from the magnet. A magnetic field may be
mapped out by lines of force which show the direction of
the magnetic force or the direction in which a single north
pole would travel if it could exist independently.
Magnetic Flux Density.
Defined as the total number of lines of force per unit area
perpendicular to the area. The magnetic flux density is an
indication of the strength of the magnetic field.
Symbol : B
S.I. Units : Tesla ( T )
Magnetic Field Lines Map of the magnetic fields.
Bar magnet
North Pole Facing South Pole
North pole facing North Pole
Theory of magnetism.
Magnetic materials have some magnetism before they are
magnetised. It is thought that there are regions within the
ferromagnetic materials which contain dipoles lined up in
one direction. These domains can be made visible using
high powered microscopes.
Unmagnetised piece of iron the direction of the magnetism
of the domains is random.
On magnetisation the domains become aligned
This gives free poles at the ends of the bar which will give
rise to the poles of the magnet.
Making Magnets.
1. A piece of steel may be magnetised by drawing a
magnet pole along it in one direction only.
2. Using an electric current.
The bar to be magnetised is placed in a cylindrical coil of
wire. A current is switched on and off when the bar is
removed it is found to be magnetised.
3. Induced magnetism.
When a piece of unmagnetised steel is placed near a
permanent magnet it becomes magnetised. We say that
magnetism is induced in the material.
Demagnetisation.
1. Electrical Method. Using a solenoid with a.c. current. If
the material is slowly withdrawn it will become
demagnetised.
Magnetic Properties.
1. Susceptibility. Ease of magnetisation of a material.
2. Retentivity. Measure of the ability of a material to retain
its magnetism once it is magnetised.
Soft iron is easily magnetised but has no retentivity.
Steel Not easily magnetised but high retentivity used for
making permanent magnets.
Electromagnetism.
An electric current gives rise to a magnetic field.
1. Current in a long straight wire. Using the Right Hand
Rule if the thumb is pointing along the direction of the
current the direction of the fingers give the direction of
the magnetic field.
2. Solenoid. When an electric current is passed through a
solenoid the resultant magnetic flux is similar to that of a bar
magnet.
Electromagnet.
A solenoid is wrapped around a bar of soft iron bent into a U
shape. Using soft iron it becomes magnetised when in a
magnetic field and looses almost all of its magnetism when
the filed is removed. Therefore switching on and off the
electric current effectively switches the magnet on and off.
This is the principle of the electromagnet.
Application : The electric Bell.
Faradays Law of Electromagnetic Induction
When ever there is a change in the magnetic flux linked with a
circuit an e.m.f. is induced, the strength of which is
proportional to the rate of change of the flux linked in the
circuit. The flux is a measure of the strength of the field.
Application the Transformer. Uses two coils Primary coil
and secondary coil wrapped on a single magnetic core.
Principle of Operation.
If an a.c. current is passed through the primary coil an
alternating magnetic flux will be set up through the iron which
will induce and e.m.f. in the secondary coil.
The magnitude of the induced e.m.f. (output voltage) depends
on
1. e.m.f. applied at the primary i.e. a.c. input voltage
2. the number of turns on the primary coil
3. the number of turns in the secondary coil.
It can be shown that
Secondary emf
Number of turns in the secondary coil

Primary emf
Number of turns in the primary coil
VS
N
 S
VP N P
There are two types of transformers
1. Where the number of turns in the secondary is greater
than the number of turns in the primary and the voltage is
increased these are called step-up transformers.
2. Where the number of turns in the secondary is less the
number of turns in the primary and the voltage is
decreased these are called step-down transformers.
Features of a transformer.
The power taken from a transformer can never be greater than
the power supplied to it due to the conservation of energy. In
practice the output power is always less than the input due to
the fact that the transformer is not 100% efficient.
However assuming 100% efficiency
Powerin  Powerout
V P I P  VS I S
So therefore if the voltage is increased then the current is
decreased.
If the transformer is not 100% efficient then some of its input
energy is lost.
P 
Efficiency   out  x100
 Pin 
The energy (or power) may be lost in a number of ways
1. Heat being produced in the windings or coils
2. The core is continually being magnetised and
demagnetised which requires some energy
3. All the magnetic field is not linked
Questions.
1. A transformer steps down the voltage from 220 V to 6.3
V in a radio. If there are 1320 turns in the primary coil
how many turns are there in the secondary coil.
2. A step-up transformer is designed to operate from a 20V
supply and deliver energy at a voltage of 250V.
Determine the current flowing in the primary coil when
the output terminals are connected to a 100W lamp,
assuming that the transformer is 100% efficient.
3. If the transformer in the above question is only 90%
efficient calculate the current flowing in the primary coils.
Let us now consider case where there is both an Electric
and Magnetic Field present.
It is found the a current carrying conductor in a magnetic field
experiences a Force. Recall that a force has both a magnitude
and a direction. Consider the case where a charge q
(coulombs) is moving with a velocity v (m.s-1) through a
magnetic field of flux density B (Tesla).
It can be shown that the magnitude of the Force acting is
given by
F  Bqv sin 
Where the angle between the magnetic field lines and the
direction of the motion of the charge.
The Force is a maximum value when the angle is 90o.
F  Bqv
Direction : As the force is a vector quantity we must also
give its direction. For the direction of the field we use
Fleming’s Left Hand Rule. Place the forefinger, second
finger and thumb of the left hand mutually at right angles to
each other. If the forefinger points in the direction of the
field, the second finger along the direction of the current then
the thumb will point in the direction of the motion caused by
the force therefore the direction of the force itself.
Also recall that
Q  It
and
v
l
t
l 
F  BIt   sin   BIl sin 
t 
Questions.
1. An electron moving at a speed of 5x107m.s-1 enters a
magnetic field of 0.5T. Calculate the magnetic force on the
electron if (i) the electron is moving at right angles of the field
and (ii) if the angle between the field and the motion of the
electron is 60o.
2. A wire of length 2m has a current of 25mA flowing
through it and is placed in a magnetic field of flux density
0.5T. Calculate the magnetic force on the wire if (i) the
current is moving at right angles of the field and (ii) if the
angle between the field and the current is 30o.