Lodestone is a natural magnet
Bar Magnet and Horseshoe Magnet
Magnetic Poles
A pole is the part of a magnet
where magnetic force is very
strong.
They are North and South poles
The Law of Attraction and Repulsion
Unlike poles
attract while
like poles repel.
Note that
repulsion is the
sure test for
the pole of a
magnet
Magnetic Field (Iron fillings experiment)
This is the space around
a magnet where
magnetic force is felt.
So a magnet can be
defined as a material that
creates a magnetic field.
https://www.youtube.co
m/watch?v=j8XNHlV6Qx
g
Magnetic Field Lines
They show the direction of
the magnetic field.
The lines point out of the
north and enter the south
pole.
The field is very strong
where the lines are close.
https://www.youtube.com/
watch?v=LNxGKKOKq_U
Plotting Magnetic Field Lines
The lines can be drawn
using Iron filings or the
Plotting compass
https://www.youtube.c
om/watch?v=VTYp52Ja
YVA
MAGNETISM
Substances that show magnetic
properties can be made as
magnets.
These are Iron, Cobalt and Nickel.
They are metals.
Steel is an alloy of CARBON and
IRON which also shows magnetic
properties
The Earth As A Magnet
The earth behaves as if it
has a short magnet inside
it, with the north pole of
that magnet pointing
south.
Using Iron as ELECTROMAGNET
Differences between Iron and steel
• Iron is a soft metal, so it can be magnetized and demagnetized very
easily. Therefore it forms temporary magnets e.g electromagnets.
• Steel is hard metal, so it takes time to become magnetized but the
moment it is , it doesn’t losses it magnetism easily. So it forms a
permanent magnet.
Induced Magnetism
Magnetic Domains
A magnet is divided
into domains, with
each domain containing
dipoles.
Ways of magnetizing and demagnetizing
• Magnetizing
1. Electrical method(d.c) from a
battery (best method)
2. Stroking method
Single stroke and double
stroke
• Demagnetizing
1. Electrical method
(A.c)alternating current (best
method)
2. Hammering : Heat magnet red
hot and hammer
3. Face the magnet in the West –
East direction and hammer
Making a Magnet by Stroking
The Electromagnet
Here a piece of iron is
placed inside the coil. It is
called the core of the
electromagnet.
When the device is
switched on, the magnetism
is very strong, producing
poles at the ends.
When it is switched off, all
the magnetism is lost.
Advantages of Using Iron as the Core Over
Steel
1. The magnetism is
stronger
2. When the current is
switched off, all the
magnetism is lost,
but with steel, some
magnetism is
retained
How to Increase the Strength of the
Electromagnet
1. Increasing the
number of
windings (turns) of
the coil
2. Increasing the
current
3. Using soft iron as
the core
The Electric Bell
This is a device which uses
an electromagnet.
When the switch is pushed,
current passes through the
coil and the electromagnet
pulls the armature.
Then the hammer strikes
the gong and the sound is
heard.
The Magnetic Relay: Uses a small current
in a circuit to control a large current in
another circuit
• Explanation
• Three classes of magnetic materials are
• (i) Diamagnetic : They have weak magnetic field which acts in opposite direction when placed in a stronger
magnetic field. so the get repelled by the strong magnetic field. E.g Gold, copper, zinc
• (ii) Paramagnetic : They have weak magnetic field which acts in the-same direction when placed in a stronger
magnetic field. so the get attracted by the strong magnetic field. E.g Aluminum, oxygen , titanium, iron oxide.
NOTE: Increase in temperature leads to a decrease in their magnetism.
• (iii) Ferromagnetic: they are strongly magnetized in the direction of the magnetic field they are placed in. e.g : Iron,
Nickel, Cobalt and Steel.
NOTE: The magnetism decreases with an increase in temperature at a particular temperature called Curie
temperature which is unique to each metal.
Electricity and Magnetism Linked
A wire which has current in it
has a circular magnetic field
around it
If the wire is rolled into a coil and current
flows, then it has a magnetic field like this:
Fleming’s Left-Hand Rule
The Electric (DC) motor (VERTICAL=Fmax)
Fleming’s Left-Hand Rule: Used to Find the
Direction of this Force
The Electric (DC) Motor (Revision)
The Electric (DC) Motor
Made of:
1. Coil or armature
2. Magnetic field
3. Split rings (or commutator)
4. Carbon brush
The split rings make continuous
motion possible by constantly
reversing the current every 180
degrees
The Magnet and Coil Experiment
https://www.youtube.com/watch?
v=Hh58afwzHfA
The AC Generator (or the Alternator) (Horizontal
wire = Current max )
The Telephone Earpiece
This is another device
that uses the
electromagnet
Transformers
A transformer is a device that
changes the magnitude of an
AC voltage by mutual
induction.
It has two coils called the
primary and secondary,
wound round a laminated
soft-iron core.
It is laminated to reduce heat
losses due to eddy currents.
Symbol of the Transformer
Step-Up and Step-Down Transformers
How it Works
When an alternating emf is
connected to the primary coil,
alternatig/changing magnetic field
is produced.
This links the secondary coil
through the soft iron core and cuts
across the secondary coil to induce
alternating output voltage
Turns Ratio Formula
Question
A transformer is connected to a 250 V AC supply.
The primary coil has 5000 turns and the
secondary voltage is found to be 10 V. What is the
number of turns in the secondary coil?
The Efficiency of a Transformer
In real situations, a transformer is not 100 % efficient.
So the efficiency of a transformer is given as:
power developed in the secondary
E = power supplied to the primary x 100. So,
Is x Vs
E=
Ip x V p
x 100
Class Activity: Efficiency of a Transformer
1. A transformer supplies 12 V when connected to a
240 V mains. If this transformer takes 0.11 A from
the mains when used to light a 12 V, 24 W lamp, find
its efficiency.
2. A transformer is used to light a 40 W, 8 V lamp from
a 240 V mains supply. Calculate the (i) turns ratio of
the transformer (ii) current taken from the mains
circuit if the efficiency of the transformer is 90%.
How Energy is Lost in a Transformer
1. In form of heat (I²R) in the coils. This is minimized by using
low resistance wire for the coil
2. In form of Eddy currents in the soft-iron core. Minimized by
laminating the core
3. Due to the leakage of magnetic flux. Minimized by limiting
the air gap in the core
4. Hysteresis in the iron. Minimized by efficient core design,
using soft iron core
Advantages of AC over DC in Power
Transmission
1. It can be stepped up or down easily and
cheaply
2. It can be transmitted in large quantities
3. Heat loss is minimized over long distances
Power Generation, Transmission and Distribution
Across the Country
Step Down Transformers in Sub-stations
Street 11 kV – 220 V (Step-Down) Transformer
High-Voltage Electricity Transmission
The power loss formula is:
P = I²R.
So it is best to transmit at
high voltage and low
current, (not low voltage,
high current), to minimize
power loss.
ELECTRONS AND ELECTRONICS
CATHODE RAYS
These are beams of
electrons moving at a
high speeds.
The Maltese Cross tube
uses thermionic emission
to produce cathode rays.
The Maltese Cross Tube
Properties of Cathode Rays
1. They are negatively charged (they are electrons)
2. They move in straight lines
3. They are attracted to the anode
4. They make certain materials fluoresce
5. They are deflected by electric and magnetic fields
Deflection of An Electron Beam by an Electric
Field
Deflection of an Electron Beam by a Magnetic
Field
The Cathode-Ray Oscilloscope
This uses a narrow
beam of electrons
produced by thermionic
emission to display
waveforms and other
signals on a fluorescent
screen.
Parts of the Cathode-Ray Oscilloscope
The Transistor
The Transistor as A light-Sensitive Switch