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Form 5
Physics
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The study of matter
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Chapter 3:
Electromagnetism
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Physics: Chapter 3
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Objectives:
(what you will learn)
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1)
magnetic effect of current-carrying conductor
2)
force on current-carrying conductor in
magnetic field
3)
electromagnetic induction
4)
transformers
5)
generation & transmission of electricity
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Line of Force
A line of force in magnetic field represents path of
free N-pole in magnetic field.
Direction of line of force: N-pole
S-pole
Line of force
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Stargazers
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Magnetic field around a bar magnet
Magnetic field around the Earth
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Magnetic effect
When current flows in a conductor, a magnetic
field is produced around it.
Magnetic field can be observed by sprinkling iron
filings around wire on a piece of cardboard.
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The direction
of field can be
obtained by
moving a
compass
around the
wire.
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Magnetic effect
The 2-dimensional view of magnetic field due
to current in straight wire is easier to draw.
Current up: Current coming out of paper
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Current down: Current going into paper
As distance from wire
increases, magnetic field gets
weaker (as shown by increasing
distance between lines).
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Magnetic effect
Without
compass, the
direction of
magnetic field
can be
obtained using
Right-Hand
Grip Rule.
Right-Hand Grip Rule
Grip wire with the right hand and with the thumb
pointing in the direction of current. The other fingers
point in the direction of magnetic field.
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Solenoid
Current, I in circular coil
creates magnetic field where
it is strongest along the axis.
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Solenoid is formed from many
circular coils of wire uniformly
wound in the shape of a cylinder
through which electric current
flows.
Magnetic field pattern produced
by a current in a solenoid is
almost identical to that of a bar
magnet.
The direction of
the field, B is
determined using
right-hand grip
rule (R.H.).
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Solenoid
Solenoids are
important
because they
can create
controlled
magnetic fields
and can be used
as
electromagnets.
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To find the N-pole of
solenoid, grip it with
right hand, the fingers
curl in the direction of
current, and the
thumb points in the
direction of N-pole.
This slide for extra information only.
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Solenoid
The magnetic field inside a solenoid is given by:
B = µnI
B = magnetic field magnitude (teslas)
µ = magnetic permeability (henries/meter or newtons/ampere2)
n = turns density (number of turns/meter)
I = current (amperes)
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n=N/h
N = number of turns
h = length of solenoid (meters)
µ = ku0
magnetic constant or
permeability of free space,
µ0 = 4π x 10-7 H/m
k = relative permeability
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Electromagnet
An
electromagnet
is made by
winding a coil of
wire around a
soft iron core,
which loses its
magnetism when
the current is
switched off,
unlike steel
which is
magnetized
permanently.
In electromechanical devices, direct current is used
to create strong magnetic field for drawing iron core
or plunger into it, such as in switches and relays.
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Electromagnet
The strength of the electromagnet increases
• significantly with the use of soft iron core (µ)
• when the number of turns per unit length of the
coil is increased (n)
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• when the current in the coil is increased (I)
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B = µnI
where µ = ku0
µ0 = 4π x 10-7 H/m (or N/A2)
k = relative permeability of iron is about 200, steel over 800
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Electromagnets are used in electric bells, circuit breakers,
electromagnetic relays, telephone earpieces, etc.
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The direction of the force
F on the conductor can
be obtained using
Fleming’s left-hand
motor rule.
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Magnetic force
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Field, B
Current, I
Force, F
(Motion)
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Electromagnetic induction
Electromagnetic induction is the production of
induced e.m.f. in conductor when there is relative
motion between conductor and magnetic field.
Faraday’s law of electromagnetic induction
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The e.m.f. induced in a conductor is directly
proportional to the rate of change of magnetic flux
through the conductor.
An e.m.f. is induced if wire cuts across magnetic field.
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No e.m.f. is induced if the wire moved parallel to magnetic
field; the magnetic lines of forces are not cut by the wire.
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The direction of e.m.f.
induced or the induced
current I can be obtained
using Fleming’s right-hand
dynamo rule.
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Electromagnetic induction
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Force, F
(Motion)
Field, B
Current, I
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Electromagnetic induction
Lenz’s law
The direction of the induced current produces an
effect that opposes the change in the magnetic flux.
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An e.m.f. is induced in a solenoid when a magnet is
moved into or out of solenoid. The direction of
induced current is obtained using Lenz’s law.
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Induced current produces N-pole to repel the N-pole of magnet
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Transformers
Transformer is an application of electromagnetic induction.
It consists of a primary coil and a secondary coil wound on a
soft iron core.
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Transformer is used to step-up or step-down the voltage of an
a.c. supply, depending on where the a.c. source is applied.
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Generation of Electricity
Many sources of energy are used to generate electricity, each
with their own advantages and disadvantages.
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Examples:
Hydro
Potential energy of water in a dam converted to kinetic energy
Natural gas, diesel, coal
Used as fuel to heat water in boilers to produce steam
Biomass
Waste material used as fuel, or decomposition of waste for
methane gas for use as fuel.
Nuclear energy
Nuclear fission of uranium releases heat used to heat water.
Sunlight
Solar cells convert sunlight into electricity.
Wind
Strong wind rotates windmill-like blades to rotate turbines.
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Generation of Electricity
Many sources of energy are used to generate electricity, each
with their own advantages and disadvantages.
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Examples:
Hydro
Potential energy of water in a dam converted to kinetic energy
Natural gas, diesel, coal
Used as fuel to heat water in boilers to produce steam
Biomass
Waste material used as fuel, or decomposition of waste for
methane gas for use as fuel.
Nuclear energy
Nuclear fission of uranium releases heat used to heat water.
Sunlight
Solar cells convert sunlight into electricity.
Wind
Strong wind rotates windmill-like blades to rotate turbines.
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Transmission of Electricity
Alternating voltage is generated at power station as its
voltage can be transformed with transformers.
A step-up transformer changes voltage to 320 kV or 500 kV.
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Transmission at high voltage reduces current in cables; thus
reducing power loss greatly.
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Power loss as heat in cables = I2R
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Transmission of Electricity
Voltage is stepped down in stages to, say 240 V using
transformers before supplying to consumers.
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The National grid network is an interconnection of various
power stations in the country.
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It ensures:
• minimal disruption to power supply through fast backups
• efficient power generation by matching demand with supply
• that power stations can shut down for regular maintenance
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Summary
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What you have learned:
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1.
magnetic effect of current-carrying conductor
2.
force on current-carrying conductor in
magnetic field
3.
electromagnetic induction
4.
transformers
5.
generation & transmission of electricity
Thank You