Chapter 3: Electromagnetism

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Chapter 3 Electromagnetism
Form 5
Physics
Next gt
The study of matter
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Objectives (what you will learn) 1) magnetic
effect of current-carrying conductor2) force on
current-carrying conductor in magnetic
field3) electromagnetic induction4) transforme
rs5) generation transmission of electricity
Physics Chapter 3
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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
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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.
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.
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Magnetic effect
Without compass, the direction of magnetic field
can be obtained using Right-Hand Grip Rule.
Right-Hand Grip RuleGrip 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.
Solenoid is formed from many circular coils of
wire uniformly wound in the shape of a cylinder
through which electric current flows.
The direction of the field, B is determined using
right-hand grip rule (R.H.).
Magnetic field pattern produced by a current in a
solenoid is almost identical to that of a bar
magnet.
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Solenoid
Solenoids are important because they can create
controlled magnetic fields and can be used as
electromagnets.
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.
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This slide for extra information only.
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) n N / h N
number of turns h length of solenoid (meters)
µ ku0 magnetic constant or permeability of
free space, µ0 4p 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)

• when the current in the coil is increased (I)

B µnI where µ ku0 µ0 4p 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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Magnetic force
The direction of the force F on the conductor can
be obtained using Flemings left-hand motor rule.
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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.
Faradays law of electromagnetic induction 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.
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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Electromagnetic induction
The direction of e.m.f. induced or the induced
current I can be obtained using Flemings
right-hand dynamo rule.
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Electromagnetic induction
Lenzs law The direction of the induced current
produces an effect that opposes the change in the
magnetic flux.
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 Lenzs 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. 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 Stro
ng 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. 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 Stro
ng 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.
Transmission at high voltage reduces current in
cables thus reducing power loss greatly. 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.

• The National grid network is an interconnection
  of various power stations in the country.
• 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
What you have learned

1. magnetic effect of current-carrying conductor

2. force on current-carrying conductor
inmagnetic field
lt Back
3. electromagnetic induction
4. transformers
5. generation transmission of electricity
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Thank You