Transcript ELECTRODYNAMICS

ELECTRODYNAMICS
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Force on a current-carrying wire in a
magnetic field.
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When a wire carrying a current is placed inside a
magnetic field, it experiences a force that causes
the wire to move.
The force is the result of the interaction between
the magnetic field of the magnets and the
magnetic field of the wire.
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The force is at a maximum when the wire
moves at 90o to the magnetic field and the force
is zero when the wire moves parallel to the
magnetic field.
The direction of the force can be found by
using right hand screw rule
Conventional current is used,
that is, current that flows from
the positive towards the
negative terminal.
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The Direct Current (DC) Motor
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Electrical energy
(current) in the motor
is converted into
mechanical energy
(the movement of the
motor).
Using the Left Hand
Motor Rule, the left
side is forced up and
the right side is forced
down.
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A simple DC motor consists of a rectangular
coil of wire mounted on an axle that can rotate
between the two poles of a magnet.
Each end of the coil is connected to half of a
split-ring commutator that consists of two
copper segments that rotate with the coil.
Two carbon blocks, the brushes, are pressed
lightly against the split rings.
The brushes are connected to the power
supply.
The split-ring commutator ensures that the
coil turns in one direction only.
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Step 1 shows the coil in the horizontal position. ab
experiences an upward force and cd a downward force.
Step 2. The torque causes the coil to rotate into a
vertical position. Now the openings between the halfrings of the split-ring commutator are opposite the
brushes and the commutator loses contact with the
brushes. The current stops flowing through the coil.
However, the momentum of the coil carries it past the
vertical position.
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Step 3: The commutator makes contact with the
brushes again, but the current in the coil is
reversed (in the direction abcd). This allows the
torque to continue acting in the same direction.
The side ab now experiences a downward force,
and side cd an upward force.
Step 4: The coil continues to rotate until it
reaches a vertical position again and the current
is broken.
The rotating shaft is usually connected to other
rotating parts in the system, by means of gears
or pulleys.
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The turning effect on the coil can
be increased by:
Increasing the current in the coil.
 Increasing the number of turns on the coil.
 Increasing the strength of the magnetic
field.
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Motors in everyday life
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The changing force experienced by the coil as it
rotates through 360o, results in the simple motor
not running smoothly.
In practice, motors turn very smoothly and at high
speeds.
In these motors the coil consists of a soft iron core,
surrounded by many loops.
Such a coil is called an armature.
Most armatures have many coils which are placed at
different angles.
Each coil in the armature has its own commutator.
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Some motors, such as those in electric drills,
can run on AC, because they contain
electromagnets, rather than permanent
magnets.
As the current flows through the coil, the
magnetic field changes direction to match it.
This enables the motor to keep turning in the
same direction.
Electric motors have almost limitless useful
applications. These vary from the tiny motors
found in moving electric toys and disc players, to
the driving force behind water pumps – all the
way to the giant motors that drive the winches
on construction cranes.
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The design of an electric motor depends on the
task it must perform – sometimes it must turn
fast as in the dentist drill, or slower as in a clock
and sometimes in steps as in the motor in a
printer which feeds the paper line by line.
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ELECTROMAGNETIC INDUCTION
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When a magnet moves near a
conductor or when a wire
moves in the magnetic field of
a magnet, the change in the
magnetic field induces an emf
and a current flows in the
conductor.
This phenomenon is called
electromagnetic induction.
The induced current will be
maximised when the motion of
the conductor is perpendicular
to the direction of the magnetic
field, and minimised when it
travels parallel to the field.
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Moving a conductor in a magnetic field
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Faraday’s Law
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The size of the induced current is directly
proportional to the rate of change of
magnetic flux linkage.
What this means is that the induced current is
most effectively produced when the number of
magnetic field lines being ‘cut’ by the conductor
is greatest.
The size of the induced emf (and hence the
induced current) can be increased by
Moving the conductor faster
Using stronger magnets
Increasing the length of the conductor (more
turns on the coil)
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THE AC GENERATOR
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N & S are the field
magnets that provide the
magnetic flux.
abcd indicates one turn of
a rectangular coil of
insulated copper wire and
represents the armature.
S1 & S2 are a pair of sliprings consisting of
copper around an
insulated cylinder. Each
end of the coil remains
connected to its own slip
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ring.
B1 & B2 are carbon or copper brushes for
‘collecting’ the induced current.
 A is an insulated shaft that enables the coil
and slip-rings to rotate as a single unit.
 The handle stresses the fact that kinetic
energy must be supplied to the armature
and the lamp,L, represents the electrical
device in which the generated current is
used.
 Generators use mechanical energy to
produce electrical energy.
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Working of an AC generator
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When the coil is in the
vertical position, no
magnetic field lines are
cut by ab and dc; no emf
is induced and no
current flows.
While the coil rotates it
cuts the field lines and
causes a change in
magnetic flux, an emf is
induced which causes an
electric current flow in
the circuit.
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As the coil stats moving
to the vertical position
again, the induced emf
decreases so the current
also decreases, till it is
zero.
When the coil is rotated
further, there is a change
in magnetic flux again,
but now the induced
current flows in the
opposite direction.
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When the coil of an AC generator rotates in a magnetic
field, a constantly varying emf is induced across the
coil’s ends.
One complete change in the direction of the current
during one revolution of the coil is called a cycle of
alternating current.
The induced emf varies according to a sine wave and
the induced current is slightly less than the induced emf.
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Uses of the AC generator
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Alternators: in cars, the movement of the car’s engine
produces electricity via the alternatort to recharge the
car’s battery.
Back-up power: generators are used to produce
electricity where there is a power outage. Places that are
left vulnerable or cannot function in the event of a power
cut, such as hospitals, fresh produce distributors and
high security areas, use generators.
Power stations: huge dynamos are turned using steam
from heated water, to produce electricity.
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The DC Generator (dynamo)
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The slip-rings of the AC generator are replaced
by a split-ring commutator to convert the AC
generator to a DC generator.
The DC generator produces current that flows in
one direction only.
The carbon brushes are arranged in such a way
that contact is broken between the coil and the
brushes for a brief instant when the coil is
vertical.
When contact is re-established, the brushes
come into contact with a different part of the coil.
So the current continues to flow through the
brushes with no switch in its direction.
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Uses of AC Generators
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The main generators in nearly all electric power plants
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are AC generators. This is because a simple
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electromagnetic device called a transformer makes it
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easy to increase or decrease the voltage of alternating
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current. Almost all household appliances utilize AC.
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Uses of DC Generators
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Factories that do electroplating and those that produce
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aluminium, chlorine, and some other industrial
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materials need large amounts of direct current and use
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DC generators. So do locomotives and ships driven by
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diesel-electric motors. Because commutators are
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complex and costly, many DC generators are being
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replaced by AC generators combined with electronic
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rectifiers.
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Alternating Current
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Alternating current (AC) is an electric current
which reverses its direction of flow fifty times
every second – it has a frequency of 50 Hz.
Why do we use AC and not DC?
Electricity needs to be distributed through the
country at high voltages to reduce energy losses
in the power cables.
To increase the voltage at the power stations
and reduce it again before it reaches your home,
transformers must be used.
Transformers can only work on AC, since a
changing magnetic field is required to induce a
current.
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In addition, generating AC at the power stations is easier
– no complex modifications to the AC generator is
necessary.
 Most appliances can operate on AC, including all those
with a heating element (light bulbs, kettles, toasters,
stoves), but some require the AC to be converted to DC
first as they are direction sensitive (laptops, cell phone
chargers)
Characteristics of AC
 One of the characteristics of alternating current is that it
causes self-inductance in the wires that carry it.
 It means that when the current changes direction, the
magnetic field associated with it is in such a way as to
oppose that change (Lenz’s Law).
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Because of self-inductance, some of the
electrical energy of the current is wasted –
appliances will get a lower maximum voltage
than the peak value.
This lower maximum voltage is known as the
root-mean-square value (RMS value)
VRMS 
Vmax
I RMS 
I max
2
2
Prms  I rmsVrms
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In SA our mains supply is 220V (rms) AC
(50 Hz).
What is the peak or maximum voltage?
Vmax  2  Vrms
 2  220V
 311.13V
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