Prepared By: Shakil Raiman
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If a current passed through a piece
of wire held at right angles to the
magnetic field of a magnet the
wire will move. This motion is the
result of a force created by
overlapping magnetic fields
around the wire and the magnet.
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When a current flows along a wire a cylindrical magnetic field is created
around the wire. If the wire is placed between the poles of a magnet, the
two fields overlap.
In certain places, the fields are in the same direction and so reinforces
each other, producing strong magnetic field. In other places, the fields are
in opposite directions, producing a weaker field.
The wire experiences a force, pushing it from the stronger part of the field
to the weaker part. This is called motor effect.
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If the direction of the current or the
direction of the magnetic field is reversed,
the wire experiences a force in the opposite
direction. We can predict the direction of
the motion of the wire by using Fleming’s
left-hand rule.
How to Increase the force:
 by increasing the current
 by using stronger magnet
Moving Coil Loudspeaker:
 Signals from a source, such as an amplifier,
are fed into the coil of the speaker as
currents that are continually changing in
size and direction. This produces varying
magnetic field around the coil which
interacts with the magnetic field of the
permanent magnet. This causes rapidly
varying forces on the wires of the coil,
which causes the speaker cone to vibrate.
These vibrations create the sound waves we
hear.
Electric Motor:
 Motor converts electrical energy to kinetic energy.
 As current passes around the loop of wire, one side
of it will experience a force pushing it upwards. The
other side will feel a force pushing it downward, so
the loop will rotate.
 Current following through the loop changes
direction at every half turn which ensures the
continuous rotation.
 How to increase the rotation or force:
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Increase the number of turns or loops of wire
Increase the strength of the magnetic field
Increase the current flowing through the loop of wire
Difference of Practical Electric Motor from the previous one:
 The permanent magnets are replaced with curved electromagnets capable
of producing strong magnetic field for longer time which increases the
rotational effect.
 The single loop is replaced with several coils of wire wrapped on the same
axis. This makes the motor more powerful and allows it to turn more
smoothly.
 The coils are wrapped on a laminated soft iron core which causes more field
passes through the coil. This makes the motor more efficient and more
powerful.
Electric Motor:
 If we move a wire across a magnetic field at right
angles, a voltage is induced or generated in the
wire. If the wire is part of a complete circuit, a
current flows. This Phenomenon is called
electromagnetic induction.
 The size of the induced voltage and current can be
increased by:
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moving the wire more quickly
using a stronger magnet
wrapping the wire into a coil so that more pieces of wire move
through the magnetic field.
Faraday’s Law of Electromagnetic Induction:
 The size of the induced voltage across the ends of a
wire (coil) is directly proportional to the rate at
which the magnetic lines of flux are being cut.
Electric Generator:
 Generator converts kinetic energy to electrical
energy.
 As the coil rotates, its wires cut through the
magnetic field lines. So, there is rate of change of
flux linkage which produces induced current in
the coil.
 In the figure the wire of the coil moves up
through the field and moves down through the
field for each turn. As a result, the current
induced in the coil flows first in one direction and
then in the opposite direction.
 This kind of current is called alternating current.
A generator produces alternating current is
called an alternator.
Electric Generator:
 A generator which produces direct current is
called a dynamo.
Electric Transformer:
 When alternating current is passed through
primary coil, the magnetic field around it is
continuously changes. This changing
magnetic filled is passed towards the
secondary coil by iron core.
 The secondary coil cuts the field lines.
 So, there is rate of change of flux cut by
secondary coil. So, there is induced voltage in
the secondary coil.
 The relationship between voltage and no of
turns is shown on right side:
Electric Transformer:
 There are two kinds of transformer. Step-up transformer to make
voltage high and step down transformer to make voltage low.
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The relationship between voltage and no of turns is :
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If a transformer is 100% efficient:
Pin=Pout
VP×IP=Vs×Is
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After the generator or before transmission line step-up transformer is used to make the
voltage high. In a step up transformer the no of turns in the secondary coil is more.
After the transmission line or before household and industry step-down transformer is used
to make the voltage low. In a step-down transformer the no of turns in the secondary coil is
less than primary coil.
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2
At transmission line power is lost as heat energy as P=I R
If voltage is increased, current will decrease.
So, power loss will be decreased.
More efficient.
Less current needs thin wire, which cost less.
So, building transmission line becomes cheaper.
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Wish you all very good luck and excellent
result.