Transcript EM induction

Electromagnetic Induction
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Electromagnetic Induction
Practical and predict direction of the Iinduced
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• Flemings right hand generator rule:
• First finger (Index) point to the direction of
magnetic Field
• Thumb is the direction that the conductor is
moving
• SeCond (Middle) finger=direction of the
induction Current flow
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The Fleming’s Right Hand Rule
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• Are there any readings on
Galvanometer if we move the
metal bar in the direction which
is parallel to the B lines?
No!
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v
a
B
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I
b
a
v
B
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E = Blv sinθ
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An airplane with a wing span of 30.0 m
flies parallel to the Earth’s surface at a
location where the downward component
of the Earth’s magnetic field is 0.60 ×10-4 T.
Find the difference in potential between the
wing tips is the speed of the plane is 250m/s。
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Any other methods can create Iinducted?
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Switch on/off
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Electromagnetic induction
When a magnet is pushed into a coil of wire an electric
current is produced in the wire. We say that a current
was induced in the wire.
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EM Induction explained
•The coils of wire are full of ‘free’ electrons randomly
moving from atom to atom.
•When the magnetic field of the magnet moves past the
coils of wire the magnetic field exerts a force on the
‘free’ electrons.
•We call this force an EMF ‘Electro Motive Force’.
•The force on the electrons causes them to move
through the wire, this is an electric current.
•The changing magnetic field induces an electric
voltage across the circuit.
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Moving up and down
When we change the
direction of the magnet
we are changing the
direction of the force
on the ‘free’ electrons
in the wire.
The electric current is
induced in the opposite
direction.
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Electromagnetic induction
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Stationary magnet = no current
•When the magnet is stationary
there is no electric current.
•This is common sense we cannot
generate electrical energy without
some energy going into the system.
•Without the kinetic energy of the
moving magnet the electrical
energy of the current will not be
created.
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Standard equipment
A suitable meter, either
an a.c. meter or a
galvanometer,
•A coil,
•Two connecting wires,
•A magnet.
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Factor’s affecting induction
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Increasing the speed
•Moving a magnet in and out at a
faster speed will increase the
voltage.
•The overall electrical energy
from each movement will be the
same but there will be more
movements per second and hence
more power. We will need to put
in more kinetic energy every
second.
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Increasing the magnetism
•If we increase the strength of
the magnet passing into the coil
there will be a more voltage.
•However, if this makes the
magnet heavier we will need to
provide more kinetic energy.
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Increasing the coils
•If we increase the number or
area of the coils there will be
more voltage.
•With more coils of wire we
will have more ‘free’ electrons,
there will be a greater current;
but a longer wire also means
more resistance – and
something called Lenz’s law
(not in examination).
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Increasing the voltage
•There are three mains ways
to increase the size of the
induced electric voltage.
• Use a stronger magnet,
• Use more coils of wire
(sometimes called turns),
• Move the magnet quicker.
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What’s the conditions for creating
induced current /e.m.f. ?
Change of magnetic field in the loop
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Lenz’s law
• The direction of the induced e.m.f. is
such as to cause effects to oppose the
change producing it.
• The INDUCED current creates an
INDUCED magnetic field of its own
inside the conductor that opposes
change of the original magnetic flux
density.
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1/φo point to downwards
2/φo increased , △φo>0
3/BInduced point to upwards
4/IInduced Anti-clockwise
IInduced
1/Direction of the original magnetic flux density
2/Change of the original magnetic flux density
3/Direction of the INDUCED magnetic field
4/By using right hand grip rule, find out Iinduced.
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IInduced
IInduced
IInduced
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D
C
A
B
D
C
F
A
B
Iinduced
F
B
Iinduced
B
Any magnetic force
on the coil?
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• the current is always induced in
such a direction that the
magnetic force it produces
opposes the relative motion
between the conductor and the
magnetic field.
Magnitude of E?
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Metal bar ab keep in
touching with the P and Q,
Mass of ab is m， length of
it is L，there is a resistor R
in the circuit，the resistance
in the wire is negligible ,the
length of the P and Q is
infinite , ab fall down in a
uniform magnetic field, find
out its maximum speed.
R
a
× × L× ×
m
× × × ×
b
× × × ×
B
× × × ×
× × × ×
P
Q
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Electromagnetic Induction
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•An e.m.f. and a current induced in a circuit
by changing magnetic flux
I1
I2
I1
I2
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Induction cooker
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Applications
• Moving coil microphone
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A microphone works when sound
waves enter the filter of a
microphone. Inside the filter, a
diaphragm is vibrated by the
sound waves which in turn moves
a coil of wire wrapped around a
magnet. The movement of the wire
in the magnetic field induces a
current in the wire. Thus sound
waves can be turned into
electronic signals and then
amplified through a speaker.
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Applications
• Electric Guitar Pickups
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AC and DC
A quick review
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A.C. and D.C.
•Direct current is present in one direction. The
charge flows in one direction only, e.g. a battery.
•Alternating current is present in both directions.
Charge flows first in one direction and then in the
opposite direction. It has a typical frequency of 50
Hz (UK) or 60 Hz (Bermuda and the USA).
•There are advantages and disadvantages of both
types of electrical power.
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A.C. and D.C.
•Batteries always supply direct current.
•Mains electricity is always supplied as alternating current.
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Alternating Current
•An electric current is called an alternating current if the
charge flows in one direction then changes to flow in the
opposite direction.
•When looking at a diagram it is important to see if the
line goes both above and below the zero line.
Forwards
Zero line
Backwards
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a.c. sine wave
+
0 Volts
When electricity is generated in power stations it takes the
form of a sine wave. Many electrical signals look like a
sine wave.
House current is 120 volts, 60 Hz in the US and Bermuda.
House current is 240 volts, 50 Hz in the UK.
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Is this a.c. or d.c.
+
0 Volts
Although this electrical signal has a sine wave shape it
is still a d.c. signal.
The wave increases and decreases but never reverses
direction. Think about a car moving along speeding
up and then slowing down.
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Generators
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The dynamo
A dynamo has a
rotating magnet
inside a coil.
Simple examples
can be found in old
bicycle lamps.
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Generator
•Rather than move a magnet inside of a coil we can
turn the situation around.
•This time we will move a coil inside of a magnetic
field.
•We call this arrangement a generator.
•Most generators produce a.c. electricity
consequently they are sometimes called alternators.
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a.c. generator
Moving a coil in
a circle between
two magnets
produces an
induced electric
current in the
coil.
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Overall picture
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One circle = one wave
When the coil is turned through one complete
circle one complete sine wave of alternating
current is produced.
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Generator animation
coil
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Generator
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Generators
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Generator theory
•As the coil turns it cuts across the magnetic field,
•An EMF (Electro-Motive Force) pushes free
electrons along the wire.
•This is an induced electric current.
•An a.c. is transmitted through the brushes to the
rest of the circuit.
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Generation by hand
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Generators
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Power Production
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Transformers
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Applications
• My never dying flashlight
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What are the four ways in which the (induced)
current from an AC generator can be increased?
faster movement
1. _________________________
stronger magnetic field
2. _________________________
3. _________________________
more coils
4. _________________________
larger area of coils
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Summary - Induction
•When a conductor moves through a permanent magnetic
field a voltage is induced in the conductor.
•When a magnet spins inside a coil of wire the magnetic
field at any place in the coil is changing. The changing
magnetic field induces a current in the wire. coil.
•Induced voltage can be increased by using a stronger
magnetic field, using more coils of wire or by moving the
generator faster.
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