Download - Chapter 28 Electromagnetic Induction
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
Chapter 28Electromagnetic Induction
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
28.1 Induced e.m.f. and induced current
• An e.m.f. is induced whenever– the conductor cuts through magnetic field lines due to the
relative motion between the conductor and the magnet.
Induction by the relative motion between a coil and a magnet
Experiment 28.1
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• An e.m.f. is induced whenever– the conductor cuts through magnetic field lines due to the
relative motion between the conductor and the magnet.
Induction by moving a wire across magnetic field lines
Experiment 28.2
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28 Electromagnetic Induction
• An e.m.f. is induced whenever– the conductor cuts through magnetic field lines due to the
relative motion between the conductor and the magnet.– the magnetic field through a coil changes.
Experiment 28.3Induction caused by a changing magnetic field
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28 Electromagnetic Induction
• This phenomenon is called electromagnetic induction.
• Such an e.m.f. is called induced e.m.f.
low potential
high potential
induced e.m.f.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• This phenomenon is called electromagnetic induction.
• Such an e.m.f. is called induced e.m.f.
• The current produced is called an induced current.
induced current if a closed loop is formed
wire frame
induced current
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
Faraday’s law of electromagnetic inductionThe magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes.
smaller
larger
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
Faraday’s law of electromagnetic inductionThe magnitude of the induced e.m.f. is directly proportional to the rate at which the conductor cuts through the magnetic field lines, or the field through the coil changes.
Checkpoint (p.358) O
largersmaller
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28 Electromagnetic Induction
magnet approaching
N S
Lenz’s lawAn induced current always flows in a direction so as to oppose the change producing it.
Ring that cannot get through
opposing the change
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
magnet withdrawing
S N
Ring that cannot get through
Lenz’s lawAn induced current always flows in a direction so as to oppose the change producing it.
opposing the change
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Fleming’s right hand rule can also be used to determine the
direction of an induced current.
Example 28.2Experiment 28.5 Checkpoint (p.365) O
Conducting loop moving across a magnetic field
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
28.2 Faraday’s law and motional e.m.f.
• Magnetic flux is a measure of the number of magnetic field
lines through a surface.
Magnetic flux
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The magnetic flux through a planar surface in a magnetic fi
eld B is
• Magnetic flux is a scalar quantity with the unit weber (Wb).• The magnetic field is also called the magnetic flux density.
Example 28.3
= BA cos
Checkpoint (p.370) O
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Faraday’s law can be expressed in mathematical form:
• For an N-turn coil, if the magnetic flux through each turn is
the same, the total flux through the coil, called the magnetic
flux linkage, is N.
t
The negative sign is another way to state Lenz’s law.
Example 28.6
Example 28.5Example 28.4
Checkpoint (p.374) O
Mathematical form of Faraday’s law
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• A motional e.m.f. is the induced e.m.f. arises from the motion
of a conductor in a magnetic field.
uniform magnetic field B
+++++
–––––
vmotional e.m.f.
magnetic force
Motional e.m.f.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• A motional e.m.f. is the induced e.m.f. arises from the motion
of a conductor in a magnetic field.
+++++
–––––
v
magnetic forceFB = qvB
electric forceFE = qE = qV / l
FB = FE
l
uniform magnetic field B
= Blv
At equilibrium,
motional e.m.f.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• If the velocity v makes an angle with the magnetic field B,
the motional e.m.f. is
Example 28.7
= Blv sin
Checkpoint (p.377) O
v
uniform magnetic field B
straight conductor rod of length l
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28 Electromagnetic Induction
28.4 Applications of electromagnetic induction and generators
• Electromagnetic induction is used in
– moving-coil microphones
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
Writing data Reading data
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
– electric guitars
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
– electric guitars
– electrical generators
Generators in a power stationA bicycle alternator
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• A generator is a device that
converts mechanical energy
into electrical energy.• The ones generating
alternating currents are called
a.c. generators (or alternators).• Those generating direct
currents are called d.c.
generators (or dynamos). A simple a.c. generator
Simple a.c. generator
Electrical generator
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• The slip rings in an a.c. generator are used to prevent the
twisting of wires during the rotation of the coil.
slip rings
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The slip rings in an a.c. generator are used to prevent the
twisting of wires during the rotation of the coil.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The slip rings in an a.c. generator are used to prevent the
twisting of wires during the rotation of the coil.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The slip rings in an a.c. generator are used to prevent the
twisting of wires during the rotation of the coil.
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The slip rings in an a.c. generator are used to prevent the
twisting of wires during the rotation of the coil.
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Variation of the induced e.m.f. in a simple a.c. generator
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Electricity and MagnetismElectricity and Magnetism
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• The commutator of a d.c. generator is used to reverse the co
nnection to the external circuit when the direction of the induc
ed e.m.f. in the coil reverses.
Simple d.c. generator
A simple d.c. generator
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• The commutator of a d.c. generator is used to reverse the co
nnection to the external circuit when the direction of the induc
ed e.m.f. in the coil reverses.
Variation of the induced e.m.f. in a simple d.c. generator
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28 Electromagnetic Induction
• The induced e.m.f. of a generator can be increased by
– increasing the rotational speed of the coil,
– using stronger magnets,
– winding more turns of wire on the coil,
– increasing the area of the coil within the field, and
– winding the coil on a soft iron core.
Example 28.9Experiment 28.6
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• In some generators, it is the magnets that rotate but not the
coil.
A bicycle alternator
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• For an a.c. generator in a power station, there is an
electromagnet, called rotor, rotating in a set of fixed coils,
called the stator. Thus, e.m.f. is induced in the stator coils.
Checkpoint (p.390) O
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28 Electromagnetic Induction
28.5 Eddy currents
• Eddy currents are induced currents circulating through a
conducting plate when the plate cuts through magnetic field
lines or is placed in a changing magnetic field.
uniform magnetic field
eddy currents
Coin detectorFalling magnet
Conducting plate leaving the field
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• Eddy currents are induced currents circulating through a
conducting plate when the plate cuts through magnetic field
lines or is placed in a changing magnetic field.
uniform magnetic field
eddy currents
Coin detectorFalling magnet
Conducting plate entering the field
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Electricity and MagnetismElectricity and Magnetism
28 Electromagnetic Induction
• In general, magnetic braking effect arises whenever eddy
currents are induced due to the relative motion between a block
of conductor and a magnetic field.
uniform magnetic field
external forcemagnetic force
Experiment 28.7
Braking effect
decelerates
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• Eddy currents can heat up the conductor itself as a result of
the heating effect of current.• This phenomenon is called induction heating.
Checkpoint (p.397) O
Induction heating
A floating aluminium foil
What makes the brass gong rotate