Transcript Electromagnetic Induction

Electricity and Magnetism
Chapter 28
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Electromagnetic Induction
Electromagnetic Induction
Electricity and Magnetism
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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Electromagnetic Induction
Electricity and Magnetism
• 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
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Electromagnetic Induction
Experiment 28.2
Electricity and Magnetism
• 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.
Induction caused by a
changing magnetic field
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Electromagnetic Induction
Experiment 28.3
Electricity and Magnetism
• This phenomenon is called electromagnetic induction.
• Such an e.m.f. is called induced e.m.f.
high potential
induced e.m.f.
low potential
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Electromagnetic Induction
Electricity and Magnetism
• 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.
wire frame
induced
current
induced current
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Electromagnetic Induction
if a closed loop
is formed
Electricity and Magnetism
Faraday’s law of electromagnetic induction
The 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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Electromagnetic Induction
Electricity and Magnetism
Faraday’s law of electromagnetic induction
The 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
Checkpoint (p.358) O
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Electromagnetic Induction
Electricity and Magnetism
Lenz’s law
An induced current always flows in a direction so as to
oppose the change producing it.
N
opposing
the change
magnet approaching
Ring that cannot get through
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Electromagnetic Induction
S
Electricity and Magnetism
Lenz’s law
An induced current always flows in a direction so as to
oppose the change producing it.
S
opposing
the change
magnet withdrawing
Ring that cannot get through
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Electromagnetic Induction
N
Electricity and Magnetism
• Fleming’s right hand rule can also be used to determine the
direction of an induced current.
Experiment 28.5
Example 28.2
Checkpoint (p.365) O
Conducting loop moving across a magnetic field
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Electromagnetic Induction
Electricity and Magnetism
28.2 Faraday’s law and motional e.m.f.
Magnetic flux
• Magnetic flux F is a measure of the number of magnetic field
lines through a surface.
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Electromagnetic Induction
Electricity and Magnetism
• The magnetic flux F through a planar surface in a magnetic
field B is
F = BA cos q
• Magnetic flux is a scalar quantity with the unit weber (Wb).
• The magnetic field is also called the magnetic flux density.
Example 28.3
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Electromagnetic Induction
Checkpoint (p.370) O
Electricity and Magnetism
Mathematical form of Faraday’s law
• Faraday’s law can be expressed in mathematical form:
The negative sign is another
way to state Lenz’s law.
F
 
t
• For an N-turn coil, if the magnetic flux F through each turn is
the same, the total flux through the coil, called the magnetic
flux linkage, is NF.
Example 28.4
Example 28.6
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Electromagnetic Induction
Example 28.5
Checkpoint (p.374) O
Electricity and Magnetism
Motional e.m.f.
• A motional e.m.f. is the induced e.m.f. arises from the motion
of a conductor in a magnetic field.
++
++
+
magnetic
force
v
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––
––
–
uniform
magnetic field B
Electromagnetic Induction
motional e.m.f. 
Electricity and Magnetism
• A motional e.m.f. is the induced e.m.f. arises from the motion
of a conductor in a magnetic field.
At equilibrium,
++
++
+
FB = FE
magnetic force
FB = qvB
 = Blv
l
v
motional e.m.f. 
electric force
FE = qE = qV / l
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Electromagnetic Induction
––
––
–
uniform
magnetic field B
Electricity and Magnetism
• If the velocity v makes an angle f with the magnetic field B,
the motional e.m.f. is
 = Blv sin f
straight conductor
rod of length l
v
f
uniform magnetic field B
Checkpoint (p.377) O
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Electromagnetic Induction
Example 28.7
Electricity and Magnetism
28.4 Applications of electromagnetic
induction and generators
• Electromagnetic induction is used in
– moving-coil microphones
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Electromagnetic Induction
Electricity and Magnetism
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
Writing data
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Reading data
Electromagnetic Induction
Electricity and Magnetism
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
– electric guitars
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Electromagnetic Induction
Electricity and Magnetism
• Electromagnetic induction is used in
– moving-coil microphones
– magnetic storage
– electric guitars
– electrical generators
A bicycle alternator
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Electromagnetic Induction
Generators in a power station
Electricity and Magnetism
Electrical generator
• 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
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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
Variation of the induced e.m.f. in a simple a.c. generator
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Electromagnetic Induction
Electricity and Magnetism
• The commutator of a d.c. generator is used to reverse the
connection to the external circuit when the direction of the
induced e.m.f. in the coil reverses.
A simple d.c. generator
Simple d.c. generator
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Electromagnetic Induction
Electricity and Magnetism
• The commutator of a d.c. generator is used to reverse the
connection to the external circuit when the direction of the
induced e.m.f. in the coil reverses.
Variation of the induced e.m.f. in a simple d.c. generator
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Electromagnetic Induction
Electricity and Magnetism
• 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.
Experiment 28.6
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Electromagnetic Induction
Example 28.9
Electricity and Magnetism
• In some generators, it is the magnets that rotate but not the
coil.
A bicycle alternator
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Electromagnetic Induction
Electricity and Magnetism
• 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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Electromagnetic Induction
Electricity and Magnetism
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.
eddy
currents
Conducting plate leaving the field
uniform magnetic field
Falling magnet
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Electromagnetic Induction
Coin detector
Electricity and Magnetism
• 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.
eddy
currents
Conducting plate entering the field
uniform magnetic field
Falling magnet
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Electromagnetic Induction
Coin detector
Electricity and Magnetism
Braking effect
• 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.
external force
magnetic force
decelerates
uniform magnetic field
Experiment 28.7
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Electromagnetic Induction
Electricity and Magnetism
Induction heating
• Eddy currents can heat up the conductor itself as a result of
the heating effect of current.
• This phenomenon is called induction heating.
A floating aluminium foil
What makes the brass gong rotate
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Electromagnetic Induction
Checkpoint (p.397) O