Magnetism: Faraday's Law and Electromagnetic Induction
L 28 Electricity and Magnetism [6]
•
magnetism
•
Faraday’s Law of Electromagnetic
Induction
–
induced currents
–
electric generator
–
eddy currents
•
Electromagnetic Waves (Maxwell & Hertz)
1
Basic facts of Magnetism
•
Oersted
discovered that a compass needle
responded to the a current in a loop of wire
•
Ampere
deduced the law describing how a
magnetic field is produced by the current in a wire
•
magnetic field lines are always closed loops
– no
isolated magnetic poles; magnets always have a
north and south pole
•
permanent magnets
: the currents are
atomic
currents
– due to electrons spinning in atoms -
these currents are always there
•
electromagnets:
currents in wires produce
magnetic fields
2
Faraday’s Law of
Electromagnetic
Induction
•
Faraday wondered if the magnetic
field due to the current in one coil
could regulate the current in an
adjacent coil.
•
He was correct, with one
important qualification:
the magnetic field must be
changing
in some way to
produce a current
•
the phenomenon that a changing
magnetic field can produce a
current is called
electromagnetic
induction
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1
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1
-
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7
)
3
Induced currents (a)
•
When a current is turned on or off in
coil A
, a
magnetic
field
is produced which also passes through
coil B
.
•
A
c
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B
•
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c
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n
t
.
•
The current in B is only present when the current in A is
turned on or off, that is, when the current in A is
changing
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f
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e
s
current
indicator
switch
battery
B
4
Induced currents (b)
a)
No current is induced if the magnet is stationary.
b)
When the magnet is pushed toward the coil or pulled
away from it, an induced current appears in the coil.
c)
The induced current only appears when the magnet is
being moved
(a)
(b)
(c)
5
Induced currents (c)
•
If an AC (time varying)
current is used in the
primary circuit, a current is
induced in the secondary
windings.
•
If the current in the
primary windings were
DC, there would be NO
induced current in the
secondary circuit.
•
Levitated coil demo
6
electric generators
When a coil is rotated in a magnetic field, an
induced current appears in it.
This is how electricity
is generated.
Some external source of energy is
needed to rotate the turbine which turns the coil.
7
The transformer
The voltage on the secondary depends on the number
of turns on the primary and secondary.
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Eddy currents
•
Eddy currents are induced in conductors if time-
varying magnetic fields are present
•
As the magnet falls the magnetic field strength at
the plate increases
9
Falling
magnet
Eddy
currents
Induced
magnetic
field
Copper
plate
Eddy currents
application
An induction stove uses
eddy currents to cook food
Only the metal pot gets hot, not the
glass pot or the stove.
10
slotted
copper pipe
bar
magnet
Floating magnet – induced currents
•
As the magnet falls, it
induces currents in the
copper pipe known as eddy
currents.
•
These eddy currents
produce a magnetic field
that
opposes
the field of the
falling magnet, so the
magnet does not accelerate
but descends slowly.
11
The Laws of Electricity and Magnetism
•
Laws of electricity
–
electric charges produce
electric
fields
(
Coulomb
)
–
electric fields begin and end on charges
•
Laws of magnetism
–
currents produce
magnetic
fields (
Ampere
)
–
magnetic field lines are closed loops
–
a changing magnetic field can produce a
current (
induced currents
) (
Faraday
)
–
A changing electric field can produce a
magnetic field (
Maxwell
)
12
E
L
E
C
T
R
O
M
A
G
N
E
T
I
C
(
E
M
)
W
A
V
E
S
Faraday
laid the groundwork with the discovery of
electromagnetic induction,
Maxwell added the last piece
.
H
e
i
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r
i
c
h
H
e
r
t
z
s
h
o
w
e
d
experimentally
in 1886
that EM waves do exist.
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E
M
waves should exist.
EM
EM
WAVES
WAVES
LIGHT
LIGHT
13
Electromagnetic (EM) waves
•
Mechanical wave
: a disturbance that
propagates in a
medium
(eg, water, strings, air)
•
An
electromagnetic wave
is a combination
of
electric
and
magnetic
fields that oscillate
together in space (
no medium
) and time in a
synchronous manner, and propagate at the
speed of light 3
×10
8
m/s or 186,000 miles/s.
•
EM waves include radio, microwaves, x-rays,
light waves, thermal waves, gamma rays
14
the generation of an electromagnetic wave
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m
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t
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The time varying
electric field
generated the time
varying
magnetic field
which generates the time
varying electric field and so on and so on . . . .
15
EM waves: transverse
•
the electromagnetic wave is a
transverse
wave
, the
electric
and
magnetic
fields
oscillate in the direction perpendicular to
the direction of propagation
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Electromagnetic waves
•
the EM wave propagates because the
electric field recreates the magnetic field
(
Maxwell
) and the magnetic field
recreates the electric field (
Faraday
)
•
The EM wave is self-sustaining
•
an
electromagnetic wave
has an electric
field and a magnetic field component,
which are perpendicular to each other
and to the direction of propagation.
17
How radio waves are produced
Dipole
Antenna
transmission
line
High Frequency
Oscillator
18
An oscillating voltage applied to the antenna
makes the charges in the antenna vibrate
up and down sending out a synchronized
pattern of electric and magnetic fields.
Electromagnetic Waves
Antenna:
emits waves
EM WAVE: time and space
varying electric and magnetic
fields moving through space
at the speed of light, c =
3 x 10
8
m/s = 186,000 miles/sec
19
Radio antenna
the EM wave
causes the
electrons in the
receiving antenna
to oscillate at the
same frequency
the amplifier
converts the
electrical signal
to sound waves
Sound waves are
transformed to an
electrical signal
which is amplified
and sent to the
transmitter
20
•
The periodic wave relation:
c =
f
•
c = 3 ×10
8
m/s is the speed of light
•
Example:
–
What is the wavelength of an electromagnetic
wave having a frequency
f
= 1 MHz (10
6
Hz)?
–
Solution:
21
The periodic wave relation applies to
electromagnetic waves
Electromagnetic spectrum
Visible light
22
•
A
M
r
a
d
i
o
:
5
3
5
K
H
z
–
1
.
7
M
H
z
•
F
M
r
a
d
i
o
:
8
8
–
1
0
8
M
H
z
•
G
P
S
:
1
2
2
7
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n
d
1
5
7
5
M
H
z
•
C
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l
p
h
o
n
e
s
:
8
2
4
M
H
z
–
2
G
H
z
•
1 vibration per second = 1 Hertz (Hz)
•
1 KHz (kilohertz) = 10
3
Hz
•
1 MHz (megahertz) = 10
6
Hz
•
1 GHz (gigahertz) = 10
9
Hz
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y
b
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s
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Microwaves
•
are in the frequency range of a few billion Hz or
wavelengths of about several cm (about the
same range as radar
the
“Radarange”
•
H
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d
o
m
i
c
r
o
w
a
v
e
s
h
e
a
t
w
a
t
e
r
?
•
Remember that the water molecule has a
positive end and a negative end.
•
The electric field of the microwave grabs onto
these charges and shakes them violently a few
billion times each second
•
all this shaking energizes the molecules making
the water hotter and hotter.
24
Explore the fascinating world of magnetism and electromagnetism with insights into Faraday's Law of Electromagnetic Induction, induced currents, and the basic principles of magnetism. Discover how a changing magnetic field can produce a current and unravel the key concepts behind electromagnetic waves. Delve into the contributions of Oersted, Ampere, and Faraday in shaping our understanding of magnetic fields and currents.
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Presentation Transcript
L 28 Electricity and Magnetism [6] magnetism Faraday s Law of Electromagnetic Induction induced currents electric generator eddy currents Electromagnetic Waves (Maxwell & Hertz) 1
Basic facts of Magnetism Oersted discovered that a compass needle responded to the a current in a loop of wire Ampere deduced the law describing how a magnetic field is produced by the current in a wire magnetic field lines are always closed loops no isolated magnetic poles; magnets always have a north and south pole permanent magnets: the currents are atomic currents due to electrons spinning in atoms - these currents are always there electromagnets: currents in wires produce magnetic fields 2
Faradays Law ofElectromagneticInduction Faraday wondered if the magnetic field due to the current in one coil could regulate the current in an adjacent coil. He was correct, with one important qualification: the magnetic field must be changing in some way to produce a current the phenomenon that a changing magnetic field can produce a current is called electromagnetic induction Michael Faraday (1791-1867) 3
Induced currents (a) A B magnetic field lines battery current indicator switch When a current is turned on or off in coil A, a magnetic field is produced which also passes through coil B. A current then briefly appears in coil B The current in coil B is called an induced current. The current in B is only present when the current in A is turned on or off, that is, when the current in A is changing 4
Induced currents (b) (c) (b) (a) a) No current is induced if the magnet is stationary. b) When the magnet is pushed toward the coil or pulled away from it, an induced current appears in the coil. c) The induced current only appears when the magnet is being moved 5
Induced currents (c) If an AC (time varying) current is used in the primary circuit, a current is induced in the secondary windings. If the current in the primary windings were DC, there would be NO induced current in the secondary circuit. Levitated coil demo 6
electric generators When a coil is rotated in a magnetic field, an induced current appears in it. This is how electricity is generated. Some external source of energy is needed to rotate the turbine which turns the coil. 7
The transformer The voltage on the secondary depends on the number of turns on the primary and secondary. Step-up the secondary has more turns than the primary Step-down the secondary has less turns than the primary 8
Eddy currents Eddy currents are induced in conductors if time- varying magnetic fields are present As the magnet falls the magnetic field strength at the plate increases Falling magnet Copper plate Eddy currents Induced magnetic field 9
Eddy currents application An induction stove uses eddy currents to cook food Only the metal pot gets hot, not the glass pot or the stove. 10
Floating magnet induced currents As the magnet falls, it induces currents in the copper pipe known as eddy currents. These eddy currents produce a magnetic field that opposes the field of the falling magnet, so the magnet does not accelerate but descends slowly. bar magnet slotted copper pipe 11
The Laws of Electricity and Magnetism Laws of electricity electric charges produce electric fields (Coulomb) electric fields begin and end on charges Laws of magnetism currents produce magnetic fields (Ampere) magnetic field lines are closed loops a changing magnetic field can produce a current (induced currents) (Faraday) A changing electric field can produce a magnetic field (Maxwell) 12
ELECTROMAGNETIC (EM) WAVES Faraday laid the groundwork with the discovery of electromagnetic induction, Maxwell added the last piece. EM WAVES LIGHT James Clerk Maxwell in 1865 predicted theoretically that EM waves should exist. Heinrich Hertz showed experimentally in 1886 that EM waves do exist. 13
Electromagnetic (EM) waves Mechanical wave: a disturbance that propagates in a medium (eg, water, strings, air) An electromagnetic wave is a combination of electric and magnetic fields that oscillate together in space (no medium) and time in a synchronous manner, and propagate at the speed of light 3 108 m/s or 186,000 miles/s. EM waves include radio, microwaves, x-rays, light waves, thermal waves, gamma rays 14
the generation of an electromagnetic wave electric field wave emitter e.g. antenna magnetic field The time varying electric field generated the time varying magnetic field which generates the time varying electric field and so on and so on . . . . 15
EM waves: transverse the electromagnetic wave is a transverse wave, the electric and magnetic fields oscillate in the direction perpendicular to the direction of propagation Efield direction of propagation Bfield 16
Electromagnetic waves the EM wave propagates because the electric field recreates the magnetic field (Maxwell) and the magnetic field recreates the electric field (Faraday) The EM wave is self-sustaining an electromagnetic wave has an electric field and a magnetic field component, which are perpendicular to each other and to the direction of propagation. 17
How radio waves are produced An oscillating voltage applied to the antenna makes the charges in the antenna vibrate up and down sending out a synchronized pattern of electric and magnetic fields. transmission line High Frequency Oscillator Dipole Antenna 18
Electromagnetic Waves EM WAVE: time and space varying electric and magnetic fields moving through space at the speed of light, c = 3 x 108 m/s = 186,000 miles/sec Antenna: emits waves 19
Radio antenna the EM wave causes the electrons in the receiving antenna to oscillate at the same frequency Sound waves are transformed to an electrical signal which is amplified and sent to the transmitter the amplifier converts the electrical signal to sound waves 20
The periodic wave relation applies to electromagnetic waves The periodic wave relation: c = f c = 3 108 m/s is the speed of light Example: What is the wavelength of an electromagnetic wave having a frequency f = 1 MHz (106 Hz)? 300, 000, 000 1, 000, 000 / c f m s Hz Solution: = = = 300 m 21
Electromagnetic spectrum = c 22 Visible light
Common frequency bands 1 vibration per second = 1 Hertz (Hz) 1 KHz (kilohertz) = 103 Hz 1 MHz (megahertz) = 106 Hz 1 GHz (gigahertz) = 109 Hz AM radio: 535 KHz 1.7 MHz FM radio: 88 108 MHz GPS: 1227 and 1575 MHz Cell phones: 824 MHz 2 GHz 23
Microwaves are in the frequency range of a few billion Hz or wavelengths of about several cm (about the same range as radar the Radarange How do microwaves heat water? Remember that the water molecule has a positive end and a negative end. The electric field of the microwave grabs onto these charges and shakes them violently a few billion times each second all this shaking energizes the molecules making the water hotter and hotter. 24
