Electromagnetic waves He predicted electromagnetic wave … · 2003-02-27 · EM waves carry energy...
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Electromagnetic waves James Clerk Maxwell (1831-1879) He predicted electromagnetic wave propagation
Transcript of Electromagnetic waves He predicted electromagnetic wave … · 2003-02-27 · EM waves carry energy...
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
A singular theoretical achievement of the 19th century
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
A singular theoretical achievement of the 19th century
It was know that time changing B induced E (Faraday’s law)
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
A singular theoretical achievement of the 19th century
It was know that time changing B induced E (Faraday’s law)
Another symmetry argument
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
A singular theoretical achievement of the 19th century
It was know that time changing B induced E (Faraday’s law)
Another symmetry argument
Maxwell hypothesized: a time changing E must induce B
Electromagnetic waves
James Clerk Maxwell(1831-1879)
He predicted electromagnetic wave propagation
A singular theoretical achievement of the 19th century
It was know that time changing B induced E (Faraday’s law)
Another symmetry argument
Maxwell hypothesized: a time changing E must induce B
and there was light
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
“L”
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
“L”
“C”
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
“L”
“C”
“L*”
“C*”
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
“L”
“C”
“L*”
“C*”
MHz1000 ≈f
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
**0CL2
1LC2
1
π=
π=f
EM wave
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
EM wave
Hertz apparatus
Heinrich Hertz (1857-1894) in 1887 generated & detected electromagnetic (EM) waves in lab
EM waves are generated because charges are accelerating
Moving charges produce currents which in turn produce B
Moving charges produce currents which in turn produce B
E and B at right angles to each other and both are transverse to wave propagation direction
Transverse wave
E and B at right angles to each other and both are transverse to wave propagation direction
Transverse wave
00
1c
εµ=
00
1c
εµ=λ
wavelength
Transverse wave
E and B at right angles to each other and both are transverse to wave propagation direction
00
1c
εµ=λ
wavelength
Transverse wave
E and B at right angles to each other and both are transverse to wave propagation direction
c=λ⋅f
Properties of electromagnetic waves
Speed = c = 3.00×108 m/s
Properties of electromagnetic waves
Speed = c = 3.00×108 m/s
Maxwell proved: cBE
=
Properties of electromagnetic waves
Speed = c = 3.00×108 m/s
Maxwell proved: cBE
=
EM waves carry energy and linear momentum
Properties of electromagnetic waves
Speed = c = 3.00×108 m/s
Maxwell proved: cBE
=
EM waves carry energy and linear momentum
Because E and B fields store energy in them. In fact, energies stored in E & B fields are equal.
Let S = “intensity” = average rate at which EM-wave energy passes through a unit area perpendicular to direction of wave = W/m2.
( ) ( )2max00
2max
0
maxmax B2
cc2
E2
BES
µ=
µ=
µ=
Let S = “intensity” = average rate at which EM-wave energy passes through a unit area perpendicular to direction of wave = W/m2.
( ) ( )2max00
2max
0
maxmax B2
cc2
E2
BES
µ=
µ=
µ=
Let S = “intensity” = average rate at which EM-wave energy passes through a unit area perpendicular to direction of wave = W/m2.
Radio station with total power P: what is S at distance r from station?
( ) ( )2max00
2max
0
maxmax B2
cc2
E2
BES
µ=
µ=
µ=
Let S = “intensity” = average rate at which EM-wave energy passes through a unit area perpendicular to direction of wave = W/m2.
Radio station with total power P: what is S at distance r from station?
2r4P
Sπ
=
( ) ( )2max00
2max
0
maxmax B2
cc2
E2
BES
µ=
µ=
µ=
Let S = “intensity” = average rate at which EM-wave energy passes through a unit area perpendicular to direction of wave = W/m2.
Radio station with total power P: what is S at distance r from station?
2r4P
Sπ
=
Once you know S, you can find Emax and Bmax
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
( ) ( ) cS2E cS2E c2
ES 0max0
2max
0
2max µ=⇒µ=⇒µ
=
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
( ) ( ) cS2E cS2E c2
ES 0max0
2max
0
2max µ=⇒µ=⇒µ
=
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
( ) ( ) cS2E cS2E c2
ES 0max0
2max
0
2max µ=⇒µ=⇒µ
=
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
( ) ( ) cS2E cS2E c2
ES 0max0
2max
0
2max µ=⇒µ=⇒µ
=
m/V 870Emax ≈
Since the sun delivers at the surface of the earth S = ~1 kW/m2, find Emax and Bmax.
( ) ( ) cS2E cS2E c2
ES 0max0
2max
0
2max µ=⇒µ=⇒µ
=
T 109.2c
EB 6max
max−×==m/V 870Emax ≈
Detecting EM waves
Detecting E-field part E
Detecting EM waves
Detecting E-field part
C L
Antenna
E
Detecting EM waves
Detecting E-field part
C L
Antenna
E
Detecting B-field part
•×B•
Detecting EM waves
Detecting E-field part
C L
Antenna
E
Detecting B-field part
C L
•×B•
Loop antenna
Detecting EM waves
Detecting E-field part
C L
Antenna
E
Detecting B-field part
C L
Inducedemf due to changing B-flux in antenna
•×B•
Loop antenna
Linear momentum carried by EM wave
“light pressure” P (N/m2)
Linear momentum carried by EM wave
“light pressure” P (N/m2)
Complete absorption: cS
=P
Linear momentum carried by EM wave
“light pressure” P (N/m2)
=⋅⋅
=⋅== 2
222
mN
smmsmN
smmsJ
s/mm/W
cS
Complete absorption: cS
=P
Linear momentum carried by EM wave
“light pressure” P (N/m2)
=⋅⋅
=⋅== 2
222
mN
smmsmN
smmsJ
s/mm/W
cS
Complete absorption: cS
=P
Complete reflection:cS2 ⋅
=P
Linear momentum carried by EM wave
“light pressure” P (N/m2)
=⋅⋅
=⋅== 2
222
mN
smmsmN
smmsJ
s/mm/W
cS
Complete absorption: cS
=P
Complete reflection:cS2 ⋅
=P
Light bouncing off mirror imparts 2× more momentum to mirror
The effect of sunlight pressure on dust in our solar system(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
(Applying Physics 21.2, p. 672)
Light not reflected
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
(Applying Physics 21.2, p. 672)
Light not reflected
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
Light not reflected
Light output power of sun
(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
2
3sun
2sun
gravR
r34
MG
RmMG
F
π⋅ρ⋅⋅
=⋅⋅
=
Light not reflected
Light output power of sun
(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
2
3sun
2sun
gravR
r34
MG
RmMG
F
π⋅ρ⋅⋅
=⋅⋅
=
Light not reflected
Light output power of sun
(Applying Physics 21.2, p. 672)
dust particle of radius r and mass min orbit around sun
Sun
R
The effect of sunlight pressure on dust in our solar system
cR4rP
cr
R4P
rcS
rF
2
2sun
2
2sun
22light
⋅=
ππ
=
π=π⋅= P
2
3sun
2sun
gravR
r34
MG
RmMG
F
π⋅ρ⋅⋅
=⋅⋅
= density ofdust particle
Light not reflected
Light output power of sun
(Applying Physics 21.2, p. 672)
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
2
21126
sun30
sunkg
mN 108.6G W; 107.3P kg; 100.2M
⋅×=×=×= −
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
2
21126
sun30
sunkg
mN 108.6G W; 107.3P kg; 100.2M
⋅×=×=×= −
33 kg/m 103 g/cc 3 ×=≈ρFor dust particle
ComputeFlight/Fgrav
ratio
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
2
21126
sun30
sunkg
mN 108.6G W; 107.3P kg; 100.2M
⋅×=×=×= −
µ
µ=
m)(rm 18.0
F
F
grav
light
ComputeFlight/Fgrav
ratio
33 kg/m 103 g/cc 3 ×=≈ρFor dust particle
cR4rP
F 2
2sun
light⋅
=2
3sun
gravR
r34
MGF
π⋅ρ⋅⋅
=
ρπ
=ρπ
=r1
c16GMP3
R3r4GMcR4rP
F
F
sun
sun23
sun
22sun
grav
light
2
21126
sun30
sunkg
mN 108.6G W; 107.3P kg; 100.2M
⋅×=×=×= −
µ
µ=
m)(rm 18.0
F
F
grav
light
ComputeFlight/Fgrav
ratio
⇒ For r < 0.18 µm, dust particle will pushed out of solar system!
33 kg/m 103 g/cc 3 ×=≈ρFor dust particle
