csmsp11 lecture02 emwaves(asgiven) · 1/18/11 2 s e⋅da= q incl closedε 0 urface ∫∫...
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PH300 Modern Physics SP11
1/18 Day 2: Ques)ons? Review E&M Waves and Wave Equa)ons
“From the long view of the history of mankind – seen from, say, ten thousand years from now – there can be liGle doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scien)fic event of the same decade.” – Richard Feynman
Next Time: Interference Polariza)on
Double-‐Slit Experiment 2
Today & Thursday: “Classical” wave-‐view of light & its interac)on with maGer • Pre-‐quantum • S)ll useful in many situa)ons.
Next week: Special Rela)vity è Universal speed of light
Length contrac)on, )me dila)on
Thursday: HW01 due, beginning of class; HW02 assigned
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Maxwell’s Equa)ons: Describe EM radia)on
E ⋅dA∫ =
Qincl
ε0
B ⋅dA∫ = 0
E ⋅dl∫ = −
dΦB
dt
B ⋅dl
∫ = µ0Ithrough + ε0µ0dΦE
dt
B
E
• Aim is to cover Maxwell’s Equa)ons in sufficient depth to understand EM waves
• Understand the mathema)cs we’ll use to describe WAVES in general
E ⋅dA
Some surface, A∫
B ⋅dl
Some path, l∫
‘Flux’ of a vector field through a surface:
The average outward-‐directed component of a vector field mul)plied by a surface area.
‘Circula)on’ of a vector field around a path:
The average tangen)al component of a vector field mul)plied by the path length.
E ⋅dA =Qincl
ε0Closedsurface
∫∫
An example:
Gauss’ Law Electric flux through a closed surface tells you the total charge inside the surface.
A)
B)
C)
D) Something else
If the electric field at the surface of a sphere (radius, r) is radial and of constant magnitude,
what is the flux out of the sphere? E ⋅dA∫
E ⋅πr2
E ⋅ 4πr2
E ⋅43πr3
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E ⋅dA =Qincl
ε0Closedsurface
∫∫
An example:
Gauss’ Law Electric flux through a closed surface tells you the total charge inside the surface.
E =14πε0
Qr2
E ⋅ 4πr2 = Qε0
So:
Coulomb’s Law is contained in Gauss’ Law
An example:
Faraday’s Law Electric circula)on around a closed path tells you the (nega)ve of) the )me change of flux of B through any open surface bounded by the path.
E ⋅dl∫ = −
dΦB
dt
ΦB (t) =
B ⋅dA∫
B ⋅dA∫ = 0
E ⋅dA∫ =
Qincl
ε0
E ⋅dl∫ = −
dΦB
dt
B ⋅dl
∫ = µ0Ithrough + ε0µ0dΦE
dt
Consider the following configura)on of field lines. This could be…
A) …an E-‐field B) …a B-‐field C) Either E or B D) Neither E) No idea
B ⋅dA∫ = 0
E ⋅dA∫ =
Qincl
ε0
Consider the following configura)on of field lines. This could be…
A) …an E-‐field B) …a B-‐field C) Either E or B D) Neither
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E ⋅dl∫ = −
dΦB
dt
B ⋅dl
∫ = µ0Ithrough + ε0µ0dΦE
dt
Maxwell’s Equa)ons in Vacuum
The words “in vacuum” are code for “no charges or currents present”
The equa)ons then become:
ε0 = 8.85 ×10−12 farad/meterµ0 = 4π ×10−7 henry/meter
E ⋅dA∫ = 0
B ⋅dA∫ = 0
E ⋅dl∫ = −
dΦB
dt
B ⋅dl
∫ = ε0µ0dΦE
dt
E ⋅dA∫ = 0 →
∇ ⋅E = 0
B ⋅dA∫ = 0 →
∇ ⋅B = 0
E ⋅dl∫ = −
dΦB
dt →
∇ ×E = −
∂B∂t
B ⋅dl
∫ = ε0µ0dΦE
dt →
∇ ×B = ε0µ0
dEdt
Maxwell’s Equa)ons: Differen)al forms Maxwell’s Equa)ons: Differen)al forms
Each of these is a single par)al differen)al equa)on.
Each of these is a set of three coupled par)al differen)al equa)ons.
∇ ⋅E = 0
∇ ⋅B = 0
∇ ×E = −
∂B∂t
∇ ×B = ε0µ0
dEdt
Maxwell’s Equa)ons: 1-‐Dimensional Differen)al Equa)ons
∂Ey (x,t)∂x
= −∂Bz (x,t)
∂t
−∂Bz (x,t)
∂x= µ0ε0
∂Ey (x,t)∂t
∂2Ey
∂x2=1c2
∂2Ey
∂t 2∂2Bz∂x2
=1c2
∂2Bz∂t 2
OR
∂2Ey
∂x2=1c2
∂2Ey
∂t 2
1-‐Dimensional Wave Equa)on
Solu)ons are sines and cosines:
Ey = Asin(kx −ωt) + Bcos(kx −ωt)
k2 = ω 2
c2c = λ
T
2πλ
2πT
…with the requirement that:
or
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Sinusoidal waves: Wave in )me: cos(2πt/T) = cos(ωt) = cos(2πr)
Wave in space: cos(2πx/λ) = cos(kx)
k is spa)al analogue of angular frequency ω. One reason we use k is because it’s easier to write sin(kx) than sin(2πx/λ).
λ = Wavelength = length of one cycle
k = 2π/λ = wave number = number of radians per meter
x
T = Period = )me of one cycle
ω = 2π/T = angular frequency = number of radians per second
t
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Waves in space & )me: cos(kx + ωt) represents a sinusoidal wave traveling…
A) …to the right (+x-‐direc)on). B) …to the ler (-‐x-‐direc)on).
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At t=0
Light is an oscilla)ng E-‐field
E
E(x,t) = Emaxsin(ax-‐bt)
Func)on of posi)on (x) and )me (t):
• Oscilla)ng ELECTRIC and magne)c field • Traveling to the right at speed of light (c)
X
c
Snap shot of E-‐field in )me:
Electromagne)c radia)on
A liGle later in )me
Emaxsin(ax+bt)
∂2Ey
∂x2=1c2
∂2Ey
∂t 2
1-‐Dimensional Wave Equa)on
Ey = A1 sin(k1x −ω1t) + A2 cos(k2x −ω2t)
A specific solu)on is found by applying boundary condi)ons
The most general solu)on is:
1-‐Dimensional Wave Equa)on
hGp://www.walter-‐fendt.de/ph14e/stwaverefl.htm
Complex Exponen)al Solu)ons
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E = Acos(kx −ωt)Recall solu7on for the Electric field (E) from the wave equa7on:
exp[i(kx −ωt)] = cos(kx −ωt) + isin(kx −ωt)Euler’s Formula says:
E = Aexp[i(kx −ωt)]For convenience, write:
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Complex Exponen)al Solu)ons
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E(x, y, z,t) =
E0 exp i
k ⋅ r −ωt( )⎡⎣ ⎤⎦
Constant vector prefactor tells the direc)on and maximum strength of E.
Complex exponen)als for the sinusoidal space and )me oscilla)on of the wave.
E(x, y, z,t) =
E0 exp i kxx + kyy + kzz −ωt( )⎡⎣ ⎤⎦
E(x, y, z,t) =
E0 exp(ikxx)exp(ikyy)exp(ikzz)exp(iωt)
E(x, y, z,t) =
E0 exp i
k ⋅ r −ωt( )⎡⎣ ⎤⎦
The electric field for a plane wave is given by:
The vector tells you… k
A) The direc)on of the electric field vector B) The direc)on of the magne)c field vector C) The direc)on in which the wave is not varying D) The direc)on the plane wave moves E) None of these
A) 0 B) C)
D) E) None of these.
Complex Exponen)al Solu)ons
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E(x, y, z,t) =
E0 exp i
k ⋅ r −ωt( )⎡⎣ ⎤⎦
Constant vector prefactor tells the direc)on and maximum strength of E.
Complex exponen)als for the sinusoidal space and )me oscilla)on of the wave.
∇ ⋅E(x, y, z,t) = ∂Ex
∂x+∂Ey
∂y+∂Ez
∂z= ?
ik ⋅E i
k ×E
kE
ik ⋅E0 = 0
ik ⋅B0 = 0
ik ×E0 = iω
B0
ik ×B0 = −iωµ0ε0
E0
Complex Exponen)al Solu)ons The complex equa)ons reduce Maxwell’s Equa)ons (in vacuum) to a set of vector algebraic rela)ons between the three vectors , and , and the angular frequency :
k E0
B0 ω
• Transverse waves. and are perpendicular to . E0
B0
k
• and are perpendicular to each other. E0
B0
ik ⋅E0 = 0
ik ⋅B0 = 0
ik ×E0 = iω
B0
ik ×B0 = −iωµ0ε0
E0
Complex Exponen)al Solu)ons The complex equa)ons reduce Maxwell’s Equa)ons (in vacuum) to a set of vector algebraic rela)ons between the three vectors , and , and the angular frequency :
k E0
B0 ω
ω 2
k2 =
1µ0ε0
= c2
E0B0
= c E
B
k
, , and form a ‘right-‐handed system’, with the wave traveling in the direc)on , at speed c. E B
k
k
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How do you generate light (electromagne)c radia)on)?
A) Sta)onary charges B) Charges moving at a constant velocity C) Accelera)ng charges D) B and C E) A, B, and C
Accelera)ng charges à changing E-‐field and changing B-‐field (EM radia)on à both E and B are oscilla)ng)
Sta)onary charges à constant E-‐field, no magne)c (B)-‐field
+
E
Charges moving at a constant velocity à Constant current through wire creates a B-‐field But B-‐field is constant I
B
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How do you generate light (electromagne)c radia)on)?
A) Sta)onary charges B) Charges moving at a constant velocity C) Accelera)ng charges D) B and C E) A, B, and C
Answer is (C) Accelera'ng charges create EM radia7on.
Surface of sun- very hot! Whole bunch of free electrons whizzing around like crazy. Equal number of protons, but heavier so moving slower, less EM waves generated.
+ +
+
+
The Sun
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What does the curve tell you? A) The spa)al extent of the E-‐field. At the peaks and troughs the E-‐
field is covering a larger extent in space B) The E-‐field’s direc)on and strength along the center line of the
curve C) The actual path of the light travels D) More than one of these E) None of these.
OR
EM radiation often represented by a sinusoidal curve.
radio wave sim
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What does the curve tell you?
Making sense of the Sine Wave
-‐ For Water Waves? -‐ For Sound Wave? -‐ For E/M Waves?
wave interference sim
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EM radia)on oren represented by a sinusoidal curve. What does the curve tell you? Correct answer is (B) – the E-‐field’s direc)on and strength along the center line of the curve.
X
Only know E-‐field, along this line.
At this )me, E-‐field at point X is strong and in the points upward.
Path of EM Radia)on is a straight line.
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What stuff is moving up and down in space as a radio wave passes? A) Electric field B) Electrons C) Air molecules D) Light ray E) Nothing
Snapshot of radio wave in air.
Answer is (E): Nothing Electric field strength increases and decreases – E-‐field does not move up and down.
Length of vector represents strength of E-‐field Orienta)on represents direc)on of E-‐field
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What is moving to the right in space as radio wave propagates?
A) Disturbance in the electric field B) Electrons C) Air molecules D) Nothing
Snapshot of radio wave in air.
Answer is (A). Disturbance in the electric field.
At speed c
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Review :
• Light interacts with maGer when its electric field exerts forces on electrons.
• In order to create light, we need both changing E and B fields. Can do this only with accelera)ng charges.
• Light has a sinusoidally changing electric field Sinusoidal paGern of vectors represents increase and decrease in strength of field, nothing is physically moving up and down in space
End here -‐ 10/18
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Spectrum: All EM waves. Complete range of wavelengths. Electromagne7c Spectrum
Wavelength (λ) = distance (x) un)l wave repeats
λ
λ
λ Blue light
Red light
Cosmic rays
SHORT LONG
Frequency (f) = # of )mes per second E-‐field at point changes through complete cycle as wave passes
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How much )me will pass before this peak reaches the antenna? c = speed of light
Distance = speed * )me Time = distance/speed = d/c
Frequency = (1 )/Period (Hertz) = # of cycles per second
A) cd B) c/d C) d/c D) sin(cd) E) None of these
d
Period (seconds/cycle) = λ/c
How much )me does it take for E-‐field at point (X) to go through 1 complete oscilla)on?
f λ=c 42
How far away will this peak in the E-‐field be before the next peak is generated at this spot?
Electron oscillates with period of T=1/f
A) cλ B) c/λ C) λ/c D) sin(cλ) E) None of these
Answer is (A): Distance = velocity * )me Distance = cλ = c/f = 1 wavelength so c/f = λ