Shengwang Du 2015, the Year of Light
Dec 12, 2015
! 3.1 Basic Laws of EM Theory
!FE = q
!E
!FM = q
!v ×!B
!F = q
!E + q!v ×
!B
A charge in the EM field !E
q
What is the EM field? It can be sensed by an electric charge.
! 3.1.1 Faraday Induction Law
“Convert magnetism into electricity”, Michael Faraday, 1822
!E ⋅d!l = − d
dtC"∫!B ⋅d!S
A∫∫ = −∂!B∂t⋅d!S
A∫∫
!E ⋅d!l =
C"∫ (∇×!E) ⋅d
!S
A∫∫
∇×!E = −∂
!B∂t
!E
!B
Stokes’ theorem
! 3.1.3 Gauss’s Law - Electric
!E ⋅d!S
A"∫∫ =Qε0=1ε0
ρ dVV∫∫∫
!E ⋅d!S
A"∫∫ = (∇⋅!E)dV
V∫∫∫
∇⋅!E = ρ
ε0
Stokes theorm
! 3.1.4 Ampere’s Law
“Convert electricity into magnetism”, James Maxwell
!B ⋅d!l = µ0C!∫
"J +ε0
∂!E∂t
$
%&
'
()⋅d!S
A∫∫
∇×!B = µ0
!J +ε0
∂!E∂t
$
%&
'
()
! Maxwell Equations - General
∇×!E = −∂
!B∂t
∇×!B = µ0
!J +ε0
∂!E∂t
$
%&
'
()
∇⋅!E = ρ
ε0
∇⋅!B = 0
Question: Do the above equations apply for vacuum or/and medium?
! Maxwell Equations in a Medium
∇×!E = −∂
!B∂t
∇×!B = µ0
!J +ε0
∂!E∂t
$
%&
'
()
∇⋅!E = ρ
ε0∇⋅!B = 0
ρ = ρ f + ρb
!J =!J f +!Jb
!P!M
Polarization
Magnetization
ρb = −∇⋅!PBounded charge density
Bounded current density !Jb =∇×
!M +
∂!P∂t
! Maxwell Equations in a Medium
∇×!E = −∂
!B∂t
∇×!H =!J f +
∂!D∂t
∇⋅!D = ρ f
∇⋅!B = 0
!D = ε0
!E +!P
!H =
!Bµ0
−!M
!P!M
Polarization
Magnetization
ρb = −∇⋅!PBounded charge density
Bounded current density !Jb =∇×
!M +
∂!P∂t
! Maxwell Equations in a Linear Medium
!D = ε0
!E +!P = ε
!E
!H =
!Bµ0
−!M =
!Bµ
!P!M
Polarization
Magnetization
ρb = −∇⋅!PBounded charge density
Bounded current density !Jb =∇×
!M +
∂!P∂t
∇×!E = −∂
!B∂t
∇×!B = µ
!J f +ε
∂!E∂t
$
%&
'
()
∇⋅!E =
ρ f
ε∇⋅!B = 0
!
Maxwell Equations (Vacuum, No Source)
∇×!E = −∂
!B∂t
∇×!B = µ0ε0
∂!E∂t
∇⋅!E = 0
∇⋅!B = 0
ρ = 0!J = 0
∇× (∇×) =∇(∇⋅) ∇2!E = µ0ε0
∂2!E
∂t2
∇2!B = µ0ε0
∂2!B
∂t2
! 3.2 Electromagnetic Waves
∇2!E = µ0ε0
∂2!E
∂t2=1c2∂2!E
∂t2
∇2!B = µ0ε0
∂2!B
∂t2=1c2∂2!B
∂t2
Recall: Ch2, Wave Motion ∇2Ψ =1v∂2Ψ∂t2
c = 1ε0µ0
≈ 3×108m / s
Maxwell: Light is EM wave!
! 3.2.1 Transverse Wave
0 =∇⋅!E =∇⋅ iEoxe
i(!k ⋅!r−ωt+φx ) + jEoye
i(!k ⋅!r−ωt+φy ) + kEoze
i(!k ⋅!r−ωt+φz )$
%&'
!E = iEoxe
i(!k ⋅!r−ωt+φx ) + jEoye
i(!k ⋅!r−ωt+φy ) + kEoze
i(!k ⋅!r−ωt+φz )
0 = Eox∂∂xei(!k ⋅!r−ωt+φx ) +Eoy
∂∂yei(!k ⋅!r−ωt+φy ) +Eoz
∂∂zei(!k ⋅!r−ωt+φz )
!k ⋅ !r = kx x + ky y + kz z
0 = ikxEoxei(!k ⋅!r−ωt+φx ) + ikyEoye
i(!k ⋅!r−ωt+φy ) + ikzEoze
i(!k ⋅!r−ωt+φz ) = i
!k ⋅!E
!k ⋅!E = 0 Transverse wave
!k ⊥!E
!k ⋅!B = 0
!k ⊥!B
! EM Wave: Transverse Wave
∇×!E = −∂
!B∂t
!E = iEoxe
i(kz−ωt+φEx ) + jEoyei(kz−ωt+φEy )
!B = iBoxe
i(kz−ωt+φBx ) + jBoyei(kz−ωt+φBy )
∇⇔ i!k ∂
∂t⇔−iω
!k ×!E =ω
!B
Recall: Ch2, Wave Motion
c = ωk
!B = 1
ω
!k ×!E = 1
ccω
!k ×!E = 1
cs ×!E
k = ωc
s = cω
!k =!kk
E =CB