Maxwell's Equations Explained for Electromagnetic Fields and Waves
1. EEEC6430310 ELECTROMAGNETIC FIELDS AND WAVES
Maxwellβs Equation
FACULTY OF ENGINEERING AND COMPUTER TECHNOLOGY
BENG (HONS) IN ELECTRICALAND ELECTRONIC ENGINEERING
Ravandran Muttiah BEng (Hons) MSc MIET
2. Maxwellβs Equation
I. Maxwellβs Equation For Linear Media
π» Γ π¬ = βπ
ππ―
ππ‘
Faradayβ²
s Law
π» Γ π― = π± + π
ππ¬
ππ‘
Ampereβ²
s Law
π» β π¬ =
πf
π
Gaussβ²
s Law
π» β π― = 0 Gaussβ²
s Law
where, π» = vector differential (del) operator
π¬ = electric field intensity
π― = magnetic field intensity
π± = electric current density
πf = free electric charge density
1
4. 3
V
π» β π¬ Γ π― dπ =
S
π¬ Γ π― β dπΊ
S
π¬ Γ π― β dπ +
d
dπ‘ V
1
2
π π¬ 2
+
1
2
π π― 2
dπ = β
π
π¬ β π± dπ
πΊ = π¬ Γ π― Poynting Vector
Watt
m2
π = V
1
2
π π¬ 2
+
1
2
π π― 2
dπ Electromagnetic Stored Energy
πd = V
π¬ β π± dπ Power dissipated if π½ β π¬ > 0
e.g., π± = ππ¬ β π½ β π¬ = π π¬ π
Power source if π½ β π¬ < 0
πout =
S
π¬ Γ π― β dπ =
S
πΊ β dπ
πout +
dπ
dπ‘
= βπd
π€e =
1
2
π π¬ 2
Electric energy density in
Joules
m3
π€m =
1
2
π π― 2
Magnetic energy density in
Joules
m3
5. 4
Figure 1: The circuit power into an N terminal network π=1
π
πππΌπ equals the
electromagnetic power flow into the surface surrounding the network, β s
π¬ Γ π― Β· dπΊ
B. Power In Electric Circuits
E
H
π = πΈ Γ π»
π1
π2
π3
ππβ1
ππ
πΌ1
πΌ2
πΌ3
πΌπβ1
πΌπ
π
6. 5
Outside circuit elements
C
π¬ β dπ β 0, π» Γ π¬ = 0 β π¬ = βπ»Π€ (Kirchoffβs Voltage Law π π£π = 0)
π» Γ π― = π± β π» β π± = 0, S
π± β dπΊ = 0 (Kirchoffβs Current Law π ππ = 0)
πin = β
S
π¬ Γ π― β dπΊ
= β
V
π» β π¬ Γ π― dπ
π» β π¬ Γ π― = π― β π» Γ π¬ β π¬ β π» Γ π― = βπ¬ β π± = π»Π€ β π±
π» β π±Π€ = Π€π» β π± + π± β π»Π€
π» β π¬ Γ π― = π± β π»Π€ = π» β Π€π±
πin = β
V
π» β π¬ Γ π― dπ = β
V
π» β π±Π€ dπ = β
S
π±Π€ β dπΊ
On π, Π€ = voltages on each wire, π± is non-zero only on wires.
πin = β
S
π±Π€ β dπΊ = β
π=1
π
π£π
S
π± β dπΊ =
π=1
π
π£πππ
0
0
βππ
7. C. Complex Poyntingβs Theorem (Sinusoidal Steady State, ejππ‘
)
6
π¬ π, π‘ = Re π¬ π ejππ‘
=
1
2
π¬β
π ejππ‘
+ π¬β
π eβjππ‘
π― π, π‘ = Re π― π ejππ‘
=
1
2
π―β
π ejππ‘
+ π―β
π eβjππ‘
The real part of a complex number is
one-half of the sum of the number
and its complex conjugate
11. III. Transverse Electromagnetic Waves (π± = 0, πf = 0)
10
The great success of Maxwellβs equations lies partly in the simple prediction of
electromagnetic waves and their simple characterization of materials in terms of
conductivity π, permittivity π, and permeability π. In vacuum we find π = 0, π = πo
and π = πo. Therefore, π± = ππ¬ = 0 and πf = 0.
A. Wave Equation
π» Γ π¬ = βπ
ππ―
ππ‘
π» Γ π― = π
ππ¬
ππ‘
π» β π¬ = 0
π» β π― = 0
In contrast the nano-structure of media can be quite complex and requires quantum
mechanics and for its full explanation. Fortunately, simple classical approximations
to atoms and molecules suffice to understand the origin of π, π and π.
18. 17
C. Normal Incidence Onto A Perfect Conductor
Incident Fields: π¬i π§, π‘ = Re π¬iej ππ‘βππ§
ππ₯
π―i π§, π‘ = Re
π¬i
π
ej ππ‘βππ§
ππ¦
Reflected Fields: π¬r π§, π‘ = Re π¬rej ππ‘+ππ§
ππ₯
π―r π§, π‘ = Re β
π¬r
π
ej ππ‘+ππ§
ππ¦
π = π ππ, π =
π
π
The boundary conditions require that,
πΈπ₯ π§ = 0, π‘ = πΈπ₯, i π§ = 0, π‘ + πΈπ₯, r π§ = 0, π‘ = 0
π¬i + π¬r = 0 β π¬r = βπ¬i
19. 18
Figure 2: Time varying electromagnetic phenomena differ only in the scaling of time
(frequency) and size (wavelength). In linear dielectric media the frequency and
wavelength are related as πΞ» = π (π = ππ), where π =
1
ππ
is the velocity of light in the
medium.
sin πz
π =
2Ο
π
π =
2Ο
π
2Ο
π
2Ο
π
π§
π‘
Ο
π
Ο
π
β1
β1
1
1 sin ππ‘
0 102
104 106
108 1010
1012
1014 1016 1018
1020
Circuit Theory Microwaves
f (Hz)
Visible Light Ultraviolet X-Rays Gamma Rays
Power Infrared (Heat)
Radio and Television
AM FM
3 Γ 106
Ξ» meters 3 Γ 104 3 Γ 102 3 3 Γ 10β2
3 Γ 10β4
3 Γ 10β6 3 Γ 10β8
3 Γ 10β10 3 Γ 10β12
20. 19
For π¬i = πΈi real we have:
πΈπ₯ π§, π‘ = πΈπ₯, i π§, π‘ + πΈπ₯, r π§, π‘ = Re π¬i eβjππ§
β ejππ§
ejππ‘
= 2πΈi sin ππ§ sin ππ‘
π»π¦ π§, π‘ = π»π¦, i π§, π‘ + π»π¦, r π§, π‘ = Re
π¬i
π
eβjππ§
β ejππ§
ejππ‘
=
2πΈi
π
cos ππ§ cos ππ‘
πΎπ§ π§ = 0, π‘ = π»π¦ π§ = 0, π‘ =
2πΈi
π
cos ππ‘
Radiation pressure in free space π = πo, π = πo
Forceπ§
Area π§=0
=
1
2
π² Γ πoπ― =
1
2
πoπΎπ₯π»π¦
π§=0
ππ§ =
1
2
πoπ»π¦
2
π§ = 0 ππ§
=
2πoπΈi
2
πo
2 cos2
ππ‘ ππ§
=
2πo
πo
πo
πΈi
2
cos2
ππ‘ ππ§
= 2πo πΈi
2
cos2
ππ‘ ππ§
21. 20
Figure 3: A uniform plane wave normally incident upon a perfect conductor has zero
electric field at the conducting surface thus requiring a reflected wave. The source of this
reflected wave is the surface current at π§ = 0, which equals the magnetic field there. The
total electric and magnetic fields are 90Β° out of phase in time and space.
x
z
y
πΈi = Re π¬iej ππ‘βππ§
ππ₯
π»i = Re
π¬i
πo
ej ππ‘βππ§
ππ¦
π =
π
π
ππ§
π»r = Re β
π¬r
πo
ej ππ‘+ππ§
ππ¦
πΈr = Re π¬rej ππ‘+ππ§
ππ₯
πr = β
π
c1
ππ§
πo, πo πo = πo=
πo
πo
π»π¦ π , π‘ =
2π¬i
πo
cos ππ cos ππ‘
πΈπ₯ π , π‘ = 2π¬isin ππ sin ππ‘
π β β
22. 21
Figure 4: A uniform plane wave normally incident upon a dielectric interface separating
two different materials has part of its power reflected and part transmitted.
IV. Normal Incidence Onto A Dielectric
x
z
y
π1, π1 π1 = π1 =
π1
π1
, π1 =
1
π1π1
π2, π2 π2 =
π2
π2
, π2 =
1
π2π2
πΈi = Re π¬iej ππ‘βπ1π§
ππ₯
πΈt = Re π¬tej ππ‘βπ2π§
ππ₯
π»t = Re
π¬t
π2
ej ππ‘βπ2π§
ππ¦
π»r = Re β
π¬r
π1
ej ππ‘+π1π§
ππ¦
π»i = Re
π¬i
π1
ej ππ‘βπ1π§
ππ¦
πΈr = Re π¬rej ππ‘+π1π§
ππ₯
πi = π1ππ§ =
π
π1
ππ§
πr = βπ1ππ§ =
βπ
π1
ππ§
πt = π2ππ§ =
π
π2
ππ§
23. 22
π¬i π§, π‘ = Re π¬iej ππ‘βπ1π§
ππ₯ , π1 = π π1π1
π―i π§, π‘ = Re
π―i
π1
ej ππ‘βπ1π§
ππ¦ , π1 = π
π1
π1
π¬r π§, π‘ = Re π¬rej ππ‘+π1π§
ππ₯
π―r π§, π‘ = Re β
π¬r
π1
ej ππ‘+π1π§
ππ¦
π¬t π§, π‘ = Re π¬tej ππ‘βπ2π§
ππ₯ , π2 = π π2π2
π―t π§, π‘ = Re
π¬t
π2
ej ππ‘βπ2π§
ππ¦ , π2 = π
π2
π2