Skin Depth Power Flow - empossible.net · Notes on Skin Depth and AC Resistance • High...
Transcript of Skin Depth Power Flow - empossible.net · Notes on Skin Depth and AC Resistance • High...
3/5/2020
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Electromagnetics:
Electromagnetic Field Theory
Skin Depth & Power Flow
Lecture Outline
• Skin Depth 𝛿•Power Flow
Slide 2
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Slide 3
Skin Depth
Skin Depth
Slide 4
Waves in good conductors attenuate very quickly. The distance over which they decay by a factor of 1 𝑒⁄ is called the skin depth 𝛿.
z
ze
0E
0E e
Definition of Skin Depth
1 m
In Terms of Fundamental Parameters
2 1
f
Relation to Impedance
1 245
j
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Skin Depth for Various Materials
Slide 5
https://en.wikipedia.org/wiki/Skin_effect
DC Resistance, RDC
Slide 6
J
Current density is uniform throughout the conductor so the entire conductor
contributes to current flow.
2DCConductor Area A r
DC 2DC
RA r
r
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AC Resistance, RAC
Slide 7
J
Current density is NOT uniform throughout the conductor so only part of the
conductor contributes to current flow.
ACConductor Area 2A r
ACAC 2
RA r
r
for r
22ACA r r
Conductor Area for AC Resistance
Slide 8
AC
Effective Conductor Area
2 for A r r
Area Described by
2 2 22r r r
2 r
22 r
2 2 22r r r for r
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Notes on Skin Depth and AC Resistance
• High frequencies experience so much loss that they do not penetrate very far into a conductor.• The depth of penetration is called skin depth .• Due to the skin depth at high frequencies, only part of the conductor contributes to current flow. This makes resistance increase as a function of frequency.• Drawbacks• High frequencies experience more loss.• Signals get distorted
• Benefits• Conductors can be made hollow – cheaper, lighter, etc.• Can make inner part of conductor out of a different material.
Slide 9
Slide 10
Power Flow
John Henry Poynting1852 – 1914 Academic Advisor: James Clerk Maxwell
https://en.wikipedia.org/wiki/John_Henry_Poynting
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Poynting’s Theorem
Slide 11
Poynting’s theorem is a conservation of power equation.
The total power leaving a volume must be equal to the rate of decrease of the total energy stored in the field plus the energy lost due to heat (or something else).
Power leaving
Power enteringOhmic Loss
Stored magnetic energy
Stored electric energy
2 2 21
2 VVS
H E dE H ds Evt
dv
Total power leaving volume
Rate of decrease of stored electric and magnetic energy
Ohmic power dissipated
Poynting Vector
Slide 12
From Poynting’s theorem, the term responsible for power leaving the volume is identified.
2 2 21
2S V V
E H ds H E dv E dvt
Here flux is being integrated to get total power. The argument must be power density (W/m2). This term is called the instantaneous Poynting vector.
t E t H t
Due to the cross product, the Poynting vector ℘ is
perpendicular to both 𝐸 and 𝐻. For LHI materials, the Poynting vector is in the same direction as the wave vector.
E k H
E H
|| k
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Instantaneous Poynting Vector ℘ 𝑡
Slide 13
Recall the electric and magnetic field components of a plane wave travelling in the +zdirection can be written in the time‐domain as
0 ˆ, coszxE z t E e t z a
0 ˆ, coszy
EH z t e t z a
Substituting these expressions into the definition of the instantaneous Poynting vector gives
00 ˆ ˆcos cosz z
x y
EE e t z a e t z a
Constant power flow
Rapidly oscillating fluctuation in power flow
2 2
2 20 0ˆ ˆcos cos 2 22 2
z zz z
E Et e a e t z a
t E t H t
Animation of Instantaneous Power Flow
Slide 14
Constant power flow
Rapidly oscillating fluctuation in power flow
2 2
2 20 0ˆ ˆcos cos 2 22 2
z zz z
E Et e a e t z a
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Average Poynting Vector ℘
Slide 15
The instantaneous power flow is rarely of interest because the rapidly fluctuating term does not transport any net power. The more practical and useful quantity is the time‐average
Poynting Vector ℘ .
To obtain the time‐average Poynting vector ℘ , integrate over one wave cycle 𝜏.
2 2
2 20 0
0 0
1 1ˆ ˆcos cos 2 2
2 2z z
z z
E Ee a dt e t z a dt
2
20avg ˆcos
2z
z
Ee a
avg
0
1t dt
Integrating cosine over one wave cycle equals zero because cosine is both negative and positive equally.
2
0 2 ˆ ˆz jx y
Ee e a a
Complex Poynting Vector ℘
Slide 16
For time‐harmonic signals, the frequency‐domain Poynting vector is complex.
*E H
The field expressions for the plane wave are
0 ˆz xE z E e a
0 ˆz jy
EH z e e a
Substituting these into the definition of complex Poynting vector gives
220 ˆ z j
z
Ee e a
*
00 ˆ ˆz z j
x y
EE e a e e a
*0
0 ˆ ˆz j z z j z jx y
EE e e a e e e a
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2
20 ˆcos2
zz
Ee a
RMS Poynting Vector ℘
Slide 17
The complex Poynting vector ℘ is like the instantaneous Poynting vector ℘ 𝑡 and contains the rapidly varying fluctuations in power flow.
A more meaningful quantity is the root‐mean‐square (RMS) power flow that is easily calculated from the complex Poynting vector.
*avg
1 1Re Re
2 2E H
2
20avg ˆcos
2z
z
Ee a
avg
1Re
2
2201
ˆRe2
z jz
Ee e a
Total Power
Slide 18
The Poynting vector is a power density with units of W/m2.
To calculate total power flow through some area, integrate the Poynting vector over that area.
S
P t t ds
The average power flow is simply calculated from the average Poynting vector.
avg avg
S
P ds
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Index Ellipsoids and Power Flow
Slide 19
Isotropic Materials
k
x
y
k
Anisotropic Materials
x
y
Phase propagates in the direction of 𝑘. Therefore, the refractive index 𝑛 derived from |𝑘| is best described as the phase refractive index 𝑛 . Velocity here is the phase velocity �⃗� .
Power propagates in the direction of the Poynting vector ℘ which is always normal to the surface of the index ellipsoid. From this, we can define a group refractive index 𝑛 and group velocity �⃗� .
P
P
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