bmayer@chabotcollege.edu engr-45_lec-10_dielectrics.ppt 1 bruce mayer, pe engineering-45: materials...
Post on 18-Dec-2015
222 Views
Preview:
TRANSCRIPT
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt1
Bruce Mayer, PE Engineering-45: Materials of Engineering
Bruce Mayer, PERegistered Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering 45
ElectricalElectricalPropertiesProperties
-3-3
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt2
Bruce Mayer, PE Engineering-45: Materials of Engineering
Learning Goals – DielectricsLearning Goals – Dielectrics
Understand the fundamentals of Electrical Capacitance
How Certain Materials can Dramatically Increase the Electrical Capacity
Understand Dipoles and Polarization Learn the Types of Polarization Dielectric-Constant vs Frequency
Behavior
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt3
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electrical CapacitanceElectrical Capacitance Consider Two
Conductive Plates Separated by a Small & Empty Gap With a Voltage Applied (right)
Since No Current Can Flow Across The Gap• Positive Charges
Accumulate on Top
• Negative Charges Accumulate on Bot
The Quantity of the Separated Charge, Q, is Proportional to V
VQ Look for Constant of
Proportionality, C
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt4
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electrical Capacitance cont.Electrical Capacitance cont. The Value of C can
Found from an Expression that is Analogous to Ohm’s Eqn
• For || plates in a Vacuum C is proportional to the Plate AREA, and the inverse Separation LENGTH
CVQ • Where
– Q Charge (A-s or Coulombs)
– V Elect. Potential (V)– C Capacitance
(A-s/V or Coul/V or Faradays [Farads, F])
l
AC
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt5
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electrical Capacitance cont.2Electrical Capacitance cont.2
• Filling The Gap with a NONconductive Material INCREASES the Charge Accumulation Thru the DiElectric Effect
l
Aε C 0
• Where– A Plate Area (sq-m)– l Plate Distance (m) 0 Permittivity of
Free Space (vacuum) = 8.85x10−12 F/m
Introducing a Constant of proportionalitybetween C & A/ℓ
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt6
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electrical Capacitance cont.3Electrical Capacitance cont.3 For a DiElectric
Filled Cap
• Sometimes called “k”, the Dielectric Constant is ALWAYS Positive with a Magnitude greater than Unity
l
Aε C
• Where Permittivity of the
Dielectric Medium (F/m)
Using 0 as a BaseLine, Define a Material’s RELATIVE Dielectric Constant
0εε r
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt7
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electrical TermsElectrical Terms Electric Field is the
ratio of a Voltage Drop to Distance over Which the Drop Occurs; to whit
and Current will Flow
Now as V Increases toward at Some Point the Dielectric will “Break Down”
mV
lVΕ
/units
mV
lVΕ flowibd
/units
Thus the Dielectric E-Field Strength
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt8
Bruce Mayer, PE Engineering-45: Materials of Engineering
ExamplesExamples
r = 1.00059
• Ebd = 3 x 106 V/m (75 V/mil)
For Air at RoomConditions
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt9
Bruce Mayer, PE Engineering-45: Materials of Engineering
Electric DiPoleElectric DiPole What is a “DiPole”?
• DiPole Refers to the Physical SEPARATION of TWO, OPPOSITE-polarity, and thus Attractive, “Charge Entities”
Two Classical Types• Electric DiPole
– “+” & “-” Charges Separated
• Magnetic DiPole– “North” and “South”
“Poles” Separated Note: These Entities
ALWAYS exist in Tandem; There is NO Magnetic MonoPole
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt10
Bruce Mayer, PE Engineering-45: Materials of Engineering
Field Vectors contField Vectors cont Consider an Electric
DiPole with Charge, q, and Separation, d
The DiPole Moment, p, is Quantified• Magnitude = q•d
• Direction Neg→Pos
We call this a “Moment” because of the the DiPole can be Twisted • The Torque Can Be
applied with an Electric Field
• The Process of Pole Alignment is called“polarization”
Not Aligned
→Torque Aligned →NO Torque
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt11
Bruce Mayer, PE Engineering-45: Materials of Engineering
Field Vectors cont.2Field Vectors cont.2 Consider again the
||-Plate Cap The Areal Density of
Charges on Each Plate, D
• Where & E from Before– D Charge Density
(Coul/sq-m)
• Since a Cap Configuration “Displaces” Charges from one Plate to Another, The Quantity D is also Called the DIELECTRIC (charge) DISPLACEMENT
DEl
VV
lD
A
Q
Vl
ACVQ
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt12
Bruce Mayer, PE Engineering-45: Materials of Engineering
Origins of DiElectric ConstantOrigins of DiElectric Constant Consider Two Caps: One in a
Vacuum, and one with a Dielectric Material Between the Plates
Charge on the Vacuum Plates = Q0
Then The Dielectric Slides Between the Plates and DiPoles Align to the E-Field• i.e. The DiElectric Becomes Electrically
POLARIZED – See (b)
Adding the DiElectric Increases the Plate Charge to Q0+Q’
The Dielectric Charges Nearest the Plates Orient Oppositely to the Added Plate Charge – See (c)
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt13
Bruce Mayer, PE Engineering-45: Materials of Engineering
Origins of DiElectric Const cont.Origins of DiElectric Const cont.• Note that Regions Removed from the
Dielectric Surface Do Not Contribute to the ElectroStatic Balance, and thus this region is Electrically NEUTRAL
The Dielectric Surface Charge Tends to Cancel the Vacuum Charge• Hence the Battery Must Supply added
Charge to Bring the interface Regions to Electrical Neutrality– This Occurs withOUT an increase in V;
and to the Q/V quotient (C) increases
Quantify the Increase in D as
PED 0
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt14
Bruce Mayer, PE Engineering-45: Materials of Engineering
Origins of DiElectric Const contOrigins of DiElectric Const cont
• Capital-P Units Should be Coul/sq-m AND dipole-moments/cu-m
PED 0
EP r 10
P vs p Units Analysis
33
22
m
p
m
mCoulP
m
m
m
Coul
m
CoulP
mCouldqp
• Where– P is the DiElectric POLARIZATION
charge, (Coul/sq-m)
In Concept, P → TOTAL DiPole Moment Per Unit-Volume for the Dielectric Material
For Many DiElectrics
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt15
Bruce Mayer, PE Engineering-45: Materials of Engineering
Polarization TypesPolarization Types Electronic
Orientation • Occurs Only in Materials
that have PERMANENT Dipole Moments (atomic or molecular)
• The Field Polarizes the Originally Randomly oriented Dipoles
• The Applied Field Displaces the e- “cloud” relative to the Nucleus, resulting in noncoincident charge centers– Occurs to some Extent
in all Atoms
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt16
Bruce Mayer, PE Engineering-45: Materials of Engineering
Polarization Types cont.Polarization Types cont. Ionic
• The Applied Field Causes Relative Displacement of the Anion and Cation Charge Centers Which Causes a Net Dipole Moment
• The Magnitude of The Dipole Moment for each ion pair:
ii dqp • Where
– di Relative Displacement (m)
Total Polarization for any Material is the Sum of the Three Constituent Types
oietot PPPP
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt17
Bruce Mayer, PE Engineering-45: Materials of Engineering
Frequency DependenceFrequency Dependence AC Electric signals
Are often Applied at High Frequencies to Capacitive Materials
Since Dipole Alignment MUST have some FINITE Relaxation Time, r, Expect some Dielectric Frequency Dependence
• At Frequencies, fr, That exceed 1/r DiPoles CanNOT keep Up with the Applied Field; Reducing the Dielectric Effect
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt18
Bruce Mayer, PE Engineering-45: Materials of Engineering
rr Comparison Comparison Relaxation Frequency, fr,
progression• Fastest → Electronic• Medium → Ionic• Slowest → Orientation
BMayer@ChabotCollege.edu • ENGR-45_Lec-10_Dielectrics.ppt19
Bruce Mayer, PE Engineering-45: Materials of Engineering
All Done for TodayAll Done for Today
ElectricalCapacity
top related