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Chapter 6 Electrostatic Boundary-Value Problems

Lecture by Qiliang Li

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§6.1 Introduction

The goal is to determine electric field E• We need to know the charge distribution– Use Coulomb’s law

or – Use Gauss’s law

• Or We need to know the potential V

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§6.1 Introduction

• However, we usually don’t know the charge distribution or potential profile inside the medium.

• In most cases, we can observe or measure the electrostatic charge or potential at some boundaries

We can determine the electric field E by using the electrostatic boundary conditions

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§6.2 Poisson’s and Laplace’s Equations

• Poisson’s and Laplace’s equations can be derived from Gauss’s law

Or

This is Poisson’s Eq.If , it becomes Laplace’s Eq.

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Continue 6.2

The Laplace’s Eq. in different coordinates:

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§6.3 Uniqueness Theorem

Uniqueness Theorem: If a solution to Laplace’s equation can be found that satisfies the boundary conditions, then the solution is unique.

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§6.4 General Procedures for Solving Poisson’s or Laplace’s Equations

1. Solve L’s Eq. or P’s Eq. using (a) direct integration when V is a function of one variable or (b) separation of variables if otherwise. solution with constants to be determined

2. Apply BCs to determine V3. V E D 4. Find Q induced on conductor , where

C=Q/V R

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Example 6.1 (page 219)

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Example 6.2

Details in P222

Example 6.3

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§6.5 Resistance and Capacitance

R’s definition (for all cross sections)

Procedure to calculate R:1. Choose a suitable coordinate system2. Assume V0 as the potential difference b/w two ends

3. Solve to obtain V, find E from , then find I from 4. Finally, obtain

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(continue)

Capacitance is the ratio of magnitude of the charge on one of the plates to the potential difference between them.

Two methods to find C:1. Assuming Q and determining V in terms of Q

(involving Gauss’s law)2. Assuming V and determining Q interms of V

(involving solving Laplace’s Eq.)

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First methods QV, procedure:1. Choose a suitable coordinate system2. Let the two conductor plates carry Q and –Q3. Determine E by using Gauss’s law and find V

from . (Negative sign can be ignored. We are interested at absolute value of V)

4. Finally, obtain C from Q/V

(continue)

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(continue)

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(continue)

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(continue)

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(continue)

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Example 6.8: a metal bar of conductivity σ is bent to form a flat 90o sector of inner radius a, outer radius b, and thickness t as shown in Figure 6.17. Show that (a) the resistance of the bar between the vertical curved surfaces at ρ=a and ρ=b is (b) the resistance between

the two horizontal surface

at z=0 and z=t is

(continue)

z

x

yba

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(a) Use Laplace’s Eq. in cylindrical coordinate system:

So, ,, , ,,

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(b) Use Laplace’s Eq. in cylindrical coordinate system:

So, ,, , ,, R=V0/I=?

(please also use conventional method for (b))

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Example 6.9: a coaxial cable contains an insulating material of conductivity σ. If the radius of the central wire is a and that of the sheath is b, show that the conductance of the cable per unit length is Solve:Let V(ρ=a)=0 and V(ρ=a)=V0

, R per unit length =V/I/LG=1/R= …

ab

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Example 6.10: find the chargein shells and the capacitance.Solve:Use Laplace’s Eq. (spherical)And BCs, So E=-dV/dr ar, Q=?, C=Q/V0

a

b

GND

V0=100V

Єr

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Example 6.11: assuming V and finding Q to derive Eq. (6.22): Solve:From Laplace’s Eq.:From BCs: V(0)=0 and V(x=d)=V0

So, , the surface charge: S, so:

x

d

0

V0

0

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Example 6.12: determine the capacitance of each of th Єr1=4 e capacitors in Figure 6.20. Take Єr2=6, d=5mm, S=30 cm2.

Solve:(do it by yourself)

Єr1

Єr2

Єr1 Єr2

d/2

d/2

w/2 w/2

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Example 6.13: A cylindrical capacitor has radii a=1cm and b=2.5cm. If the space between the plates is filled with and inhomogeneous dielectric with Єr=(10+ρ)/ρ, where ρ is in centimeters, find the capacitance per meter of the capacitor.

Solve:

Use Eq. 6.27a, set the inner shell with +Q and outer shell with –Q. C=Q/V=…=434.6 pF/m

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§6.6 Method of Images

• The method of images is introduced by Lord Kelvin to determine V, E and D, avoiding Poison’s Eq.

• Image Theory states that a given charge configuration above an infinite grounded perfect conducting plane may be replaced by the charge configuration itself, its image, and an equipotential surface in place of the conducting plane.

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Conducting planegrounded

Image chargeSo that the potential at the plane position = 0 V

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In applying the image method, two conditions must always be satisfied:1. The image charge(s) must be located in the

conducting region (satify Poisson’s Eq.)2. The image charge(s) must be located such that on the

conducting surface(s) the potential is zero or constant

Perfect conducting surface grounded

Equipotential V = 0

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A. A point charge above a grounded conducting plance