chapter 2 electrostatics
DESCRIPTION
Chapter 2 Electrostatics. 2.1 The Electrostatic Field. 2.0 New Notations. 2.2 Divergence and Curl of Electrostatic Field. 2.3 Electric Potential. 2.4 Work and Energy in Electrostatics. 2.5 Conductors. 2.0 New Notations. R. 2.1 The Electrostatic Field. 2.1.1 Introduction - PowerPoint PPT PresentationTRANSCRIPT
Chapter 2 Electrostatics
2.0 New Notations
2.1 The Electrostatic Field
2.2 Divergence and Curl of Electrostatic Field
2.3 Electric Potential
2.4 Work and Energy in Electrostatics
2.5 Conductors
2.0 New Notations
R
2.1 The Electrostatic Field
2.1.1 Introduction
2.1.2 Coulomb’s Law
2.1.3 The Electric Field
2.1.4 Continuous Charge Distribution
2.1.1 Introduction
The fundamental problem for electromagnetic to solve is to calculate the interaction of charges in a given configuration. That is, what force do they exert on another charge Q ? The simplest case is that the source charges are stationary.
Principle of Superposition: The interaction between any two charges is completely unaffected by the presence of other charges.
1 2 3F F F F
R
is the force on Q due to iqiF
2.1.2 Coulomb’s Law
the permittivity of free space
The force on a charge Q due to a single point charge q is given by Coulomb`s law
20
212
0 2
1 ˆ ˆ 4
8.85 10
Q qqQ
F R R r r R RR
c
N m
2.1.3 The Electric Field
1 2
1 21 22 20 1 2
31 21 2 32 2 20 1 2 3
1 ˆ ˆ 4
ˆ ˆ ˆ 4
F F F
q Q q QR R
R R
qq qQR R R
R R R
F QE
20 1 1
1 ˆ( )4
Ni
ii
qE P R
R
the electric field of the source charge
2.1.4 Continuous Charge Distributions
( )1
i
nq
i
~ ~~ ( ) dlline
( ) dasurface
( ) dvolume
20 1
1 ˆ4
Ni
ii i
qE P R
R
20
ˆ14
line
RE P dR
20
ˆ14
surface
RE P daR
20
ˆ14
volume
RE P dR
12 2 2 2
32 2 20 2
ˆˆ ˆ( , , )
ˆˆ ˆ
ˆ
1, ,4 [ ]volume
x x i y y j z z kx y z dx dy dz
x x y y z z
R x x i y y j z z k
RR x x y y z z R
R
E x y z
2.1.4 (2)
20
ˆ1( , , ) ( , , )4 volume
RE x y z x y z dx dy dzR
, ,p x y z , ,x y z the source point is atthe test point is atExample:
20
ˆ1 ( , , )4 volume
R R x y z dx dy dzRR
30
1 ( , , )4 volume
R x y z dx dy dzR
2.1.4 (3)
Example 2.1 Find the electric field a distance z above the midpoint of a straight of length 2L, which carries a uniform line charge
20
1ˆ2 ( )cos
4
dxdE z
R
2 2 3 200
2 2 200
2 20
1 2
4 ( )
2
4
1 2
4
L
L
zE dx
z x
z x
z z x
L
z z L
Solution:
(1) z>>L2
0
1 2
4
LE
z
(2) L
0
1 2
4E
z
R
2.2 Divergence and Curl of Electrostatic Fields
2.2.1 Field Lines and Gauss’s Law
2.2.2 The Divergence of E
2.2.3 Application of Gauss’s Law
2.2.4 The Curl of E
A single point charge q, situated at the origin
2.2.1 Fields lines and Gauss’s law
20
1ˆ( )
4
qE r r
r
Because the field falls off like ,the vectors get shorter as I go father away from the origin,and they always point radially outward. This vectors can be connect up the arrows to form the field lines. The magnitude of the field is indicated by the density of the lines.
21 r
strength length of arrow strength density of field line
1.Field lines emanate from a point charge symmetrically in all directions.2.Field lines originate on positive charges and terminate on negative ones.3.They cannot simply stop in midair, though they may extend out to infinity.4.Field lines can never cross.
2.2.1 (2)
Since in this model the fields strength is proportional to the number of lines per unit area, the flux of ( ) is proportional to the the number of field lines passing through any surface . The flux of E through a sphere of radius r is:
22
0 0
1 1ˆ ˆ( ) ( sin )
4
qE da r r d d r q
r
The flux through any surface enclosing the charge is According to the principle of superposition, the total field is the sum of all the individual fields:
0q
E da
01 1
1( ) ( )
n n
i ii i
E da E da q
,
2.2.1 (3)
E
A charge outside the surface would contribute nothing to thetotal flux,since its field lines go in one side and out other.
1
n
ii
E E
Turn integral form into a differential one , by applying the divergence theorem
0 0
( )
1 ( )
1
surface volume
volumeenc
E da E d
Q d
0
1E
Gauss’s law in differential form
2.2.1 (4)
Gauss’s Law in integral form 0
1encE da Q
2.2.2 The Divergence of E
20
ˆ1( ) ( )
4all space
RE r r d
R
,R r r
The r-dependence is contained in
20
ˆ1( ) ( )
4
RE r d
R
From3
2
ˆ( ) 4 ( )
R
RR
Thus
3
0 0
1 14 ( ) ( ) ( )
4E r r r d r
Calculate the divergence of E directly
R
2.2.3 Application of Gauss’s Law
0
1enc
surface
E da Q
surface
E da E da
(b)E is constant over the Gaussian surface
24surface surface
E da E da E r
Thus 2
0
14E r q
20
1ˆ
4
qE r
r
(a) E point radially outward ,as does
Sol:
Example 2.2 Find the field outside a uniformly charged sphere of radius
ad
a
1. Spherical symmetry. Make your Gaussian surface a concentric sphere (Fig 2.18)2. Cylindrical symmetry. Make your Gaussian surface a coaxial cylinder (Fig 2.19)3. Plane symmetry. Use a Gaussian surface a coaxial the surface (Fig 2.20)
2.2.3 (2)
Example 2.3 Find the electric field inside the cylinder which contains charge density as Solution:
kr
0
1enc
surface
E da Q
The enclosed charge is
2 30
2( )( ) 2
3
rencQ d kr r dr d dz k l r dr klr
2E da E da E da E rl
3
0
1 22
3E rl klr
2
0
1ˆ
3E kr r
thus
2.2.3 (3)
(by symmetry)
Example 2.4 An infinite plane carries a uniform surface charge. Find its electric field.
Solution: Draw a ”Gaussian pillbox Apply Gauss’s law to this surface
0
1
surfaceencE da Q
By symmetry, E points away from the planethus, the top and bottom surfaces yields
2E da A E
(the sides contribute nothing)
0
12A E A
0ˆ
2E n
2.2.3 (4)
Example 2.5 Two infinite parallel planes carry equal but opposite uniform charge densities .Find the field in each of the three regions.
2.2.3 (5)
Solution: The field is (σ/ε0 ), and points to the right, between the plane elsewhere it is zero.
0( )2
0( )2
0( )2
0
( )2
0( )2
0
( )2
2.2.4 The Curl of EA point charge at the origin. If we calculate the line integral of this field from some point to some other point b :
b
aE dl
20 0 0
1 1 1
4 4 4
b
a
rb b
a a a br
q q q qE dl dr
r r rr
This integral is independent of path. It depends on the two end points
0E dl
( if ra = rb )Apply by Stokes’ theorem, 0E
In spherical coordinate
ˆ ˆˆ sindl dr r rd r d
20
1
4
qE dl dr
r
a
2.2.4
The principle of superposition
1 2E E E
1 2 1 2( ) ( ) ( ) 0E E E E E
so
must hold for any static charge distribution whatever.
0E
2.3 Electric potential
2.3.1 Introduction to Potential
2.3.2 Comments on Potential
2.3.3 Poisson’s Equation and Laplace’s Equation
2.3.4 The Potential of a Localized Charge Distribution
2.3.5 Electrostatic Boundary Conditions
2.3.1 introduction to potential
Any vector whose curl is zero is equal to the gradient of somescalar. We define a function:
Where is some standard reference point ; V depends only on the point P. V is called the electric potential.
The fundamental theorem for gradients
( ) ( ) ( )b
aV b V a V dl
( )b b
a aV dl E dl
so
( )
pV p E dl
( ) ( )b a b
aV b V a E dl E dl E dl
E V
2.3.2 Comments on potential
(1)The name Potential is not potential energy
F qE q V
U F X
V : Joule/coulomb U : Joule
(2)Advantage of the potential formulation V is a scalar function, but E is a vector quantity
( )V r ˆ ˆ ˆx y zE E x E y E z
If you know V, you can easily get E: .E V
0E
so
2.3.2 (2)
, ,x y zE E E are not independent functions
, ,y yx xz zE EE EE E
y x y z z x
2
y z x z y x z x y x z y x y z y x z z x x zE E E E E E E E
1 3
2 1 3
Adding a constant to V will not affect the potential difference between two point:
(3)The reference point Changing the reference point amounts to adds a constant tothe potential
( ) ( )p p
V p E dl E dl E dl K V p
(Where K is a constant)
2.3.2 (3)
( ) ( ) ( ) ( )V b V a V b V a
Since the derivative of a constant is zero:For the different V, the field E remains the same.
Ordinarily we set
V V
( ) 0V
2.3.2 (4)
(4)Potential obeys the superposition principle
1 2F F F F QE
1 2E E E
Dividing through by Q
,
Integrating from the common reference point to p ,
1 2V V V (5)Unit of potential Volt=Joule/Coulomb
:
: :qV
XF X
Vq
F newton F x Joule
F qE q V
Joule/Coulomb
solution:
0 0
1 14 4
rr q q
V r E dr r
for r>R:
for r<R:
0inE
201
4ˆq
out rE r
0
14
qV r V R
R 0
14
qV R
R
0inE
2.3.2 (5)
Example 2.6 Find the potential inside and outside a spherical shell of radius R, which carries a uniform surface charge (the total charge is q).
_
2.3.3 Poisson’s Eq. & Laplace’s Eq.
0
2E V V
E V
Laplace’s eq.0
Poisson’s Eq.0
2V
2 0V
pR r r
i i pR r r 0 1
1( )4
ni
ii
qV P
R
2.3.4 The Potential of a Localized Charge Distribution
• Potential for a point charge
• Potential for a collection of charge
E V r
V V Edr 0V
20 0 0
1 1 1( )4 4 4
rr q q q
V r drr rr
R
Ri
0
1( )4
qV P
R
• Potential of a continuous distribution for volume charge for a line charge for a surface charge
0
1( )4
V P dR
0
1( )4
V P dR
0
1( )4
V P daR
q d q d q da
20
1 ˆ( )4
rE P dR
20
ˆ14
( ) r dR
E P
20
ˆ14
( ) r daR
E P
• Corresponding electric field 2
ˆ1 RR R
[ ]
2.3.4 (2)
2.3.4 (3)Example 2.7 Find the potential of a uniformly charged spherical shell of radius R.
0
1( ) ,
4V r da
r
r2 2 2 2 cosr R z Rz
2
2 2
2
2 20
2 2 2
0
2 2 2 2
2 2
sin4 ( )
2 cos
sin2
2 cos
12 2 cos
22 2
2( ) ( )
( )
( )
[ ]
R d dV z
R z Rz
R dR z Rz
R R z RzRz
RR z Rz R z Rz
zR
R z R zz
Solution:
2
0 0
0 0
( ) [( ) ( )] ,2
( ) [( ) ( )] ,2
R RV z R z z R outsidez z
R RV z R z R z insidez
2.3.4 (4)
2.3.5 Electrostatic Boundary Condition
Electrostatic problem
The above equations are differential or integral. For a unique solution, we need boundary conditions. (e.q. , V()=0 )(boundary value problem. Dynamics: initial value problem.)
•Superposition
•Coulomb law
2.3.5 (2)
B.C. at surface with charge
0 0
encQ A
surface
E da
above belowE A E A
0above belowE E
=
∥:
0E dl
0E
E‖ above= E‖ below
+∥0
ˆabove belowE E n
:
Potential B.C.
2.3.5 (3)
00
babove below a
abV V E d
0ˆabove belowE E n
E V
ˆV V nn
0above belown nV V
above belowV V
2.4 Work and Energy in Electrostatics 2.4.1 The Work Done in Moving a Charge
2.4.2 The Energy of a Point Charge Distribution
2.4.3 The Energy of a Continuous Charge Distribution
2.4.4 Comments on Electrostatic Energy
2.4.1 The Work Done in Moving a charge
A test charge Q feels a force Q E
(from the stationary source charges).
To move this test charge, we have to apply a force
conservative
F QE
The total work we do is
b b
a aW F d Q E d Q V b V a
WV b V aQ
So, bring a charge from to P, the work we do is
( ) ( ) ( )W Q V P V QV P
( ) 0V
2.4.2 The Energy of a Point Charge Distribution
It takes no work to bring in first charges
1 0W
Work needed to bring in q2 is :
1 12 2 2
0 12 0 12
1 1[ ] ( )4 4
q qW q q
R R
1 2 1 23 3 3
0 13 0 23 0 13 23
1 1 1[ ] ( )4 4 4
q q q qW q q
R R R R
Work needed to bring in q3 is :
Work needed to bring in q4 is :
31 24 4
0 14 24 34
1 [ ]4
qq qW q
R R R
for q1
2.4.2
Total work
W=W1+ W2+ W3 +W4
1 3 2 3 3 41 2 1 4 2 4
0 12 13 23 14 24 34
14
q q q q q qq q q q q qR R R R R R
01
14
nj
iijj
qV P
R
j i
0 1 1
14
n ni j
iji j
q q
R
ij jiR R
01 1
1 12 4
n nj
iiji j
R
0 1 1
18
n ni j
iji j
q q
R
1
12
n
i ii
W q V P
Dose not include the first charge
j i j i
2.4.3 The Energy of a Continuous Charge Distribution
0ρ E
q d
Volume charge density
0 0τ2 2
E Vd EV d E V d
surface
VE da
20 ( )2
surface volume
VE da E d
F.T.forE
1τ
2W Vd
surface 20
τ2
all space
W E d
EV E V E V
2.4.3
0
1
4
qV
R24q R Sol.1 :
2
0 0
1 1 1 1 1
2 2 2 4 8
q q qW Vd Vda da
A R R
Example 2.8 Find the energy of a uniformly charged spherical shell of total charge q and radius R
Sol.2 : 20
1ˆ
4
qE r
r
22
2 40(4 )
qE
r
2
2 20 02 4
1sin
2 2 (4 )
outer
spaceo
qW E d r d d dr
r
2 2
2 200
1 12 2
832 R
q qdr
Rr
2.4.4 Comments on Electrostatic Energy
1 A perplexing inconsistent
20 Energy 02 all space
W E d
1
1( )
2
n
i ii
W q V P
0 1 1
1
4
n ni j
iji jj i
q qW
r
or
Only for point charge 2
1 20 12
1, 0
4
qq q q q W
r
1 2( )q q
Energy 0 or 0
and are attractive
20 τ2 all space
W E d
is more complete
the energy of a point charge itself,
22
202 4 2000
1( ) sin θ θ φ
8 π2(4 )
qqW r d d dr dr
r r
1
1
2
n
i ii
W q V P
, ( )iV P does not include iq
1
2W Vd is the full potential
2.4.4 (2)
There is no distinction for a continuous distribution,because 0
( ) 0d
d p
( )V P,
(2)Where is the energy stored? In charge or in field ?
Both are fine in ES. But,it is useful to regard the energy as being stored in the field at a density
2
0 2
E Energy per unit volume
2.4.4 (3)
(3)The superposition principle,not for ES energy
201 12
W E d 20
2 2 τ2
W E d
20
1 2( )2totW E E d
1 2 0 1 2( )W W E E d
2 201 2 1 2( 2 )
2E E E E d
2.5 Conductors
2.5.1 Basic Properties of Conductors
2.5.2 Induced Charges
2.5.3 The Surface Charge on a Conductor; the Force on a Surface Charge
2.5.4 Capacitors
2.5.1 Basic Properties of Conductors
e are free to move in a conductor
E
otherwise, the free charges that produce
will move to make 0E
inside a conductor
( ) ( ) 0
( ) ( )
b
aV b V a E dl
V b V a
0E
0 0E
0E
inside a conductor(1)
(2) ρ = 0 inside a conductor
(3) Any net charge resides on the surface
(4) V is constant, throughout a conductor.
(5) is perpendicular to the surface, just outside a conductor. Otherwise, will move the free charge to make in terms of energy, free charges staying on the surface have a minimum energy.
uniform
0inE 0inW
2
0
1
8
qEnergy
R
uniform
0inE 0Win
2
0
3
20
qEnergy
R
2.5.1 (2)
E
E
0E
0
0in
enc induced
induecd
E
Q q q
q q
Example : A point charge q at the center of a spherical conducting shell. How much induced charge will accumulate there?
charge conservation2 24 4b ab a
21
4bq
b
2
0
4
in
a
E
a q
21
4aq
a
q induced
Solution :
2.5.1 (3)
2.5.2 Induced Charge
Example 2.9 Within the cavity is a charge +q. What is the field outside the sphere?
-q distributes to shield q and to make 0inE i.e., surfaceV constant
from charge conservation and symmetry.+q uniformly distributes at the surface
20
1ˆ( )
4
qE P r
r
a,b are arbitrary chosen
0
0b
a
E dl
E dl
0incavityE
a Faraday cage can shield out stray E
2.5.3 The Surface Charge on a Conductor
0ˆabove belowE E n
0 or 0in belowE E
0ˆaboveV E n
0
V
n
if we know V, we can get
or
why the average?
0
ˆ2above otherE E k
0
ˆ2below otherE E k
1( )
2other above below averageE E E E 0
electrostatic pressure2
20
02 2P E
In case of a conductor2
0
1ˆ
2 2f E nabove
2.5.3
1( )
2average above belowf E E E
Force on a surface charge,
2.5.4 Capacitors
Consider 2 conductors (Fig 2.53)
The potential difference
V V V E dl
20
ˆ1
4
rE d
r
Define the ratio between Q and V to be capacitanceQ
CV
a geometrical quantity
in mks 1 farad(F)= 1 Coulomb / volt
inconveniently large ; 6
12
10 :
10 :
F microfarad
F picofarad
Q E V ��������������
double double double double
(V is constant.)
Example 2.10 Find the capacitance of a “parallel-plate capacitor”?
0 0
QE
A
0
QV E d d
A
0AC
d
2.5.4 (2)
+Q
-QAd
Solution:
Example 2.11 Find capacitance of two concentric spherical shellswith radii a and b .
20
1ˆ
4
QE r
r
20 0
1 1 1
4 4
a a
b b
Q QV E dl dr
a br
04Q ab
CV b a
-Q+Q
2.5.4 (3)
Solution:
The work to charge up a capacitor
( )q
dW Vdq dqC
22
0
1 1 1( )
2 2 2
Q QqW dq QV CV
C C
2.5.4 (4)