Physics 2102 Physics 2102 Lecture: 03 TUE 26 JAN Lecture: 03 TUE 26 JAN
Electric Fields IIElectric Fields IIMichael Faraday
(1791-1867)
Physics 2102
Jonathan Dowling
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What Are We Going to What Are We Going to Learn?Learn?
A Road MapA Road Map• Electric charge
- Electric force on other electric charges- Electric field, and electric potential
• Moving electric charges : current • Electronic circuit components: batteries,
resistors, capacitors• Electric currents - Magnetic field
- Magnetic force on moving charges• Time-varying magnetic field Electric Field• More circuit components: inductors. • Electromagnetic waves - light waves• Geometrical Optics (light rays). • Physical optics (light waves)
2q−12F1q+ 21F
12r
Coulomb’s LawCoulomb’s Law
212
2112
||||||r
qqkF =
2
212
00
1085.8with 41
mNCk −×== επε
2
291099.8
CmN×k =
For Charges in aVacuum
Often, we write k as:
E-Field is E-Force Divided by E-E-Field is E-Force Divided by E-ChargeCharge
Definition of Electric Field:
€
r E =
r F q
€
|r F 12 |= k | q1 | | q2 |
r122
+q1 –q2
€
r F 12
P1 P2
€
|r E 12 |= k | q2 |
r122
–q2
€
r E 12
P1 P2
Units: F = [N] = [Newton] ; E = [N/C] = [Newton/Coulomb]
E-Force
on Charg
eE-Field at
Point
Force on a Charge in Electric Force on a Charge in Electric FieldField
Definition of Electric Field:
€
r E =
r F q
Force on Charge Due to Electric Field:
€
r F = q
r E
Force on a Charge in Electric Force on a Charge in Electric FieldField
E
E
Positive Charge Force in Same Direction as E-Field (Follows)
Negative Charge Force in Opposite Direction as E-Field (Opposes)
+ + + + + + + + +
+ + + + + + + + +
– – – – – – – – – –
– – – – – – – – – –
Electric Dipole in a Uniform Electric Dipole in a Uniform FieldField
• Net force on dipole = 0; center of mass stays where it is.
• Net TORQUE : INTO page. Dipole rotates to line up in direction of E.
• | | = 2(qE)(d/2)(sin ) = (qd)(E)sin = |p| E sin = |p x E|
• The dipole tends to “align” itself with the field lines.
• What happens if the field is NOT UNIFORM??
Distance Between Charges = d
Electric Charges and Electric Charges and FieldsFields
First: Given Electric Charges, We Calculate the Electric Field Using E=kqr/r3.
Example: the Electric Field Produced By a Single Charge, or by a Dipole:
Charge Produces E-Field
E-Field Then Produces Force on Another Charge
Second: Given an Electric Field, We Calculate the Forces on Other Charges Using F=qE
Examples: Forces on a Single Charge When Immersed in the Field of a Dipole, Torque on a Dipole When Immersed in an Uniform Electric Field.
Continuous Charge Continuous Charge DistributionDistribution
• Thus Far, We Have Only Dealt With Discrete, Point Charges.
• Imagine Instead That a Charge q Is Smeared Out Over A:– LINE
– AREA
– VOLUME
• How to Compute the Electric Field E? Calculus!!!
q
q
q
q
Charge DensityCharge Density
• Useful idea: charge density• Line of charge:
charge per unit length = • Sheet of charge:
charge per unit area = • Volume of charge:
charge per unit volume = r
= q/L
= q/A
r = q/V
Computing Electric FieldComputing Electric Field of Continuous Charge of Continuous Charge
DistributionDistribution• Approach: Divide the Continuous
Charge Distribution Into Infinitesimally Small Differential Elements
• Treat Each Element As a POINT Charge & Compute Its Electric Field
• Sum (Integrate) Over All Elements• Always Look for Symmetry to
Simplify Calculation!
dq
dq = dLdq = dSdq = r dV
Differential Form of Differential Form of Coulomb’s LawCoulomb’s Law
q2
€
r E 12
P1 P2
E-Field at
Point
dq2
€
dr E 12
P1
Differential dE-Field at Point
€
dr E 12 =
k dq2
r122
€
r E 12 =
k q2
r122
Field on Bisector of Charged RodField on Bisector of Charged Rod• Uniform line of charge
+q spread over length L
• What is the direction of the electric field at a point P on the perpendicular bisector?
(a) Field is 0.(b) Along +y(c) Along +x•Choose symmetrically
located elements of length dq = dx
•x components of E cancel
q
L
a
P
o
y
x
dx
€
ds E
dx
€
dr E
€
↑E
Line of Charge: QuantitativeLine of Charge: Quantitative• Uniform line of charge,
length L, total charge q• Compute explicitly the
magnitude of E at point P on perpendicular bisector
• Showed earlier that the net field at P is in the y direction — let’s now compute this!
q
L
a
P
o
y
x
Line Of Charge: Field on bisectorLine Of Charge: Field on bisector Distance hypotenuse: r = a2 + x2( )
1/2
dE =k(d)r2
Lq=Charge per unit length:
2/322 )()(cos
xaadxkdEdEy +
== q
L
a
P
ox
dE
dx
r
cos =ar=
a(a2 + x2 )1/2
AdjacentOverHypotenuse
[C/m]
Line Of Charge: Field on Line Of Charge: Field on bisectorbisector
∫− +
=2/
2/2/322 )(
L
Ly xa
dxakE λ
Point Charge Limit: L << a
€
=2kλL
a 4a2 + L2
2/
2/222
L
Laxaxak
−⎥⎦⎤
⎢⎣⎡
+=
€
Ey = 2kλLa 4a2 + L2
≅2kλ
a
Line Charge Limit: L >> a
€
Ey = 2kλLa 4a2 + L2
≅kλLa2 = kq
a2
Integrate: Trig Substitution!
Coulomb’sLaw!
€
Nm2
C1m
Cm
⎡ ⎣ ⎢
⎤ ⎦ ⎥= N
m ⎡ ⎣ ⎢
⎤ ⎦ ⎥
Units Check!
Binomial Approximation from Taylor Series:x<<1
€
2kλLa 4a2 + L2
= kλLa2 1+ L
2a ⎛ ⎝ ⎜
⎞ ⎠ ⎟2 ⎡
⎣ ⎢
⎤
⎦ ⎥
−1/ 2
≅kλLa2 1− 1
2L2a ⎛ ⎝ ⎜
⎞ ⎠ ⎟2 ⎡
⎣ ⎢
⎤
⎦ ⎥≅ kλL
a2 ; (L << a)
2kλLa 4a2 + L2
= 2kλLaL
1+ 2aL ⎛ ⎝ ⎜
⎞ ⎠ ⎟2 ⎡
⎣ ⎢
⎤
⎦ ⎥
−1/ 2
≅2kλ
a1− 1
22aL ⎛ ⎝ ⎜
⎞ ⎠ ⎟2 ⎡
⎣ ⎢
⎤
⎦ ⎥≅ 2kλ
a ; (L >> a)
€
1± x( )n≅1± nx
Example — Arc of Charge: Example — Arc of Charge: QuantitativeQuantitative
• Figure shows a uniformly charged rod of charge -Q bent into a circular arc of radius R, centered at (0,0).
• Compute the direction & magnitude of E at the origin.
y
x450
x
y
dQ = Rdd coscos 2R
kdQdEdEx ==
∫∫ ==2/
0
2/
02 coscos)( ππ
θθλθθλ dRk
RRdkEx
RkEx=
RkEy=
RkE 2= = 2Q/(πR)
E–Q
E = Ex2 + Ey
2
Example : Field on Axis of Example : Field on Axis of Charged DiskCharged Disk
• A uniformly charged circular disk (with positive charge)
• What is the direction of E at point P on the axis?
(a) Field is 0(b) Along +z(c) Somewhere in the x-y plane
P
y
x
z
Example : Arc of ChargeExample : Arc of Charge• Figure shows a uniformly
charged rod of charge –Q bent into a circular arc of radius R, centered at (0,0).
• What is the direction of the electric field at the origin?
(a) Field is 0.(b) Along +y(c) Along -y
y
x
• Choose symmetric elements• x components cancel
SummarySummary• The electric field produced by a system of charges at any point in space is the force per unit charge they produce at that point. • We can draw field lines to visualize the electric field produced by electric charges. • Electric field of a point charge: E=kq/r2
• Electric field of a dipole: E~kp/r3
• An electric dipole in an electric field rotates to align itself with the field. • Use CALCULUS to find E-field from a continuous charge distribution.
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