Physics 2220 Chapter 23 Electric Fields

1
Physics 2220Chapter 23 Electric Fields

• Quiz A proton (mass 1.67 x 10-27 kg)
  experiences a force of 9.22 x 10-8 N.
• What acceleration does it undergo as a result of
  the force?
• How long does it take to move 1 nm?
• How fast is it going after this time?

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Physics 2220Chapter 23 Electric Fields

• Quiz A proton (mass 1.66 x 10-27 kg)
  experiences a force of 9.22 x 10-8 N.
• What acceleration does it undergo as a result of
  the force?
• How long does it take to move 1 nm?
• 3) How fast is it going after this time?

a F/ m 9.22 x 10-8 N/(1.66 x 10-27 kg) 5.6
x1019 m/s/s.
t v2d/a v2(1x10-9)/5.6x1019 6.02 fs
v at (5.6 x1019 m/s/s)(6.02 x 10-15 s)
1.06 x 105 m/s 332 km/s
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Physics 2220Chapter 23 Electric Fields

• What makes sticky?

Gravity? --NO!
Another force much stronger
The Electromagnetic Force
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Physics 2220Chapter 23 Electric Fields

• What makes sticky?

-Ze
-Ze
attraction
Ze
Ze
repulsion
attraction
Attraction between the positively charged nucleus
and the negatively charged electrons of the
neighboring atom is stronger than the repulsion
of its like-charged nucleus.
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Physics 2220Chapter 23 Electric Fields

• 1' Lesson
• The electric charge q, in both positive and
  negative forms, is the agent of the electro-
  magnetic force.
• Coulombs law describes the force between two
  charges.
• The Electric Field is defined as E F/ q
• The smallest unit of charge is the charge on an
  electron or proton, e 1.602 x 10 -19 Coulomb

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Physics 2220Chapter 23 Electric Fields

• Electric Charge
• A fundamental property of matter
• Charge is of two types positive and negative
  
• Like charges repel unlike charges attract

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Physics 2220Chapter 23 Electric Fields

• Electric Charge

q1

-
Neutral

Repel
Attract
No force
q2
-
Repel
Attract
No force
No force
No force
No force
Neutral
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Physics 2220Chapter 23 Electric Fields

• Charge
• Property of material objects only.
• Conserved neutral may be due to equal numbers
  of and -.
• Quantized q Ne, e /- 1.602 x10-19C
• Moves around in conductors
• Is isolated in insulators.
• Can be induced in extending material objects.

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Physics 2220Chapter 23 Electric Fields

• Conductors

Charges move about easily in conductors.
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Physics 2220Chapter 23 Electric Fields

• Insulators

Charges are isolated in insulators.
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Physics 2220Chapter 23 Electric Fields

• Semiconductors and Semi-Insulators

Charges can move if they can be pried loose
from the atoms.
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Physics 2220Chapter 23 Electric Fields

• Electric Induction

Charging by induction
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Physics 2220Chapter 23 Electric Fields

• Thought Quiz
• How can a charged balloon stick to an uncharged
  wall?

Induction
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Physics 2220Chapter 23 Electric Fields

• Coulombs Law
• F r ke q1 q2 / r 2
• ke 8.9875 x 10 9 N?m 2 /C2
• r r/r

almost 9, times ten to the ninth, nearly 9
billion
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Physics 2220Chapter 23 Electric Fields

• Coulombs Law

r
F r ke q1 q2 /r 2
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Physics 2220Chapter 23 Electric Fields

• Practice
• What is the force acting between two charges of
  2.0 µC and 3.0 µC, separated by a distance of
  0.5 m?

2.0 µC
-3.0 µC
0.5 m
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Physics 2220Chapter 23 Electric Fields

• Practice
• What is the force acting between a charge of 1.0
  µC and two charges of 2.0 µC and 3.0 µC,
  separated by a distance of 0.5 m?

Must use vector addition to find the solution.
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Physics 2220Chapter 23 Electric Fields

• Coulombs Law - Superposition

?13
r13
?23
F13 r13 ke q1q3/r13 2
r23
F23 r23 ke q2q3/r23 2
F F13 F23
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Physics 2220Chapter 23 Electric Fields

• Coulombs Law Superposition
• F F13 F23
• Fx Fx13 Fx23 Fx ke q1q3 /r13 2sin ?13
  ke q2q3 /r23 2sin ?23 Fy Fy13 Fy23 , etc

Add separate components separately!
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Physics 2220Chapter 23 Electric Fields

• Practice
• What is the force acting between a charge of 1.0
  µC and two charges of 2.0 µC and 3.0 µC,
  separated by a distance of 0.5 m?

Must use vector addition to find the solution.
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Physics 2220Chapter 23 Electric Fields

• The Electric (Force) Field
• The Electric Field is defined to be the
  electrostatic force on a (small) test charge
  divided by the value of the test charge
• E F/qo

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Physics 2220Chapter 23 Electric Fields

• The Electric Field
• The Electric Field due to a point charge q is
  E r ke q/r 2
• The total Electric Field due to a collection of
  charges is the (vector) sum of each charge
  independently
• E S Ei , Ex S Exi , etc

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Physics 2220Chapter 23 Electric Fields

• The Electric Field due to a continuous
  distribution of charge
• Charge Density
• Volume Charge Density ? dQ/dV
• Surface Charge Density s dQ/dA
• Linear Charge Density ? dQ/dx
• Cumulative Electric Field
• E ke ? r dq/r 2
• Ex ke ? cos a dq/r 2 Ey ke ? sin a dq/r 2

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Physics 2220Chapter 23 Electric Fields

• The Electric Field due to a continuous linear
  distribution of charge
• Ex ke ? (R/r) ?dy/r 2 Ex 0

dy
R
?
r
dEx
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Physics 2220Chapter 23 Electric Fields

• The Electric Field due to a circular sheet of
  charge
• Ez ke ? (z/r) (s/ r 2)RdRdf ER 0 Ef
  0

dA RdRdf
s
r
R
z
dEz
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Physics 2220Chapter 23 Electric Fields

• The Electric Field due to a sphere of uniform
  charge density
• Ez ke ? (z/r) (?/ r 2)R2 dRsin?d?df

dV R2 dRsin?d?df
R
r
?
dEz
z
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Physics 2220Chapter 23 Electric Fields

• The Electric Field Lines
• The locus of the direction of the electric field
  at every point. A way of visualizing field.
• Begins on charge ends on charge or at 8
• Number of lines/area proportional to field
• Lines do not cross

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Physics 2220Chapter 23 Electric Fields

• Motion in Electric Field

Motion of charged particles in uniform electric
field
a F/m Eq/m
ax1 Ex q1 /m
ax2 Ex q2 /m
E
ax3 Ex (-q1 )/m
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Physics 2220Chapter 23 Electric Fields

• Electrons in a CRT

a F/m E(-e/me) e/m - 1.60x10 -19 /9.11x10
-31 C/kg -1.76 x 10 -11 C/kg
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Physics 2220Chapter 23 Electric Fields

• Summary
• The electric charge q, in both positive and
  negative forms, is the agent of the electro-
  magnetic force.
• Coulombs law describes the force between two
  charges. F r ke q1 q2 / r 2
• The smallest unit of charge is the charge on an
  electron or proton, e 1.602 x 10-19 Coulomb

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Physics 2220Chapter 23 Electric Fields

• Summary
• The electric filed is defined as E F/qo.
• The electric field due to distributed charges is
  their integrated effect.
• Electric field lines are a means to visualize the
  electric field.
• Motion of a particle of mass m and charge q in
  an electric field is given by Newtons second law
  with the force F E q.

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Physics 2220Chapter 23 Electric Fields