From Last Time

1
From Last Time

• Forces between charges

Electric dipole
2

• Exam 1
• Wed. Feb. 20, 530-7 pm

Covers Chap. 21.5-7, 22-23,25-26 lecture, lab,
discussion, HW
8 1/2 x 11 handwritten note sheet (both sides)
allowed
Students with schedule class conflicts stay
after lecture today to arrange time
Review Group/Quiz (solutions on website). Review
lab question sheets. Review sample exams on
website. HW4 (due Fri. next week) covers exam
material.
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From Last Time

• Forces between charges

Electric dipole
4
Unusual dipolesElectrogenic fish

• Dipole nearby conducting object

Some fish generate charge separation - electric
dipole. Dipole is induced in nearby (conducting)
fish Small changes detected by fish.
5
The idea of electric fields

• EM wave made up of oscillating electric and
  magnetic fields.
• But what is an electric field?
• Electric field is a way to describe the force on
  a charged particle due to other charges around
  it.
• Force charge ? electric field
• The direction of the force is the direction of
  the electric field.

6
Electric field of a point charge
Force on this charge
due to this charge
Force/unit charge Units?
N/C
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Question

• Which vector best represents the electric field
  at the red dot?

A
B
E
C
D
-
-
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Electric field

• Electric field vector defined at every point in
  space.
• Gives magnitude and direction of force on test
  particle

e.g. wind velocity (speed and direction) in
different parts of the country.
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Relationship Between F and E

• Fe qE
• This is valid for a test charge small
• enough that it does not disturb the
• source charge distribution
• If q is positive, F and E are in the same
  direction

Electric field 1Å away from proton
E
(to the right)
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Electric Field Direction

• a) q is positive, F is directed away from q
• b) The direction of E is also away from the
  positive source charge
• c) q is negative, F is directed toward q
• d) E is also toward the negative source charge

11
Superposition with Electric Fields

• At any point P, the total electric field due to a
  group of source charges equals the vector sum of
  electric fields of all the charges

• Find the electric field due to q1, E1
• Find the electric field due to q2, E2
• E E1 E2
• Remember, the fields add as vectors

12
Quick Quiz
Which is the direction of the electric field at
dot? A. Left B. Right C. Up D. Down E.
Zero
Away from positive charge (right)
y

-
x
Net E field is to right.
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Quick Quiz

• In this electric dipole, what is the direction of
  the electric field at point A?
• Up
• Down
• C) Left
• D) Right
• E) Zero

A
x-a
xa
Q
-Q
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Electric field summary

• Electric field -gt will be a force on a charged
  particle.
• This force ( and electric field) can arise from
  electric charges (via Coulombs law)
• But once electric field is known, dont need to
  know the charges that produce it.

15
Calculating dipole electric field

• On the y-axis

y

x
For
-
Since points from - charge to charge
on y-axis of dipole only
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Question electric dipole

• A and B are the same (large) distance from
  dipole. How do the magnitude of the electric
  fields at A and B compare?

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Pictorial representation of E Electric Field
Lines
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Electric field lines

• Local electric field tangent to field line
• Density of lines proportional to electric field
  strength
• Fields lines can only start on charge
• Can only end on - charge.
• Electric field lines can never cross

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Point particles
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Electric field of a dipole
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Electric field of two charges
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Question
How are the charges A and B related?

1. A, B-,
2. A-, B,
3. A, B-,
4. A-, B,
5. A, B-,

23
Hydrogen Atom Example

• The magnitude of the electric force between the
  electron and proton
• Fe kee -e / r2 8.2 x 10-8 N, r 0.53 x
  10-10 m
• The gravitational force between the electron and
  the proton
• Fg Gmemp / r2 3.6 x 10-47 N
• me 9.11 x 10-31 kg, mp 1.67 x 10-27 kg
• G 6.67 x 10-11 N m2/kg2
• Fe/Fg 2 x 1039