A review on Coulombs Law

1
Chapter 23

• A review on Coulombs Law
• Define Electric Field
• Define Electric Field Line
• Examples on How to Calculate for the Electric
  Field
• Charge Particles Experience Force in an Electric
  Field
• Quiz 1/22

2
Coulombs Law, Review

• The formula
• The units (SI)
• Charge coulomb (C)
• Distance meter
• Force Newton
• The constants
• Ke, the Coulomb constant
• ke 8.9876 x 109 N.m2/C2 1/(4p e o)
• eo the permittivity of free space
• e o 8.8542 x 10-12 C2 / N.m2

3
Electric Field the definition of this concept

• The electric force acts through space, i.e., the
  effect is produced even with no physical contact
  between objects.
• One way to offer an explanation (we met this
  situation before, what is that?), as Faraday
  initiated, is the concept of a field in terms of
  electric fields.
• An electric field is postulated to exist in the
  region of space around a charge (of called the
  source charge).
• The strength and direction of that electric field
  at a point in space is then measured by the force
  of the electric field exerts on another charge
  (often called the test charge) at that point.
• Mathematically
• The electric field, , is a vector. The test
  charge, qo, is usually a very small charge
  compared with the source charge, so that its
  existence does not distort the electrical field
  generated by the source charge.
• Unit Newton/Coulomb or N/C.

4
Electric Field Lines, a way to illustrate the
field

• Electric field is introduced to explain the fact
  the electric forces act through space.
• We use a set of specially defined lines to
  illustrate the field. The lines do not exit in
  space, but they should do in your mind, and you
  must be able to see them with your minds eyes.
• Now lets define these electric field lines
• They start from positive charge, end at negative
  charge.
• Their density in space (number of lines in unit
  volume) indicates the field strength.
• The tangent of an electric field line at a given
  point points to the direction of the field at
  that point. Hence no lines can cross.

Field lines of one point positive source charge
in space
Field lines of one point negative source charge
in space
5
More cases on how to draw electric field lines

• Electric dipole the charges are equal and
  opposite.
• The charges are equal and positive.
• Can you draw for the case charges are equal but
  negative?
• A slightly more general case the charges are not
  equal, not the same polarity.

6
An even more general case Electric Field Lines
when the source charge is not seen

• Electric field may not come from static source
  charges. So there is need to just draw electric
  field lines to represent the electric field. In
  the case in the right side figure
• The density of lines through surface A is greater
  than through surface B. So the magnitude of the
  electric field is greater on surface A than B
• The electric field strength (number of lines)
  times the surface area (A or B) is called the
  electric flux.

7
How to calculate the electric field generated by
a point source charge q

• From the definition
• Place the test charge q0. The force on q0 is
  given by Coulombs law
• Then, the electric field will be
• The electric field only depends on the source
  charge, not the test charge.

8
How to calculate electric field generated by many
charges? Superposition with electric field from
each charge.
When the charges are still point charges
Example (23.5, page 653)
from
to
If you do not feel comfortable about the math
here, raise your hand.
9
How to calculate electric field generated by many
charges? Superposition with electric field from
each charge.
When the charges are distributed over volume V
from
to
Again if you do not feel comfortable about the
math here, raise your hand.
10
Examples on how to calculate electric field from
a continuous charge distribution
Example 23.6 (page 656)
11
Examples on how to calculate electric field from
a continuous charge distribution
Example 23.7 (page 657)
12
Examples on how to calculate electric field from
a continuous charge distribution
Example 23.8 (page 658)
13
Charge Particles Experience Force in an Electric
Field

• From the definition of electric field
• We know that charge particles experience force
  in an electric field
• This formula, although a simple transformation
  from the definition, is a lot more useful.

14
Two examples
Example 23.9 (page 662)
Example 23.10 (page 663)
15
Preview sections and homework 1/22, due 2/3

• Preview sections
• Section 24.1
• Section 24.2
• Homework
• Problem 12, page 667.
• Problem 36, page 669.