Electric Fields
Required Background Knowledge Section 7.1, pages 320326
Section 7.1 Properties of Electric Charges
1. Law of electric charges, like charges repel and
opposites attract
2. Charges are due to an excess or deficit of negative
charges (electrons). Protons are positively charged
and remain in nucleus
3. Electric charge: the unit of charge (q) is the Coulomb
(C) which represents the “charge” on a group of
electrons (-) or protons (+). The charge on an electron
(qe-) is -1.6 x 10-19C and the charge on a proton (qp+)
is +1.6 x 10-19C. So 1 C=6.25 x 1018 electrons or
protons.
4. Conductors and Insulators
5. Charging of objects by the transfer of electrons
1
Coulomb’s Law
Two similarly charged
masses will exert a force
of electrostatic repulsion
(Fe) on each other.
What parameters would
affect the magnitude of
this force?
- amount of charge (q)
on each
- distance between them (r)
The forces can be analyzed by
standard FBDs and dynamics.
Henri Cavendish studied
electrostatic forces with a
torsion balance.
2
Deriving Coulomb’s Law
Using proportional techniques we can determine:
1
F  q1q2 and F  2
r
so altogether
q1q2
F  2 and then
r
qq
F  k 1 22
r
Henri Cavendish determined the constant of
proportionality (k) to be 9.0 x 109 N·m2/C2
3
Coulomb’s Law
Déjà vu - Gravity
4
Coulomb’s law can be used…
(a) Comparatively using:
F1
q1q2

F2 q1 ' q2 '
where the ' (prime sign) represents either of the two
charges being changed to a secondary value.
Example: #4, page 333
5
(b) Directly using
kq1q2
Fe  2
r
What is the electrostatic force exerted between
the two charges q1 and q2 as shown?
6
Do not forget the Vector Nature of the
electrostatic force.
Example:
Find the total
force on charge
q3 in the charge
distribution given here.
7
Example: Deals with a practical application of
forces on charges (electrons) in a conductor.
The mutual repulsion means that
electrons actually travel in the
outer regions of any conductor.
Find the force on any of the three
electrons as a result of being forced
by the other two.
Further questions - Do 1 – 10, page 333
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