Lesson 6: Coulomb’s Law
Part 1: Introduction to Coulomb’s Law
Electrostatic Forces and the Law of Charges:
Coulomb’s Law allows us to calculate the magnitude of force
between two charges:
Charles-Augustin de Coulomb
Born on June 14, 1736, in Angoulême,
France, Charles-Augustin de Coulomb
studied engineering and plied his trade
with the military before winning
accolades for his work in torsion
balances. He offered pioneering
theories in the force found between
electrical charges, as well as magnetic
attraction and repulsion. The unit of
measurement known as the coulomb is
named in his honor. He died in Paris
on August 23, 1806.
Note: Coulomb’s law gives you the magnitude of the force only. It is up to you to use the law of charges
to determine the direction.
Example: What is the force on the +3.0 µC charge?
-2.0 µC
0.15 m
+3.0 µC
Part 2: Coulomb’s Law and Graphing
There are three common graph shapes that you will encounter in Physics 30
Linear
Parabolic
Inverse Square
Coulomb’s law belongs to a family of formulas called inverse square laws
Universal Gravitation
Coulomb’s Law
In this lesson, we will use curve straightening techniques to determine Coulomb’s constant using
graphical data.
Example: The electrostatic force on a 10 µC charged pith ball is measured as a 10 µC charged rod is
brought near.
Curve straightening: turn the inverse square graph into a linear grap
Part 3: Coulomb’s Law and Vector Addition
If there are more than two charges in a problem, the electrostatic forces combine to produce a net
force. Since Force is a vector, vector addition techniques must be used to combine the forces.
Example: Calculate the force on charge 3
2
1
1.5x10-7 C
-2.3x10-7 C
1.4 m
Example: What is the force on charge 2?
1
- 7.0 C
4.0 m
2
-2.0 C
3
12 m
3.4 C
3
1.7m
-3.5x10-7 C