15.2. Coulomb’s Law http://www.ck12.org
15.2 Coulomb’s Law
Objectives
The student will:
- Understand Coulomb’s law
- Understand how to solve problems using Coulomb’s law
Vocabulary
- Coulomb’s constant:A constant of proportionality equal tok= 8. 99 × 109 N·m
2
C^2. - Coulomb’s law: The force between two charges is directly proportional to the product of the charges and
inversely proportional to the square of the distance between the two charges.
Introduction
We know that opposite electric charges attract each other, and like electric charges repel each other. The question
is, what is the amount of that force? Recall that Newton’s universal law of gravity described how masses attract and
how to calculate the force of attraction,F=Gm^1 rm 22.
In 1785, fifty-eight years after the death of Isaac Newton, the French naval engineer and physicist Charles Augustin
de Coulomb (1736-1806),Figure15.12, published a work stating the force between two charged particles was based
on a similar law.
Coulomb’s law: The force between two charges is directly proportional to the product of the charges and
inversely proportional to the square of the distance between the two charges.The amount of charge is measured in a new unit, called the coulomb after the physicist himself. It is abbreviatedC
(capitalized because it is based on his name). Using units of coulombs, the force of the charge is expressed as
F=k
q 1 q 2
r^2,
whereq 1 andq 2 are two charges measured in units of coulombs,k= 8. 99 × 109 N·m
2
C^2 is a constant of proportionality
calledCoulomb’s constant,andris the distance between the two charges.
The coulomb is a fairly big charge. Only a small fraction of a coulomb will be in most static electricity, like a
balloon sticking to a wall. A high power light bulb will pull around 1 coulomb of charge per second. Like other
metric units, coulombs are also measured in smaller units, like the millicoulomb mC= 10 −^3 C, or the microcoulomb
̄C= 10 −^6 C.