# Coulomb's Law

```Day 4 Coulomb’s Law
LO: Define and Calculate electrostatic force
Do Now:
AGENDA
Do Now
Notes
Worksheet
HW01
Due Mond
Lightning strikes the ground
and 5 &micro;C of charge are
transferred to/from the
ground. How many electrons
were transferred?
Do Now
Pick up worksheet
Get out Reference Tables
Get out Calculators
Do Now Questions
Coulomb’s Law
Material from Kinetics Books
Charles Augustin de Coulomb
• Born: 14 June 1736
• Died: 23 Aug 1806
• 1780s He Develops
theory on force of
attraction/repulsion
between charged
particles.
Coulombs Law
F k
Q1 Q2
r
2
.
Coulomb’s Law
F k
•
•
•
•
Q1 Q2
r
2
.
F is the force of attraction/repulsion in Newtons
Q1 and Q2 are the charges in Coulombs
k is the constant of proportionality in 9e9 Nm2/C2
r is the distance between the charges in “meters”
Coulomb’s Law
F k
Q1 Q2
r
2
.
• Like Newton’s Universal Law of Gravity
• This is an Inverse Square Law
Sample Problem
• What is the magnitude and direction of the force
the right charge exerts on the left charge?
Sample Problem
• Q1 = 5E-6 C Q2 = 1E-6 C r = 0.05 m
• K = 9E9 Nm2/C2
F k
Q1 Q2
r
2
.
Law of Superposition
• Forces caused by multiple charges can be added
as vectors.
• What is the NET Force on the middle charge?
Solution
•
•
•
•
•
Q1=6e C Q2 = 3e C and Q3 = -5e C
e = 1.6 x 10-19C
k = 8.99 x 109 N.m2/C2
r2,1 = 6 m and r2,3 = 3 m
Strategy
– Find F2, 1 and F2, 3
– Then Determine Net force =  F = F2,1 + F2,3
Applications of Electrostatics
• Attraction/Repulsion of Subatomic
particles
• Inkjet and Laser printers
• Spray painting cars
• Electrostatic Air Filters for home and
industry
• Capacitors – electronic equipment
```