5.2 The Electric Force
Coulomb’s Law
Charles Coulomb (1736 – 1806) using an apparatus very much like the one Henry
Cavendish used to measure the gravitational constant determined the relationship
for the electric force between two charged objects.
Coulomb found out that the force between two charges varied directly with each
charge and inversely with the square of the distance between the charges.
F =
kQ1Q2
r2
Where Coulomb’s Constant: k = 8.988 x 109 N2m2/C2
p. 167 -168
5.2 The Electric Force
The Coulomb and the Elementary Charge
The charge on an electron or a proton is called the elementary charge. The
elementary charge (determined by Robert Millikan) has a value of :
e = 1.602 x 10-19 C
And since 1 C of charge is the charge on 6.2422 x 1018 electrons:
1
1 e = 1.602 x 10-19 C =
6.2422 x 1018 C
The elementary
charge is just the
reciprocal of the # of
electrons in 1
Coulomb of charge.
p. 168
5.2 The Electric Force
The electric force like all forces are vector quantities. When determining the
net electric force you need to always consider the direction of the force. Unlike
gravitational force which has only one direction towards each mass, electric
force can either be an attractive force or a repulsive force.
The net force will the vector addition of all the electric forces acting on the
charges. For example consider the net force on the right charge due to the
other 2 charges. All three charges are identical.
F1
F2
F1
=
Fnet
Fnet
F2
p. 170 - 173
5.2 The Electric Force
Key Questions
In this section, you should understand how to solve the following key questions.
Page 172 – Practice Problem 5.2.2:
Page 175 - 176 – Review 5.2 #1,2,4,6,7, & 8