http://www.ck12.org Chapter 15. Electrostatics
FIGURE 15.12
Augustin de CoulombThe smallest unit of electric charge is the charge on electrons and protons. The charge of an electron is approximately
qe−=− 1. 60 × 10 −^19 C and the charge on a proton has the same magnitude, but is positive.
http://www.youtube.com/watch?v=b49DF-qN4n8
Illustrative Example 1
a. What is the magnitude and direction of the electrostatic force between two electrons separated by a distance of
1.00 centimeters?
Answer:For Coulomb’s law,rmust be in meters 1.00 cm= 0 .0100 m= 1. 00 × 10 −^2 m
Since we’re dealing with like charges, we know the force is repulsive. We will, therefore, ignore the sign of the
charges.
F=k
q 1 q 2
r^2
=(
8. 99 × 109
N·m^2
C^2)
( 1. 60 × 10 −^19 C)( 1. 60 × 10 −^19 C)
( 1. 00 × 10 −^2 m)^2
= 2. 30 × 10 −^24 NThe forces are of equal magnitude and directed away from each other. Since like charges repel, seeFigure15.13.
FIGURE 15.13
b. The mass of an electron isme= 9. 11 × 10 −^31 kg. What is the magnitude of the gravitational force between the
two electrons?