Chapter 18: Electric Fields & Forces
Example Problems
e  1.6  1019 C
Fnet
K  8.99  109 N  m2 /C2
FE 
Kq1q2
r2
 qE
F  F2  F2
 net
x
y
 (Fx ,Fy ) 

1

  tan (Fy /Fx )
Example 18.1
Two identical metal spheres A and B are connected by a metal rod. Both are initially
neutral. 1.0 × 1012 electrons are added to sphere A, then the connecting rod is removed.
Afterward, what is the charge of A and the charge of B?
Example 18.2
a. Two small plastic spheres each have a mass of 2.0 g and a charge of −50.0 nC.
They are placed 2.0 cm apart. What is the magnitude and direction of the electric
force between the spheres?
b. A small plastic sphere with a charge of −5.0 nC is near another small plastic sphere
with a charge of −12 nC. If the spheres repel one another with a force of magnitude
8.2 × 10-4N, what is the distance between the spheres?
Example 18.3
The drawings show three charges that have the same magnitude but may have different
signs. In all cases the distance d between the charges is the same. The magnitude of
the charges is |q| = 8.6 μC, and the distance between them is d = 3.8 mm. Determine
the magnitude & direction of the net force on charge 2 for each of the three drawings.
Example 18.4
a. A +10 nC charge is located at the origin. (i) Draw the field diagram showing the Efield vectors at these positions (cm): (x, y) = (5.0, 0), (−5.0, 5.0) and (−5.0, −5.0). (ii)
What are the strengths of the E-fields at the positions?
b. A −10 nC charge is located at the origin. (i) Draw the field diagram showing the Efield vectors at these positions (cm): (x, y) = (0, 5.0), (−5.0, 5.0) and (−5.0, −5.0). (ii)
What are the strengths of the E-fields at the positions?