Introductory Physics Volume Two

1.3 Electric Field 11
+ -
Compute the net force on each charge?
⊲ Problem 1.2
You have three charges as shown.
-
+
a
y (m)
0.2
0.1
0.0
- 0.1
- 0.2
- 0.3
1.0 µC
3.0 µC
0.2 0.4 0.6 0.8 1.0
-2.0 µC
x (m)
What is the net force on each charge?
§ 1.3 Electric Field
Definition: Electric Field
If a particle with charge q a is placed at a point in space and F a is
the net electric force on this particle due to all other charges, then
the electric field at that point in space is the ratio of the force on
the particle and the charge of the particle.
⃗E = ⃗ F a
q a
You can think of the electric field as the force per charge in the
same way that pressure is the force per area. It is important to understand
the following points about the electric field:
• The electric field does not depend on the test charge q a in any way.
• The electric field represents the effect of all the other charges.
• The electric field is different at each point in space.
We will now work a specific example in order to demonstrate these
three properties of the electric field. It is important to follow the details
of the computation in this example very closely since there is much to
learn from it. Consider the configuration of two charges shown below.