Download Electric Field Example Problems

Electric Fields
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Electric Field
• The electric field is a vector field that
describes how charges modify the space
around them.
• The electric field direction at any location is
the direction of the force experienced by a
positive test charge imagined to be placed at
that location.
How can we quantify electric field?
• Electric field is defined as
𝐹𝑡
𝐸=
𝑞𝑡
• For a single point charge (source):
𝑞𝑠 𝑞𝑡
𝑘 2
𝐹𝑡
𝑞𝑠
𝑟
𝐸= =
=𝑘 2
𝑞𝑡
𝑞𝑡
𝑟
Exercise 22, p. 557
An electron is acted on by an electric force of
3.2 x 10-14N. What is the magnitude of the
electric field at the electron’s location?
(e = 1.60 x 10-19C)
ANSWER: 2.0 x 105 N/C
Exercise 24, p. 557
What is the magnitude and direction of the
electric field at a point 0.75cm away from a
point charge of +2.0pC?
ANSWER: 3.2 x 102 N/C away from the source
Superposition Principle
For a configuration of charges, the total, or
net, electric field at any point is the vector
sum of the electric fields due to individual
charges of the configuration.
Example 15.6, p. 542
Two point charges are placed on the x-axis as in the
figure below. Find all locations on the axis where
the electric field is zero.
ANSWER: 0.20m
FOLLOW-UP EXERCISE: Repeat this example,
changing only the sign of the right-hand charge.
Example 15.7, p. 542
The figure below shows a configuration
of three point charges.
a) In what quadrant is the electric
field at the origin: (1) the first
quadrant, (2) the second quadrant,
or (3) the third quadrant? Explain
your reasoning, using the
superposition principle.
b) Calculate the magnitude and
direction of the electric field at the
origin due to these charges.
ANSWER: 1.69 x 103 N/C at 30°
What is the force on a
-1µC charge placed at
the origin? (magnitude and direction)