HW 03
Due: 11:00pm on Friday, February 18, 2011
Note:
To understand how points are awarded, read your instructor's
Grading Policy
.
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Moving a Charge
Part A
A point charge with charge
= 3.50
is held stationary at the origin. A second
point charge with charge
= -4.10
moves from the point ( 0.165
, 0) to the
point ( 0.265
, 0.260
). How much work
is done by the electric force on the
moving point charge?
Hint A.1
How to approach the problem
Hint not displayed
Hint A.2
Calculate the initial electric potential energy
Hint not displayed
Hint A.3
Calculate the final electric potential energy
Hint not displayed
Express your answer in joules. Use
= 8.99×10
9
for Coulomb's constant:
.
ANSWER:
=
-0.434
Correct
Electric Potential, Potential Energy, and Force
Learning Goal:
To review relationships among electric potential, electric potential
energy, and force on a test charge
This problem is a review of the relationship between an electric field
, its
associated electric potential
, the electric potential energy
, and the direction of
force on a test charge.

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Part A
Electric field lines always begin at _______ charges (or at infinity) and end at
_______ charges (or at infinity). One could also say that the lines we use to
represent an electric field indicate the direction in which a _______ test charge
would initially move when released from rest. Which of the following fills in the three
missing words correctly?
ANSWER:
(positive; negative; negative)
(positive; negative; positive)
(negative; positive; negative)
(negative; positive; positive)
Correct
Note that the electric field vector
is everywhere tangent to the electric field
lines. Like electric field lines, the electric field vector generally points away from
positive charges and toward negative charges.
Part B
Would a positive test charge released from rest move toward a region of higher or
lower electric potential (compared to the electric potential at the point where it is
released)?
Hint B.1
Potential, field, and force
Hint not displayed
ANSWER:
higher electric potential
lower electric potential
Correct
Part C
Now imagine that the sign of our test particle is changed from positive to negative,
but the electric potential remains the same. Which of the following statements is
correct?