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PHYS 2212 G/H Spring 2008
MP01c
Due at 9:00am on Tuesday, January 15, 2008
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Electric Force
This assignment will receive a 30% bonus if it is completed before 9:00 AM, Monday, January 14.
Charged Aluminum Spheres
Two small aluminum spheres, each of mass 0.0250 kilograms, are separated by 80.0 centimeters.
Part A
How many electrons does each sphere contain? (The atomic mass of aluminum is 26.982 grams per mole, and its atomic
number is 13.)
Hint A.1
The definition of mole and atomic number
Hint not displayed
Part A.2
How many electrons per mole of aluminum?
Part not displayed
Part A.3
How many electrons per kilogram of aluminum?
Part not displayed
Express your answer numerically.
ANSWER:
7.25
×
10
24
Part B
How many electrons would have to be removed from one sphere and added to the other to cause an attractive force between
the spheres of magnitude
(roughly one ton)? Assume that the spheres may be treated as point charges.
Hint B.1
How to approach the problem
Use Coulomb's law to find the charge needed to produce the given force. Then use the charge of an electron to determine the
number of electrons necessary to produce the calculated charge.
Part B.2
Find the relationship between the charges of the spheres
Assume that after some electrons have been removed from it, the first sphere ends up with a net charge of
. What would be
the charge on the other sphere,
, after these extra electrons are added to it?
Express your answer in terms of
and any necessary constants.
ANSWER:
=
Answer not displayed
Express your answer numerically.
ANSWER:
5.27
×
10
15
Part C
What fraction of all the electrons in one of the spheres does this represent?
Express your answer numerically.
ANSWER:
7.26
×
10
−
10
PSS 25.1: What's the Point?
Learning Goal:
To practice ProblemSolving Strategy 25.1 for problems involving electrostatic forces.
Two charged particles, with charges
and
, are located a distance
apart on the
x
axis. A third charged particle,
with charge
, is placed on the
x
axis such that the magnitude of the force that charge 1 exerts on charge 3 is equal to the
force that charge 2 exerts on charge 3.
Find the position of charge 3. Assume that all three charges are positive.
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 Spring '08
 N/A
 Physics, Charge, Force, Electric charge

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