Physics 1BL
Coulomb’s Law and Superposition
Spring 2009
1
The two balls before charging
Introduction
In this lab you will work on the ideas of Coulomb’s Law, electric field, and the
superposition of Coulomb forces. You will explore how the electrostatic force between
charges depends on distance, and demonstrate the ideas of superposition with multiple
charges. Finally, you can check your skills with Electric Field Hockey.
__________________________________________________________________________
Prelab Activity:
Read Serway & Faughn 15.3, 15.4 & 15.8
1)
Two charged balls, one with a charge of
+5.0
μ
C and the other with a charge of
+2.4
μ
C, are placed a distance d = 10.0 cm
apart from each other and rigidly held in
place. A third ball with a charge of
–
3.0
μ
C is
dangled from a string between the two
charged balls such that it remains in
equilibrium.
Calculate the distance the third
charged ball is from to the +5.0
μ
C
charge.
2)
Three charges are arranged in a square (along with point P) with sides equal to
8.0 cm as in Figure 1.
Calculate the electric field (magnitude and direction) at point
P due to the three charges.
Next, calculate the force (magnitude and direction) on a
–
0.35
μ
C charge placed at point P.
+4.0
μ
C
–
5.0
μ
C
8.0 cm
Figure 1
P
+3.0
μ
C
LAB WORK: A: Group Activity
In this white board activity, imagine that you are given two
nonconducting balls, of equal masses, each attached to an
insulating string. You also have a scale to measure weight,
a ruler, and a charging mechanism.
A1. The balls hang vertically side by side, as shown, when
they are not charged. Predict and sketch what happens
when you charge both balls equally (charges are equal in
both magnitude and sign). Assume that the excess charge
is distributed uniformly over each ball’s surface.
A2. Sketch what happens if you give a double dose of the
same sign charge to the right ball? (e.g., Q on the left and
2Q on the right).
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Coulomb’s Law and Superposition
Spring 2009
2
A3. Draw a force diagram for the left ball in situation A2.
A4. Write an expression for the electrostatic force and the charge on the left ball.
A5.
Look at each variable in your expression for charge. Which quantities can be measured
with the equipment at hand? Which quantities cannot be measured? Can you express
the quantities that cannot be measured in terms of measurable quantities?
A6.
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 Spring '08
 ANDERSON
 Charge, Force, Work, Electric charge, surface charge density, coulombs

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