Electric Charges and Forces
Equipment
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•
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acrylic rod (2)
PVC rod (2)
string
rod hanger
rayon cloth
• leaf electroscope
• computer
• Applet: 23.2 Electric Fields from
Point Charges
Applet can be found at http://www.compadre.org/Physlets/
Preparation
Read Chabay and Sherwood, “Matter and Interactions" Wiley: Sections 14.1-2
Goals of the Experiment
To understand the concept of charge and the forces between charges.
Charges and Forces
1. Take an acrylic rod and hang it from a string so that the rod and the
string are at rest. Bring a second acrylic rod close to the hanging
one. What do you observe?
2. Now rub both the hanging acrylic rod and the second acrylic rod
with a piece of rayon. Make sure that the hanging rod is at rest,
then bring the second one close to it. What do you observe? Does
the behaviour of the rods depend on the distance between them?
3. Take a PVC pipe and hang it from a string so that the rod and the
string are at rest. Bring a second PVC pipe close to the hanging one.
What do you observe?
4. Now rub both the hanging PVC pipe and the second PVC pipe
with a piece of rayon. Make sure the hanging pipe is at rest, then
bring the second one close to it. What do you observe? Does the
behaviour of the pipes depend on the distance between them?
5. Now rub the acrylic rod with rayon again, hang it from the string,
and make sure it is at rest. Rub a PVC pipe with rayon and bring it
close to the hanging acrylic rod. What do you observe?
The PVC pipe rubbed with rayon is negatively charged, and the
acrylic rod rubbed with rayon is positively charged. The rods that
were not rubbed are electrically neutral. Use this definition and
your observations recorded above to answer the following questions:
6. Is there an electrostatic force between two positive (or two negative)
charges? If so, sketch the two charged objects and the forces acting
on them in the space below.
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7. Is there an electrostatic force between a positive and a negative
charge? If so, sketch the two charged objects and the forces acting on them in the space below.
8. Is there an electrostatic force between two neutral objects? If so,
sketch the two objects and the forces acting on them in the space
below. If not, explain why not.
9. Does the electrostatic force depend on the distance between the
charges? If so, does it increase or decrease with distance? How
did you come to this conclusion?
CHECKPOINT 1: Before moving on to the next part, have your TA
check the results you obtained so far.
10. Calculate the ratio between the gravitational force and the electric
force between two protons that are at a distance of r = 10−10 m.
The mass of each proton is 1.67 × 10−27 kg, the charge of each proton is 1.6 × 10−19 C, the gravitational constant is G = 6.67 × 10−11
1
Nm2 /kg2 , and the constant in Coulomb’s law is k = 4πe
= 8.99 ×
0
9
2
2
10 Nm /C .
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11. Calculate the force between two charges of 1 C each that are 1 m
apart. When you look at the magnitude of your result, do you think
that 1 C is a charge that you will regularly find in nature? Explain
your answer.
12. Consider an object sitting on the surface of the earth that has a
weight equal in magnitude to the force calculated in the previous
question. What would be the mass of that object? The mass of the
earth is 5.98 × 1024 kg, and the radius of the earth is 6.37 × 106 m.
13. An adult blue whale – the largest animal ever to have existed on
Earth – has a mass of 190,000 kg. How many blue whales would
collectively have the same mass as the object in the previous question?
We will use the applets on your computer to illustrate what forces
are acting between several charges. For questions 14-17, open firefox
and select the bookmark "Physlets". Click on "Electromagnetism"
from the top menu (do not use the drop down menu). On the left
had side click on "Electric Fields", and then on "23.3: Electric Fields
from Point Charges".
14. Run the applet and add two 3 coulomb positive charges. Drag the
particles to the coordinates (−1, 0) and (+1, 0). Sketch the charges
and the force arrows that appear on the screen. Label them with the
magnitude | F | of the force on either charge.
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15. Observe the force between charges of equal, but opposite charge.
Clear the applet and add a 3 coulomb charge and and -3 coulomb
charge. Drag the particles to the coordinates (−1, 0) and (+1, 0).
Sketch the particles and the forces, including the signs of the charges.
16. Observe the force between two equal and negative charges. Clear
the applet and add two 3 coulomb positive charges. Drag the particles to the coordinates (−1, 0) and (+1, 0). Sketch the charges and
the force arrows that appear on the screen. Label them with the
magnitude | F | of the force on either charge.
17. Observe the force between two unequal and positive charges. Clear
the applet and a 3 coulomb positive charge, and a 1 coulomb positive charge. Drag the particles to the coordinates (−1, 0) and (+1, 0).
Sketch the charges and the force arrows that appear on the screen.
Label them with the magnitude | F | of the force on either charge.By
which factor has it increased?
18. Are the forces on the two charges still equal and opposite? Explain
why or why not.
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19. Clear the applet and once again add a 3 coulomb charge and and -3
coulomb charge. Drag the particles again to the coordinates (-1,0)
and (+1,0). Write down the value of r (the distance between them)
and | F |.
20. Move the charge on the right to the position (0,0). What is the value
of r now? By what factor has | F | changed?
CHECKPOINT 2: Before moving on to the next part, have your TA
check the results you obtained so far.
Forces due to several charges
Clear the applet and once again add a 3 coulomb charge and and -3
coulomb charge. Play around a bit by moving the charges to different positions, and watch how the force on each charge changes.
21. Move one positive charge (P1) to the approximate position (-2,0),
and the other positive charge (P2) to a point on the positive x-axis, as
shown in the figure. Write the coordinates of P2 in the figure. Find
a point on the x-axis between P1 and P2 where a negative charge
(N) can be placed so that the total force on N is (very nearly) zero.
Draw this position in the figure, and label it with its coordinates.
y
P1
P2
x
22. In the figure above, draw the forces ~FP1onN and ~FP2onN exerted by
the two positive charges on N. Label the force vectors.
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23. Explain why the total force on N cannot be zero (at least not exactly)
when N is moved to any location on the x-axis that is to the left of
P1 or to the right of P2.
24. Place N at the origin, P1 at (+2.5,0) and P2 at (0,+2.5) as shown in
the figure below. On the left-hand figure, draw a labelled vector
addition diagram to explain the direction you see for the net force
on N. Then, on the right-hand diagram, draw the corresponding
vector diagram for the force on P1.
y
y
P2
P2
P1
P1
x
N
x
N
25. Close this applet, and open the applet mystery_charge. Is the mystery charge positive or negative? Explain your answer.
CHECKPOINT 3: Before moving on to the next part, have your TA
check the results you obtained so far.
Transferring Charges
26. You previously charged acrylic rods and PVC pipes by rubbing
them with rayon. Would that work with a metal rod? Explain why
or why not. (Hint: What can charges do in a conductor?)
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This figure shows an electroscope: the metal sphere at the top is connected to the metal leaves that you can see in the glass case. Make
sure that the leaves are hanging down before you do an experiment
(if necessary, ask your TA for help).
rod
electroscope
27. Rub a acrylic rod with rayon and touch the metal sphere of the
electroscope. Do the leaves of the electroscope open up or remain
closed?
28. After you touched the electroscope with the charged acrylic rod,
does the electroscope have a charge on it? If so, how did the charge
get on the electroscope?
29. Sketch the distribution of the charges on the electroscope and the
rod in the figure.
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rod
electroscope
30. When the acrylic rod is taken away from the electroscope, do the
leaves remain apart, or do they collapse? Explain the physics of
what’s happening.
31. Charge the electroscope again. Now touch the electroscope with a
finger. Do the leaves remain apart, or do they collapse? Explain.
32. This time, rub a PVC pipe with silk and use it to charge the electroscope. Sketch the distribution of the charges on the electroscope
and the rod in the figure. What is different from the previous case?
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rod
electroscope
33. Discharge the electroscope. Touch it with a acrylic rod that has been
rubbed with rayon. Then, touch it with a PVC pipe that has been
rubbed with rayon. What do you observe? Does it correspond to
what you expected?
Final Checkpoint. Please clean your area and have your TA check
your work before proceeding to the post test.
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