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Physics 2220 (Schroeder) Name
fall 2008
Problem Set 1
(due Friday, Aug. 29)
- (a) A positively charged glass rod attracts an object suspended from a nonconducting
thread. Can you determine whether this object is charged? Please explain. (b) A
positively charged glass rod repels a similarly suspended object. Can you determine
whether this object is charged? Explain.
- Prepare two pieces of charged Scotch tape, as demonstrated in class. Estimate,
roughly, how many coulombs of charge are on each piece. You will have to make
a few assumptions and approximations, which you should state clearly. Be sure to de-
scribe fully the procedure you use to arrive at your estimate. (Hint: At what distance
does the electrostatic attraction between the two pieces of tape balance the weight of
one of them? A full spool of Scotch tape (3/4 inch by 36 yards) has a mass of about
45 grams.)
- In the figure below, three charged particles lie on a straight line and are separated by
a distance d. Charges q 1
and q 2
are held fixed. Charge q 3
is free to move but happens
to be in equilibrium (no net electrostatic force on it). Find q 1
in terms of q 2
q 1 q (^2)
d
q (^3)
d
- In the figure below, q = 1. 0 × 10
− 7 C and a = 5. 0 cm. What are the horizontal
and vertical components of the net electrostatic force on the charged particle in the
lower-left corner of the square?
q
a
a
a a
2 q
− q
− 2 q
- Physicist Richard Feynman once said that if two persons stood at arm’s length from
each other and each person had 1% more electrons than protons, the force of repulsion
between them would be enough to lift a “weight” equal to that of the entire earth.
Check (approximately) whether this is true. You may assume that a person (like
most forms of matter) is made up of roughly equal numbers of protons, neutrons, and
electrons. The data you need is all inside the covers of your textbook.
- An electric field with an average magnitude of about 150 N/C points downward in
the atmosphere near Earth’s surface. Suppose you wish to “float” an object weighing
4.4 N in this field by putting a charge on the object. What charge (both sign and
magnitude) would be needed? Why is this experiment impractical?
- Make a rough sketch of the electric field of a negative point charge. (Use arrows, not
field lines.)
- In the illustration below, q 1 = 1. 0 μC, q 2 = 3. 0 μC, and d = 10. 0 cm. Calculate the
electric field (magnitude and direction) at the point midway between the two charges.
Then calculate the electric field at a point 5 cm above the midpoint. Be sure to treat
the fields as vectors.
q 1 q (^2)
d
- In the figure below, q = 10. 0 nC and a = 5. 0 cm. What are the magnitude and
direction of the electric field at the center of the square?
q
a
a
a a
−q 2 q
− 2 q
- Thirteen identical positive point charges (each with charge q) lie on the vertices of
an equilateral 13-sided polygon. The distance from the center of the polygon to any
vertex is d. (a.) What is the electric field at the center of the polygon? Why? (b.)
Suppose that one of the 13 charges is removed. Now what is the electric field at the
center of the polygon? (Hint: There is an easy way to answer this question, without
adding up the fields of the remaining twelve charges.)
- Use the EField applet (physics.weber.edu/schroeder/software/EField/EField.html) to
make a field map of the electric field around a dipole (a positive and negative charge
of equal strength, separated by a small distance). Print your field map. (Printing is
straightforward with the Safari browser, at least on a Mac; with other browsers you’ll
need to do a screen capture and then print that.) Then pick two different field vectors
on it and explain (with a sketch) how each of these was computed by the program.
- Use the EField applet to show that the electric field of a long, uniform line of charge
falls off roughly in proportion to the distance from it (not the distance squared).
Include a printout from the program, annotated with your explanation. (Use a line of
point charges, not a single charge with the “Lines” option chosen.)
- Use the EField applet to explore the electric field inside and around a uniform cylin-
drical shell of charge. (Select the “Lines” option and then create a circle using the
Patterns menu.) Attach a printout, annotated with your observations of the features
of the electric field.
