Cabrillo College
Physics 4B
PLC Activity #2
Electric Fields
Due: see website for due dates
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
Part 1: Electric Field Physlets
Go to Chapter 23: Electric Fields. Do the following Physlet Physics exercises and
answer the questions listed.
Exploration 23.2
The animation shows two fixed charges and a test charge (position is given in meters
and time is given in seconds). The electric field lines due to the fixed charges and the
force vector on the test charge are shown. The test charge will move under the action of
the electric field when the animation is played.
a. Using Configuration A, drag the test charge to the approximate position of (-0.8 m, 0
m). Write down a prediction for the path the charge will follow after being released at
this point. After you have made your prediction, play the animation. Was your
prediction correct? If not, what caused your error?
b. Reset the applet and then drag the test charge to the approximate position of (1 m,
0.35 m). As before, write down a prediction for the path the charge will follow after
being released. If your prediction was incorrect, explain the flaw in your reasoning.
c. Repeat using Configuration B with the charge being released from the point (-0.5 m,
0.5 m).
d. Repeat using Configuration B with the charge being released from the point (0 m, 1.3 m).
Physlet Problem 23.3
Five animations show the electric field produced by a configuration of hidden
charges. The arrows represent the direction of the electric field, and the color
represents the intensity of the field. Which electric field would be produced by the
charge configuration shown below? Red represents a charge of +Q and blue represents
a charge of -Q.
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Physlet Problem 23.5
An electron is shot through four regions of constant electric field (position is given in
centimeters and time is given in seconds).
a. What is the direction of the electric field in each region?
b. Rank the magnitude of the electric fields of the four regions, smallest to greatest.
Physlet Problem 23.9
Four point charges of equal strength but two different polarities are arranged on the
corners of a square as shown. The strength of each charge is measured in nC and can
be varied using the slider. You can mouse-down to read the magnitude of the field at any
point in the animation (position is given in meters and electric field strength is given in
newtons/coulomb).
Find a formula for the electric field midway between charge 2 and 3 (indicated by the
small black dot) as a function of the strength of the charges. Include a vector diagram
that also shows the direction of the resultant field.
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Part 2: Ranking Problems
Question 1
Two identical circular nonconducting rings are centered on the
same line. For three situations, the uniform charges on rings A
and B are, respectively, (1) qO and qO, (2) qO and qO, and
(3) qO and qO. Rank the situations according to the
magnitude of the net electric field at (a) point P1 midway
between the rings, (b) point P2 at the center of ring B, and (c)
point P3 to the right of ring B, greatest first.
Question 2
The figure shows two disks and a flat
ring, each with the same uniform
charge Q. Rank the objects according
to the magnitude of the electric field
they create at points P (which are at
the same vertical heights), greatest
first.
Question 3
An electron e travels through a small hole in plate A and
then toward plate B. A uniform electric field in the region
between the plates then slows the electron without
deflecting it. (a) What is the direction of the field? (b) Four
other particles similarly travel through small holes in
either plate A or plate B and then into the region between
the plates. Three have charges +q1, +q2, and -q3. The
fourth (labeled n) is a neutron, which is electrically
neutral. Does the speed of each of those four other particles increase, decrease, or
remain the same in the region between the plates?
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Question 4
In figure (a), a circular plastic rod with
uniform charge +Q produces an electric
field of magnitude E at the center of
curvature (at the origin). In figures (b), (c),
and (d), more circular rods with identical
uniform charges +Q are added until the
circle is complete. A fifth arrangement
(which would be labeled e) is like that in d
except that the rod in the fourth quadrant
has charge –Q. Rank the five
arrangements according to the magnitude
of the electric field at the center of
curvature, greatest first.
Question 5
The figure shows three rods, each with the same charge Q spread uniformly along its
length. Rods A (of length L) and b (of length L/2) are straight, and points P are aligned
with their midpoints. Rod c (of length L/s) forms a complete circle about point P. Rank
the rods according to the magnitude of the electric field they create at points P, greatest
first.
How to get credit for this activity
Show your answers to a PLC tutor so they can check them and initial the signoff sheet.
Be prepared to answer questions about the activity or your results.
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