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Unformatted text preview: ion faithfully reprod uce? What is the specific evid ence? 48 PROBLEM #4: ELECTRIC POTENTIAL
FROM A LINE OF CHARGE
You work with an astrophysics research group investigating the origin of highenergy
particles in the galaxy. The group has just discovered a large electrically charged
nebula with an irregular shape. In order to understand how this nebula affects the
motion of charged particles passing nearby you must find the electric potential near the
nebula. Because of its complicated shape you plan to use a computer simulation. You
must determine if the simulation results match the physics you know. You decide to
test the simulation for the case of a uniformly charged rod, since the situation is simple
enough for direct calculation. You decide to calculate the electric potential at a point a
short distance from the middle of the rod along its perpendicular axis, and also at a
point a short distance from the end of the rod along its parallel axis. You will then
compare these results with those obtained using the computer simulation, to see if the
simulation can be trusted.
Instructions: Before lab, read the required reading from the textbook and the laboratory in its
entirety. In your lab notebook, respond to the warm up questions and derive a specific prediction
for the outcome of the lab. During lab, compare your warm up responses and prediction in your
group. Then, work through the exploration, measurement, analysis, and conclusion sections in
sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to
perform data analysis, rather than doing it by hand. At the end of lab, disseminate any electronic
copies of your results to each member of your group.
Read: Tipler & Mosca Chapter 23. Pay particular attention to Sections 23.1, 23.2 & 23.4
and Examples 235 & 236. EQUIPMENT
The computer program EM Field and a ruler.
If equipment is missing or broken, submit a problem report by sending an email to
[email protected] Include the room number and brief description of the
problem. WARM UP
1. Make a picture of the situation. Select an arbitrary point of interest along the
perpendicular axis of symmetry. Label relevant distances, angles, and constant
quantities. Decide on an appropriate coordinate system. Draw a representative
infinitesimal charge element dq somewhere along the rod.
2. Write an expression for the infinitesimal (scalar) electric potential dV produced by dq
at an arbitrary point of interest along the perpendicular axis of symmetry. 49 ELECTRIC POTENTIAL FROM A LINE OF CHARGE – 1302Lab2Prob4
3. Write an integral total electric potential at the point of interest in terms of dq. The total
electric potential due to a charge distribution is found by calculating the contribution from each
charge element to the total (scalar) potential field, and summing the contributions. When the
charge distribution is continuous, it may be mathematically divided into infinitesimal elements
dq; then the individual contributions are added together with an integral
4 Evaluate the integral you set up in question 3 to obtain an expression for the electric
potential at the point of interest. In order to evaluate such an integral, all terms in the
integrand must be either constants or explicit functions of the integration variable. First, choose
an appropriate integration variable. Then, rewrite all variable quantities in the integrand
(including dq) in terms of the integration variable you have chosen. Determine appropriate
limits for the integration variable you have chosen. Use the Pythagorean Theorem, trigonometry,
and the linear charge density to write your integrand(s) in a suitable form.
5. Repeat steps 14 for an arbitrary point of interest along the parallel axis. PREDICTION
Determine the physics task from the problem statement, and then in one or a few
sentences, equations, drawings, and/or graphs, make a clear and concise prediction that
solves the task. EXPLORATION
In the folder PhysLab on the desktop, open
EM Field and click anywhere in the window
for the instructions. From the Sources pulldown menu, select 3D point charges. Drag
any positive charge to the center of the
window of EM Field. From the Field and
Potential pulldown menu (shown to the
right), select Field vectors. You can reveal electric field by clicking on the locations where you would like the
electric field to remain displayed. Look at the Display dropdown menu and explore its
options. To place objects at precise points on the screen you can use the ‘show grid’ and
‘constrain to grid’ features from the display pulldown menu. Expand the display
window to fill the entire computer screen.
Measure the length of the electric field vector at several locations, as well as the distance
from the locations to the center of the charged point object. You can remove the
displayed vectors using the ‘clean up screen’ option from the Display dropdown menu. 50 ELECTRIC POTENTIAL FROM A LINE OF CHARGE – 1302Lab2Prob4 Try using differe...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
 Spring '14

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