Phys 11C - Eiteneer
Lab: Electric Field Mapping
Online version
Name: ____________________________________________________________________________
PURPOSE
To explore the relationship between electric field lines and equipotential surfaces. To practice calculating electric
field lines from equipotential surface measurements.
Read the description of the background and the experiment in the 2020 11C Lab Manual (the pages relevant to
this experiment are posted online). Notice that the copyright to these pages belongs to Department of Physics
and Astronomy, Sacramento State University). All reproductions and redistribution is prohibited without
permission by the Department of Physics and Astronomy, Sacramento State University.
PROCEDURE
1. Look at the three pictures showing the experimental setup. The Parallel Plate configurations is similar to the
Circular Electrode between Parallel Plates, but without the circular electrode in the center.
2. In your own words, describe what was done, exactly, to perform this experiment. Do not just copy out of the
lab manual. Consider discussing how the setup is connected, and how the numerical data is obtained.
3. In your own words, define electric field lines. You may use your textbook, lab manual, or lecture notes for the
definition, but the key here is to define this concept in your own words. For example, think of a classmate
who has missed the lecture and have not read the textbook. How would you explain this idea to this
classmate?
4. What feature of an electric field lines tells you about the relative magnitude of the electric field?
5. In about one or two sentences each, predict the shape of the electric field lines for each of the four
configurations. Remember, electric field lines can bend, but not form sharp angles.
a. Parallel Plate Capacitor
b. Circular Electrode between Parallel Plates
c. Dipole
d. Concentric Cylinders
6. Now look at the photographs of the equipotential curves for each of the four configurations. These are fullsize pictures: the white sheet is a standard 8.5’’ by 11’’ piece of paper, and the large circular electrode is 10’’
in diameter. For each of the four configurations, draw a minimum of 10 electric field lines. Remember that
electric field lines are smooth curves; they cannot touch, intersect, or bend at sharp angles/corners. Do not
forget directions of the field lines!
Note: to draw the electric field lines, consider printing the photographs out, or using a computer program that
would allow you to draw smooth curves on top of an existing picture.
ANALYSIS
Parallel Plate Capacitor.
7. Remember, that electric field can be written as E 
V
V
or as E 
. Which of these expressions would
x
x
be more appropriate to use for the parallel plate capacitor, and why? Warning: this question is a bit trickier
than it seems at first. Think of the fringe effects.
8. Pick 3 locations (stay between the plates), and calculate the magnitude of electric field at these three
locations. In order to do that, at each location, pick two points on two neighboring equipotential curves, and
calculate V (the difference between electric potential reading of each). You have to physically measure
x with a ruler (you may leave this value in cm). If you do not have a ruler, you can print one from the
internet, download a ruler app on your smart phone, or find a program that would allow you to superimpose
a grid on top of a picture. Depending on the scale of printing, you may have to get creative with measurement.
Remember – the white sheet is 8.5’’ by 11’’ and the large electrode is 10’’ in diameter, so you may have to
come up with a “scale factor” for your measurements. Mark the measurement locations on your picture, and
show calculations below.
9. Is the field between the plates really uniform? If your answer is “no”, what could be done to improve this
result?
Circular Electrode between Parallel Plates.
10. Where is the field most distorted? Least distorted?
11. Where is the field strongest? Weakest?
Dipole
12. Where is the field strongest? Weakest?
Concentric Cylinders
13. Where is the field strongest? Weakest?
14. What is the direction of the field? Radially outward or radially inward?
15. We want to find how V depends on r (distance from the center). Again, you need to physically measure r with
a ruler (leave in terms of cm). Create the following table in Excel:
r [cm]
ln (r)
V (volts)
16. Make the measurements of at least 8-10 points, and fill in your table. Theory predicts that the potential
between two concentric metal shells varies logarithmically with position, so that we expect V  A ln r  B ,
where A and B are constants. Create a graph in Excel, with ln(r) along the x-axis, and V along the y-axis.
Hopefully, this will get you a straight-line graph; if not, you may need to remeasure your r values. Make sure
that your graph is labeled, and the axes are labeled.
17. Fit the best-fit line (linear) to your data points. Have Excel display the equation and the R2 value If you don’t
know how to do this, ask your group, ask me during office hours, or google it. Note: R2 value is a measure of
the “goodness” of the fit. The closer the R2 value to 1, the better the fit.
18. From your equation, determine the values of the constants A and B, including units. Show your work.
What to turn in (please make a single PDF or DOC/DOCX file):
 The answers to these questions (can be typed or handwritten)
 Four configurations with electric field lines drawn on each
 Your Excel table and graph based on the data for Concentric Cylinders