PHYSICS 124 EXPERIMENT NO. 2
ELECTRIC FIELD PLOTTING
In this experiment we will plot a series of lines of equal potential for several 2dimensional charge distributions. Once we know the equipotentials, we can construct the
corresponding electric field lines, giving us an informative visual display of the field.
Equipment
3 Carbonized Acetate Sheets (“parallel plate”, “dipole”, and “dipole plus circle”),
1 dry cell (1.5V), 1 Digital Voltmeter, Galvanometer, 2 Voltage probes.
A.
Digital Voltmeter
~1.5 V Dry Cell
Wires
Parallel Plates Sheet
Voltage Probes
Platform
Method
Each carbonized sheet has a certain pattern of highly conducting areas silverpainted onto it. These painted areas are connected to the terminals of the dry cell which
supplies charge to the conductors and maintains a constant potential between them. A
current flows through the resistive carbon film from one conductor to the other. By
probing the area between the conductors with the digital voltmeter, we will find sets of
points that have zero potential difference between them, i.e. they lie on the same
equipotential lines. Each sheet has a grid of reference marks to make it easy to identify
the points between the conductors.
B.
C.
Procedure
1. Parallel Plate Configuration
Connect battery, terminals of the sheet and voltmeter as shown:
DC left out
ON pushed in
+
+




Set the voltmeter to ON, DC, V, 2, all pushed in except DC left out. Make
sure you have always the pack of 3 carbon sheets together on the tabloid with the
sheet you measure on the top. (This ensures better contact with the terminals).
The dry cell negative terminal is connected to the left “plate” on the sheet and the
positive dry cell terminal to the right “plate”. The voltmeter input “LO” is connected to
the left plate via the free standing voltage probe and the terminal “HI” is connected to the
right plate via the voltage probe with the handle, the probe you use for stepping through
the sheet. Make sure you have good contact between the terminals and the conducting
strips leading to the plates. You should see on the voltmeter ~ 1 – 1.3 V. If you see less
than ~1 V, your contact with the terminals is bad, mostly due to damage of the end of the
conducting strips. By shifting the carbon sheet as indicated below you should be able to
get ~ 1 V or more. Record the voltage on your sheet.
You improve contact by shifting the carbon sheet in this
direction
damaged end
Head of terminal clamp
watch out that you don’t slip off the other end
Follow the instructions below for your sheet.
1) Place the negative end of your voltmeter onto the outermost part of the negative
end of the silver conductor.
2) Place the positive end of your voltmeter onto the pixel (dot) that is closest to the
end of the conductor.
3) Read the voltage difference off of the voltmeter and record it on your sheet.
4) Keeping the negative voltage probe where it is, move the positive probe around on
the black part of the sheet until you find a place where the voltmeter reads zero
and record its position in your notebook.
Q1. What can you say about the electric potential of each of these two points?
5) Repeat step 4 to find a total of 3 points on both sides (a total of 6) of the negative
probe.
6) Repeat steps 1-6 moving to the next pixel away, recording the voltage and
positions of the equal-potential curves, until you reach the opposite conductor.
7) Sketch a family of equipotential lines for the entire region between the plates
8) Sketch the family of lines that are perpendicular to the equipotential lines. These
lines are the electric field lines. They begin on positive charges and end on
negative charges.
Q2. Why are electric field lines perpendicular to the equipotential lines?
(HINT: Think of a paradox that would occur if they were not perpendicular to the
equipotentials.)
Q3. Why should the electric field lines also be perpendicular to the edges of
the conductors?
The electric field in a uniform part of your diagram can be calculated from the voltage
difference V between 2 equipotential lines and the distance x between them, taken in a
direction perpendicular to them.
Q4. How would you find the magnitude of the electric field in a given region
of your diagram? Do an example.
2. Repeat the instructions for the “dipole” distribution.
3. Repeat the instructions for the “dipole plus circle” distribution.
Q5. What qualitative effect does the circle have on the field distribution?
Write-Up:
The analysis of your lab report needs to include:
1) Drawings of the three different sheets with equal potential and electric field
curves drawn with corresponding voltages
2) Answers to all 5 questions