Equipotential Lines
Equipment: Digital Multimeter (use as a voltmeter), with leads, red and black leads with
alligator clips, battery pack, conductive paper sets (parallel plate capacitor, electric dipole),
white graph paper, cork board, metal push pins, ruler, colored pencils.
Introduction: The apparatus consists of a cork board, on which is placed a sheet of black
carbonized paper imprinted with a grid. The grid has electrode configurations painted on with
metal conductive paint reinforced by aluminum foil. The painted lines act as extensions of the
positive and negative terminals of the battery when connected. The electrode configurations
provided are:
1. Two parallel linear electrodes representing a 2-D cross section of a parallel plate capacitor
2. Two dots representing the point charges of an electric dipole.
These configurations produce a 2-D electric field representative of the “real thing”.
Part 1: Parallel Plate Capacitor
1. Take the black paper with the plate capacitor drawn on it and lay it on top of the corkboard.
Push a metal push pin as far as it goes through each plate. These will act as electrodes.
Battery Pack
__
+
2. Connect the circuit as shown above. Ask your lab instructors for help. Connect the negative
terminal of the battery to the one of the push pins and also to the “com” port (black) on the
multimeter. Connect the positive battery terminal to the push pin on the other side. Use the red
probe lead and connect it to the red volt/ohm.terminal on the multimeter. Select the 200 DCV
scale.
a. Use the voltmeter to determine the potential difference between the “plates”: ________ V.
Label the plates with the potential value, using the convention shown in the following diagram.
b. Determine the magnitude and sign of the work done by the capacitor’s electric field if one
coulomb of positive charge were to move from the negative to the positive plate. Recall that the
units of volts are joules per coulomb.
__________ Joules
c. Will the positive charge gain or lose potential energy? How much?
__________ Joules
Gained/ Lost
(Circle one)
3. Use the multimeter and red probe lead to determine the potential at many different places both
inside and outside of the capacitor. Record the values and positions on the white graph paper.
4. Use the illustration above as a guide (note voltages are not the same!), and draw 5 equallyspaced equipotential lines between the capacitor plates on the WHITE graph paper (you’ll have
to draw the plates in the appropriate position). Do not draw on the black paper!
If you are using 2 D batteries, the lines should be drawn at 0.5, 1.0, 1.5, 2.0, and 2.5 V. If you
are using another power source, ask the lab instructors which lines to draw. Use a colored
pencil in the green/blue/violet range and label each line.
5. Determine the magnitude of the E field (in V/m) and draw 3 representative field lines on your
graph. Use another color (red/orange/pink). Note the graph is marked in centimeters, not meters.
Attach the completed graph to the end of the lab.
E = ____________ V/m
Part 2 Electric Dipole
1. Replace the parallel plate capacitor configuration
with that of the electric dipole and repeat the above
procedure with the modifications as listed below.
Use drawing as a guide.
2. Find the value of the potential at the midpoint.
This should approximate a vertical line on the paper.
Draw this equipotential. What is its value ?
________ Volts
3. Draw the same equipotentials, using the same
color as for part 1.
4. Draw 5 electric field lines on your graph. Recall the electric field lines are perpendicular to
the potential lines. Use the color you used for field lines in part 1. There is a diagram in your
text that may be helpful.
5. How does your diagram compare to that illustration?
Attach graph to end of the lab.