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.
Theseconfigurations 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. Determinethe magnitude and sign of the work done by the capacitor’s electric field if one coulomb of positive chargewere 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 equally-spaced 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 itsvalue ? ______Volts
3. Draw the sameequipotentials, 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.