Mapping the Electric Potential

A configuration of two equal but opposite charges in close proximity is called a "dipole". By placing the output "electrodes" of a power supply in a dilute saltwater solution, the equipotential lines can be mapped using an ordinary voltmeter.

To do this experiment, we will use a field tray, 2 electrodes, salt water, a power supply, a voltmeter and graph paper. The field tray may be constructed by placing an 8 x 11 pyrex baking dish on a piece of graph paper. The electrodes can be any conductor as long as they are fixed in place during the experiment.

Name:

Lab Partners:

When entering numeric data, use exponentials: ie., 1.6 * 10-19 = 1.6E-19.

Procedure

  1. Dissolve a pinch of table salt in one liter of water, and place in the field tray. Make sure that the salt is completely dissolved before beginning. If you have difficulty finding the points below, you probably have too much salt. Place the electrodes in the water at the points (-4, 0) and (4, 0) and connect them to either side of the power supply. Connect one lead of the voltmeter to the negative (or ground) side of the power supply. Set the power supply for 8 volts.

  2. Using the other lead of the voltmeter to probe, and rounding all voltages to 0.01 volts, identify at least 8 points in the tray with a voltmeter reading of 3 volts:

    x1: y1:

    x2: y2:

    x3: y3:

    x4: y4:

    x5: y5:

    x6: y6:

    x7: y7:

    x8: y8:

    All 8 should be in the neighborhood of one electrode. All can be in one quadrant of the tray, since the potentials should be symmetrical. Plot these points, along with the locations of the electrodes and all voltage values, on the graph paper.

    Continually monitor the electrodes for signs of corrosion (due to electrolysis). The electrodes should be kept clean.

  3. Repeat step 2 for voltage readings of 2.5 volts:

    x1: y1:

    x2: y2:

    x3: y3:

    x4: y4:

    x5: y5:

    x6: y6:

    x7: y7:

    x8: y8:

    and 3.5 volts:

    x1: y1:

    x2: y2:

    x3: y3:

    x4: y4:

    x5: y5:

    x6: y6:

    x7: y7:

    x8: y8:

    (the power supply should still be set at 8 volts). All points should be plotted on the same graph.

Analysis

  1. Connect each set of points corresponding to the same voltage reading by a smooth curve. Where are the "electric field lines?"
    Sketch them in a different color or pattern (ie., dotted lines.)

    All of the readings were relative to v = 0 at one electrode. If we think of this configuration as a dipole, you should subtract 4 volts from all of your readings, so that one electrode is at -4 volts and one is at +4 volts. Does your sketch of the equipotential lines correspond to the diagrams in the text?

©2004, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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