Lab #2 - Equipotential Lines Name: ______

Copyright R Mahoney 2012

To do this experiment, you will be shown how to use a voltmeter. Later you will learn how a voltmeter works, but for now, suffice it to say that the device measures potential difference or voltage between two points.

We discussed in class that equipotential lines, lines that have the same value of potential difference with regard to some point of reference, are always perpendicular to the corresponding electric field lines. The purpose of this lab is to map out equipotential lines, and to show their perpendicular relationship to electric field lines.

We will set up electric fields for two scenarios already discussed in class.

Scenario one

Scenario one is two point charges. The field lines between these two point charges, simulated by leads from a power supply connected to two circular pads drawn on a piece of black "resistance paper" with conductive ink, should resemble the seams of a football. The corresponding equipotential lines should then be gentle arcs at right angles everywhere to the field lines. The closer you get to either point charge, the greater the curvature of these equipotential arcs. Note that the middle equipotential line should be a straight line (zero curvature) perpendicular to the line of sight between the two point charges.

Scenario two

Scenario two is two straight and parallel conducting lines functioning as a "very thin" parallel plate capacitor (this analogy is limited). Here, as we have mentioned already in class, the electric field lines passing between the conducting lines are generally uniform (straight and not diverging) and perpendicular to the conducting lines (very thin parallel plates). It follows that the equipotential lines between the conducting lines should be parallel to the conducting lines.

1. Set up the lab for scenario one (shown in class). You will be given two white sheets of paper to record your data (voltage readings). Please don't write on the original black "resistance paper" sheets with conductive ink on them: these sheets are reused.

First make a note of the entire voltage or potential difference between the two point charges. You should be able to dial this voltage in from the DC power supply, so dial in 10 volts. When your voltmeter is across the two "point charges", it should read 10 volts.

Now look for all points where the difference in voltage is 5 volts. You will want to keep your negative lead of your voltmeter fixed to the "negative charge", and move the positive lead around. Record every point that says 5 volts on your voltmeter by making a "5" mark on one of your white data sheets. Also, be sure to mark on your data sheet where the point charges are. Now connect the "5" marks together. This is your 5 volt equipotential line. Is this what you expected?

Now do the same for 4 volts and 6 volts. Simply record your reading with "4" and "6" marks on your white data sheet. Connect the similarly numbered marks together. These are your 3 volt and 7 volt equipotential lines.

2. Now set up the lab for scenario two. Again record the locations for 4, 5, and 6 volts, and connect the similarly numbers marks on a new white datasheet. Are these the lines you expected to see? Are the equipotential lines linearly distributed? Are the equipotential lines symmetric in the vertical direction (going perpendicularly from one plate to another)?

3. Be sure to attach your equipotential graphs when presenting this worksheet for credit.

4. State at least two non-trivial systematic errors for this experiment.