University Physics, PHYS 221

Lab 1: Mapping the Electric Field and Equipotential Lines

In this lab you will investigate the electric field lines and equipotential lines in a simple 2-dimentional conducting medium.  This medium consists of carbon-impregnated paper, which allows an electric current to flow. The current enters and leaves the conducting paper through electrodes of special geometric shapes.  These shapes are painted on the paper using a conducting silver paint.  The shapes are: (A) two points, (B) two bars (like parallel plate capacitor (C) a bar and a point, as shown in the figure below.

The electrodes are connected to a source of emf, in this case a variable voltage DC power supply (VDC).  The connection is made through metal pushpins.  Make sure that the pushpins are in good contact with the conducting shapes.  The location of the equipotential lines and of field lines is determined from measurements of potential differences between any two points on the conducting paper.  Potential difference between any two points that are on a given equipotential line is zero, whereas for two points on the same field line the potential gradient is maximum.  Field lines and equipotential lines are perpendicular to each other.

EQUIPOTENTIAL LINES:

Connect the two terminals of the power supply to the pushpins, and turn up the potential difference to about 5 Volt and record it.  Some power supplies do not have a meter attached.  In this case use the voltmeter to measure the voltage across the output terminals of the power supply.  Note that, the bar graph display of the voltmeter shows the voltage simultaneously while the digital numerical display is somewhat delayed.

1)      Mark the positions of the electrodes on the white paper.

2)      Hold one of the test probes fixed at a selected point somewhere you want to explore a passing equipotential line, and determine all the other points that have zero potential with respect to this fixed point by moving the other voltmeter test probe across the conducting surface.  As you are doing this, mark the points you have found on the white paper.

3)      Connect the points by a smooth line.

4)      Move to another point and repeat the procedure 1-3. five more times(if time permits).

5)      You should have five equipotential lines.

FIELD LINES:

By holding one voltmeter test probe fixed and moving the other test probe in a circular pattern, determine point pairs of maximum potential difference.  This procedure is illustrated in the figure below.  Start with point A and find the point B, which then gets used as A (A’), and so on.  Locate one field line this way and draw on the same white paper, including the measured points.

Repeat measurements for several other field lines (5 if time allows).  If you have time left, exchange the electrodes with another group and repeat the experiment.

Questions:

1)      What are the separations of the equipotential lines in Volts?  You can mark the potential for each line by measuring it with respect to one of the electrodes.  To do this leave one test lead at the zero potential electrode (ground), and move the other through the lines you sketched and read (IF TIME PERMITS)

2)      Are the equipotential lines perpendicular to the field lines in you sketch?