EXPERIMENT 19

EQUIPOTENTIAL PLOTTING & THE ELECTRIC FIELD

To begin to understand the properties of electricity and magnetism one needs to start with the basics. Here it involves electric charge. With one or more charges or charged objects an electric field exists. Traversing along the direction of the electric field a change in the electric potential is found. It is known that these equipotential surfaces are oriented perpendicular to the electric field generated by the charges. In the general physics laboratory an electric field is generated upon electrolytic paper. Different configurations are created with different shapes and orientations of charged objects of various shapes. The electric fields of these different configurations can be mapped out by finding several equipotential lines and from these lines draw the electric field lines appropriately. A program has been created to mimic the determination of several equipotential lines. From this the student will print out these data points showing the equipotential lines and draw the electric field lines knowing them to be perpendicular to the equipotential lines and having direction from higher potential to lower potential.

Upon opening this program one will see on the screen the first configuration of electrodes.

The two larger circles represent a configuration that approximates two point charges of opposite charge. Each electrode is "connected" to a virtual battery thus establishing a potential difference between them. There is a grid across the region where the equipotential lines take place to help facilitate mapping of the equipotential lines to graph paper. The virtual voltmeter has electrodes that are movable and can be placed anywhere on the grid region. Several equipotential lines can be determined and mapped to graph paper by using these probes.

One equipotential line requires several data points to be found so that a smooth curving line can be established. To do this place one of the voltmeter probes at a strategic point on the grid by a click-and-drag method. You have just "picked out" one particular equipotential line by placing your probe at that point. To find other points so that you can establish the shape of the equipotential line leave the first probe where it is and move the second probe over the grid. You will see a display of the potential difference in a box as you drag across the grid with the second probe. When you have a potential difference of zero release the probe.

If you cannot establish zero, but can come close to it, the program will allow a data point to be established when the potential difference is less than 0.05 volts. Otherwise you will get a message stating that you are not on the equipotential line. Once you have a point on the equipotential line you will need to enter it by double clicking the mouse on this second probe. Still leaving the first probe where it is move the second probe to another place on the grid that again shows a potential difference of zero and double click this position when you have found it. Continue this process until you have enough data points representing this equipotential line. Make sure that you choose enough points that are somewhat close together to see the trend of the equipotential line. Of course, the larger the equipotential line the more data points you will have to choose to show the “smoothness” of that equipotential line.

To view the points right-click and a menu with the option “show results” will appear. Click on this and all the points for this equipotential line will show up displayed in a particular color along with the potential value in volts.

To determine a second equipotential line you now move the first probe to a new spot. Repeat the process as you did for the first line. Continue finding enough equipotential lines so that the determination of electric field lines can be drawn smoothly across these equipotential lines.

Use this same method for the two other electrode configurations available in this program. To access these click on the appropriate tab located at the top of the screen.

Print out the completed pages showing all of the data points depicting several equipotential lines about both electrodes of each configuration and draw smooth curved lines by hand through the data points for each of theses equipotential lines. Now draw by hand several lines depicting the electric fields for each configuration that exist between the two electrodes of each configuration indicating with arrows their directions.

Additional Material

In this virtual experiment there is an additional set of examples showing a representation of the electric potential and the direction of the electric field for several different configurations. In order to open these examples go to the Help menu and choose Examples. These examples are a separate program and need to be closed when you are finished.

There are four different configurations to use. Within each of these it is possible to change the position of the charged spheres (or rods in the fourth case) by the click and drag option. You will notice that you can either view these configurations’ electric potentials or their electric field lines. To choose which option you wish to view go to the File menu and click on the desired option. Take note that the electric field directions do not depict the strength of the electric field, only the direction. It is also possible to open more than one view. Doing so will enable you to do a comparison between the electric potential view and the electric field lines, or you can open views that show different configurations and compare these. You can even open views of the different examples. To do this, go to the File menu and select New. You can click and drag each view and place them conveniently about your screen. You can also resize each view to better fit the multiple views on your screen.