PHYSICS 220/230
Lab 3: Electric Fields and Potentials

Object:

To experimentally investigate the concept of the electric field and to map (represent graphically) some field lines for particular charge configurations.

Background:

The electric field intensity is defined as the electrical force per unit of charge, or E = F/q. Theoretically, the electric field is determined by using a positive test charge q and determining the force acting on it at every point in space. The direction of the field is found by the laws of vectors and the rule that tells you whether the force is attractive or repulsive. Since a free charge moves in an electric field by the action of the electric force, then work (W = F * d) is done by the field in moving charges from one point to another (e.g., point a to point b ). To move a positive charge from "b" to "a" against the electric field would require work supplied by an external force. The ratio of the work done, W, to the charge, q, in moving the charge between two points in an electric field is called the potential difference, Vab, between the points: Vab = W/q. If a charge is moved along a path at right angles (i.e., perpendicular) to the field lines, there is no work done (W = 0) since there is no force component along the path. No work means no potential difference from point to point. Hence, the potential is constant along paths perpendicular to field lines. Such paths are called equipotentials. Thus, an electric field set up by charges may be "mapped" by determining the equipotential lines (equipotential surfaces in three dimensions) that exist in the region around the charges. Potential difference is easily read by a voltmeter, whereas the measurement of forces would present numerous experimental problems.

Part 1: Measuring Equipotentials

The apparatus consists of a flat board on which is placed a sheet of carbonized conducting paper imprinted with a grid. The sheet has electrode configurations of conducting silver paint which provide an electric field when connected to a battery. The standard electrode configurations provided are two circular dots representing point charges of an electric dipole and two parallel linear electrodes representing a two-dimensional cross section of parallel plates. Place one conducting sheet on the board and connect the conductor wires from the battery terminals to the two electrodes. Make connections to the electrodes with thumbtacks to hold spade lugs in contact with the electrodes. The voltmeter measurements will be made using the Pasco Science Workshop data acquisition program. Instructions for the use of the program will be given in the laboratory.

Procedure:

Please do not write on the carbonized conducting paper.

Part 2: Computer-Aided Field Mapping

We are now going to use two software programs that calculate the potential and electric fields for various two- and three-dimensional distributions of charge. The programs can then plot the potentials and the electric fields and help you visualize them and their properties.

A: EMField

Run the mapping program EMField by using the following operations: open PY Software, then EMField.win, then Ctwinx.exe, and then open EMField.ctb.

B: Poisson

Run the mapping program Poisson by double-clicking on the following folders or files in order: "Py Software", "cups", "Cupsem", "Poisson", "Poisson.exe". If the program asks you for a place to store temporary files, just type c:\temp\ and hit enter.

Please first change the background color to white so that the printing process does not use such an enormous amount of toner.
Click on the following in order: File, Configuration, Path Temporary Files Directory, Accept, Change Colors, Reverse, OK.
Now use the PrintKey software. You now have a picture of what you have just done on the Windows clipboard. Press the Alt and Tab keys at the same time to return to the Windows desktop. Run the MS Word processor and "paste" the picture into the blank document. Now you are ready to annotate and/or print.

Remember, in order to get back into the mapping program, you now can hold down Alt and cycle through the open files with the Tab key until the MSDos icon is selected. Alternately, you may simply click the MSDos icon C:\Software\… on the bar at the bottom of the screen.

In your discussion section, comment on what you found, including how the manual mapping compared to the computer mapping.