1. Computer modeling

Field expert is a Windows application that can shed light over complex systems of charges. All images in this paper have been created with Field expert. The program takes the form of a specialized integrated environment and is designed to provide maximum ease of use:

Tasks

• To place an electric charge, click the right mouse button on the desired place on the screen; alternatively the menu Field | Add charge (^A) can be used if exact coordinates are desired.
• In the top-left corner of the window the program displays the current mouse position and the electric field strength E and potential V at that point. Multiples and submultiples are used if more appropriate: E may show up as 1.234 M, which means 1.234 * 106 V/m, or as 7.236 m, which is 7.236 * 10 -3 V/m.
• To draw field lines, click the left mouse button; depending on the current options, the program will compute and display the line of force and the equipotential passing through the specified point. You can toggle generation of lines of force or equipotentials with the corresponding items in the Options.
• To see a global map of the field, choose Draw | Field map (^M). Depending on the settings in the Options menu (as before) , the program will show elementary lines of force OR equipotentials (not both!) through the points of a grid.
• To clear the screen if it has become too cluttered, press ^R or select Draw | Clear screen.
• To remove or modify a charge, click the right mouse button on that charge.
• Use File | Save to save the configuration (not the image !) to a file; File | New will clear all charges, and File | Load will load a field configuration from a file.
• When you like the image, choose
• File | Save image to save it to a file, or
• Edit | Copy image to place it on the clipboard and paste it in other applications.
• The program currently supports the Windows DIB (Device Independent Bitmap) for disk files, and the BMP format for Clipboard selections. If another format is needed, the image can be easily filtered through Paintbrush or other specialized graphic tools.
• The Help menu is a quick reminder of the conventions and procedures used in this application.

Additional options:

• Options | Charge values: display charges as numbers (their value), rather than the usual circles with "+" and "-" inside. The is unpleasant if charges are close, so the default is off.
• Options | Time limit: sometimes it takes too long to generate a single field line; the maximum number of seconds in which the program tries to draw the field lines through a point is controlled by this option (by default 6) . The program can tell if a field line runs to infinity or gets stuck into an equilibrium point, but even "regular" field lines can be "too long".
• Options | Granularity: select granularity for the grid associated to Field map. A large value here results in coarse pictures; very small values obscure it completely; the default setting (10 pixels) works fine.

Implementation