made some changes:
redesign the control panel
added finer control of position x and y
added Q slider that remembers the state of the charges
write-up taken from http://www.opensourcephysics.org/items/detail.cfm?ID=9683
Coulomb Force Model
The EJS Coulomb Force model shows the force vectors on charges. Users can change charge of an individual charge and add more charges (maximum: 10). Users can examine the model if Ejs is installed.
Run the simulation. Move the charges around and observe the force vectors (as well as the magnitude of the force). When the charges are the same size, are the forces equal and opposite? What about when the charges have different values (the physical size of a charge in the simulation changes with the charge value as a visual cue). The drop down menu allows you to select the charge (by color) that you want to change.
Add a third charge using the Add Charge button. With three charges, how can you arrange the charges so that the force on the red charge is zero? Sketch this configuration. If you change the charge of the red one, is the force still zero? Explain.
What is wrong with the following statement from a student? "When there are three charges (q=1 for all), the force on all three charges should be bigger than when there were only two charges (q=1 on each) because there is now a bigger total charge and the Coulomb force is proportional to charge."
Add a number of charges. Can you still set things up so that the force on the red charge is zero? Explain.
Giancoli, Physics for Scientists and Engineers, 4th edition, Chapter 21 (2008).
The Coulomb Force Model was created by Anne Cox, Wolfgang Christian,and Francisco Esquembre using the Easy Java Simulations (EJS) authoring and modeling tool. Exercises written by Anne J Cox.
You can examine and modify a compiled EJS model if you run the program by double clicking on the model's jar file. Right-click within the running program and select "Open EJS Model" from the pop-up menu to copy the model's XML description into EJS. You must, of course, have EJS installed on your computer.
Information about EJS is available at: and in the OSP ComPADRE collection .