Screen shot of two charges with force vectors.

# Coulomb Force Wrong Model

The EJS Coulomb Force Wrong model asks students to figure out what is wrong with the model with multiple charges with force vectors shown. Users can change charge of an individual charge and add more charges. Users can examine and correct the model if Ejs is installed.

## Exercises:

1. Run the simulation. Move the charges around and observe the force vectors (as well as the magnitude of the force). What is correct about the model? What is your evidence?
2. Use the slider to change the charge of one of the charges (the size of the charge also changes to remind you that you changed the charge). The drop down menu allows you to select the charge (by color) that you want to change. What is wrong with the force vectors and/or magnitude reading? What is your evidence? Add at least one additional charges, using the Add Charge button, to confirm your answer.
3. If you have EJS installed, now that you have determined what is wrong with the model, correct it. The mistake is in Model->Custom->Calculate Force. Describe how you fixed it. (Background needed: arrays and for loops.)

### References:

- Giancoli, Physics for Scientists and Engineers, 4th edition, Chapter 21 (2008).

### Credits:

The Coulomb Force Wrong 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: <http://www.um.es/fem/Ejs/> and in the OSP ComPADRE collection <http://www.compadre.org/OSP/>.