Screen shot of electric field and detector.

# Point Charge Wrong Model

The EJS Point Charge Wrong model asks students to figure out what is wrong with the model of a point charge at the origin. It shows field vectors and a movable detector near a point charge. Detector values can be recorded into a data table and analyzed using a built-in data analysis tool. Users can examine and correct the model if Ejs is installed.

## Exercises:

1. Run the simulation of a point charge at the origin. Move the detector around in the electric field and observe the value of the electric field. What is wrong with this model? What is your evidence? You may need to make some measurements to help determine what is wrong. If so, click on the Record button to record the values of the electric field as measured by the detector in the Data Table.
2. Clicking on the Wrench tool opens up DataTool, a data analysis tool, with all of your recorded data in columns. Some notes on using DataTool:

- DataTool automatically draws lines between nearby points and you may find this confusing so you can click on the checkbox with a line through it to remove the connecting lines.
- DataTool can provide a Fit to the data in the first two columns. You may need to drag the columns around in order to fit data of interest.
- If you want DataTool to Fit the data to an equation not in the list, you will need to edit the current equation in Fit Builder. For example, if you want to fit the data to a/x3 (and have the program automatically find the value of a), double-clicking on the equation of the line (a*x+b) will automatically open the Fit Builder. In this case, since you only want to fit one parameter, first delete parameter "b" (from the parameter list) and then type your new Function = a/x^3.

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->Get E Value. Describe how you fixed it. (Background needed: arrays and for loops.)

### References:

- Giancoli, Physics for Scientists and Engineers, 4th edition, Chapters 21 and 23 (2008).

### Credits:

The Point Charge Wrong Model was created by Wolfgang Christian, Francisco Esquembre and Anne J Cox 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/>.