Screen shot of field vectors and a detector.

# Two Charges and Detector Model

The EJS Two Charges and Detector model shows the electric field of two charges and asks the student to determine what the movable detector is measuring. A slider can change the separation between the charges and the detector measurements can be recorded and analyzed using a built-in data analysis tool. Users can examine and edit the model if Ejs is installed.

## Exercises:

1. Run the simulation and move the detector around an observe the readings. You can see the field vectors and should be able to tell where the two charges are located. Change the slider and observe what happens. Describe the configuration.
2. The detector measures either the magnitude of the electric field or the absolute value of the potential. Which is it? 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 measured by the detector in the Data Table.
3. 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.

4. If you have EJS installed, now that you have determined what the detector measures (field or potential), change it to measure the other value. You will need to go to Model->Custom->Get Detector Value to make the changes. Describe how you changed it and how you verified your work. (Background required: for loops.)

### References:

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

### Credits:

The Two Charges and Detector 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/>.