worksheet Written by: Anne J Cox, Wolfgang Christian and Francisco Esquembre Edited by: Mario Belloni and Wolfgang Christian
Conditions of Use: This material is freely available for instructional use and may be reproduced
and distributed for non-profit, educational purposes.

Point Charge Model
The EJS Point Charge model shows you the electric field near a point charge at the origin. A movable electric field detector reports the field direction and strength.  Detector values can be recorded into a data table and analyzed using a built-in data analysis tool. Users can examine and the model if Ejs is installed.
Exercises:
Move the detector around. What do the colors represent? What color means stronger field? weaker field?
Try changing the slider and see what happens. If you double the charge, does the value at the detector double? At all points?
Now, move the detector around and recod some data (by pushing the Record button in the data table. Verify that the distance from the point charge to the detector, r, is equal to (x2+y2)1/2. Explain why r is the distance between the charge and the detector.
Record the electric field and distance data at a number of points (certainly more than five) and then click on the Wrench button to open up DataTool, a data analysis tool, with your recorded data in columns. Some notes on using DataTool:
DataTool automatically draws lines between nearby points and you may find this confusing. Click on the checkbox with a line through it (above the data) to remove the connecting lines.
If you want DataTool to Fit the data, click the Fit checkbox. Click on Auto-fit to let the computer complete the best Line fit. This is probably not a good fit.
Since the electric field decreases as you get further way, you will need to try an equation not in the list by editing 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 Line1 = a/x^3.
Try E = a/r, E=a/r2 and E=a/r3. Which is the best fit?
For your equation, if r is very large, what should the value of E be? Is this true for your fit?
If r is measured in meters and E is given in N/C, what is the charge of the yellow charge at the orgin?
References:
Giancoli, Physics for Scientists and Engineers, 4th edition, Chapters 21 (2008).
Credits:
The Point Charge 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: and in the OSP ComPADRE collection .