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November 2011

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Coulomb's Law and Electric Field Package Applet Page

Coulomb's Law and Electric Field Package: Dipole Trajectory Applet

The EJS Dipole Trajectory model shows the electric field generated by a dipole. A slider can change the separation between the charges. A test charge, with an adjustable initial position, is released from rest in the electric field.

Student Worksheet

  1.  Run the simulation and observe the dipole field due to fixed charges (red and blue) . Also note the trajectory of the test charge.* You can move the test charge anywhere on the screen (except on top of one of the stationary charges). You can also change the separation between the two fixed charges.
  2. Many students are surprised to find that the test charge does not follow the field or go along the direction of the arrows. Explain the motion of the test charge (specifically why it doesn't follow the field arrows)? Where can you place the charge initially so that it does follow the field arrows? Explain.


*Recall that a test charge is defined to feel the effects of an external electric field, but not change the field itself (therefore, there are no field vectors around it).


The Dipole Trajectory Model was created by Anne J 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
Additional information on this applet can be found in the Open Source Physics comPADRE collection.


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