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Coulomb Pendulum Model

Two charges hanging from wires, repel each other. The simulation shows the motion and reports angle (from the vertical) for each charge. You can change the charge (in μC), initial position and damp the motion (set velocity equal to zero) of the charges. Users can examine the model if Ejs is installed. 


  1. For the same initial charge on each, push the v=0 button until the charges balance. What is the angle?
  2. If you increase the value of one charge, what difference do you expect in the angles where they will balance? Will it still be symmetric? Explain and then try it. 
  3. The charge is given in μC and the support string for each is 1m in length. Determine the mass of each (they have the same mass). 


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


The Coulomb Pendulum Model was created by Anne Cox and converted to JavaScript by Fremont Teng and Loo Kan Wee using the Easy Java Simulations (EJS) authoring and modeling tool.

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 <>.


For Teachers


Software Requirements


Android iOS Windows MacOS
with best with Chrome Chrome Chrome Chrome
support fullscreen? Yes. Chrome/Opera No. Firefox/ Sumsung Internet Not yet Yes Yes
cannot work on some mobile browser that don't understand JavaScript such as..... cannot work on Internet Explorer 9 and below



Anne J Cox; Fremont Teng; Loo Kang Wee

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Sample Learning Goals


For Teachers



Charge Field Box and Slider

The charge of the particles can be set by inputting inside their respective field boxes
or by toggling their respective sliders.

Zero Velocity Button

Pressing this button will set the velocities of the two particles to zero.

Draggable Boxes

When the simulation is paused, two draggable boxes and texts will appear over the lines.
These boxes are draggable and allows the user to move and set the particle's initial position.
Note that you won't see this option if the simulation is playing.

Toggling Full Screen

Double clicking anywhere in the panel will toggle full screen.
Note that this won't work if the simulation is playing.

Play/Pause, Step and Reset Buttons

Plays/Pauses, steps and resets the simulation respectively.






Other Resources


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