Description: You have a choice between four configurations (definitions : Like/Unlike refer to the signs of the charges - Equal/Unequal refer to the magnitudes - so Like+Unequal would be two same-signed charges with different magnitudes). In the top window, the two charges are shown on the X axis, and the "test charge" (a small positive charge) is shown between them. There is a vector illustrated above the test charge (it is offset from the axis to make it easier to see) - that is the net force on the test charge from the other two charges. The directions will either be to the left or to the right (remember, this test charge is confined to the x axis - so this is only the Fx component). You can move the charge anywhere along the x axis to see the effect on the net force vector. The bottom window shows the plot of the net force along the x axis for the test charge. The magnitude of the arrow in the top window is plotted along the vertical axis of the bottom window. Also, the convention is, if the force vector (the arrow) points to the left, it is plotted positive in the graph, if it points to the right, it is plotted negative in the graph.
Things to try :
a) Choose LIKE EQUAL charges - put the test charge a little bit away from the origin (between the charges) .. what happens to it? How about UNLIKE EQUAL - can you find a similar motion? (What about UNLIKE UNEQUAL .. Is this motion as symmetric as the LIKE EQUAL case?)
b) Choose one of the UNLIKE systems - can you place the test charge in between the two and get that motion from a) and b)? - If not, why not?
a) How does the Like Unequal graph differ from the Like Equal ? (Graph 1 vs Graph 2)
b) How does the Like Unequal graph differ from the UnLike Unequal ? (Graph 1 vs Graph 3)
c) How does the Unlike Equal graph differ from the Like Equal ? (Graph 4 vs Graph 2)
d) How does the Unlike Equal graph differ from the UnLike Unequal ? (Graph 4 vs Graph 3)
Extra feature .. animation! If you put the test charge somewhere in the system, you can click the RESTART/PLAY button to "release" the test charge from rest - it will be acted upon by the other charges and move toward or away, depending on the net force on the test charge (if it hits one of the charges, there will be a "collision" and the animation will stop). Now, using that RESTART/PLAY button will start the test charge from rest! If you want to pause the motion, and not move the charge, then continue that same motion, use the RESUME FROM PAUSE button. [See, I found that if you move the charge to a new place, you want to start it from rest .. otherwise the applet would keep the old velocity from the pause - which caused weird, non-physical behavior. So, if you move the charge - use the RESTART/PLAY button ... if you just pause the motion, and then want to continue that motion without you adjusting the test charge .. use the RESUME FROM PAUSE.]
Hey, my charge disappeared! If you test charge flies off the screen to either side, or if it collides with one of the fixed charges - use the RESET button to restore the initial configuration.
Credits : Original problem concept authored by Scott Bonham and modified by Wolfgang Christian. Substantially modified by Scott Schneider.