In this applet you can move the two charges by dragging them with the mouse. You can also change their magnitudes (and signs) by clicking on the boxes.
Disclaimer: If you have an older, slower machine, this applet will work annoyingly slow. It just takes quite a number of operations to perform the calculations that are needed to display the E-field lines.
We have provided radio-buttons for you to change the precision (and with it the speed) of the calculations. The most precise setting takes about 50 times longer to compute than the quick setting. For slower machines, we recommend that you first drag the charges around using the quick setting. Then you can compute the precise E-field with the most accurate setting. But you may have to wait a few seconds for your computer to complete the precise computations.