Ejs Open Source 2 Charge Model with Field Vector, Field Lines, Potential Visualization Representation Java Applet by Hwang Fu-Kwun. I did not make this, i only slightly edited it.


reference:
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=539.new#new

You can drag either charge and find out the new electric Field Vector, Field Lines, Potential distribution, need to release the charge for simulation to recalculate.


The field line are calculated in real time. However, please remember field line is not the same as trajectory for a test charge move in the same field.  
You can add test charge if the check box were checked and watch the trajectory of test charge.

It is easy to draw those vectors which represent electric field at different positions.
Because we can calculate the electric field with $\vec{E}=\frac{kq_1}{r_1^2}\hat{r_1}+ \frac{kq_2}{r_2^2}\hat{r_2}$.
However, do you know how those field lines were calculated?

Hint: The tangential components of electric field line is the same as the direction of electric field at the same point.
Answer is
Biot–Savart law ! http://en.wikipedia.org/wiki/Biot%E2%80%93Savart_law

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