i suspect the original codes for the motion of the test mass is incorrectly modeled.

test charge by default is positive,

When both q1 and q2 are negative,

the path of the test charge defy the electric field vectors direction.

chance to update the codes to reflect the physics of force correctly on the motion of the test charge?

can you made it on a separate page say {Force} so that i can track the changes made. i been editing on the current version of your applet :)

My thoughts:
i been trying to categorize the codes into
{Field Lines} {Field Vectors} {Potential} for ease of open source sharing of codes!


in case you are wondering, my source code is attached