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

assumption:
test charge by default is positive,

condition:
When both q1 and q2 are negative,

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

request:
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!

thanks!

in case you are wondering, my source code is attached