This is fantastic applet, it will add to your collection of good simulations for physics learners.

two questions,

1. how can i automatically plot the magnetic field lines?
for example, [img];topic=740.0;attach=1065;image[/img]

2.i changed the dt from 0.5 to 2 but it looks rougher (less accurate). how to make the drawing of magnetic field lines faster n still accurate?

i edited your applet and i notice a bug, changed it for u already. it is posted here and i think i will start a new post on it here

/htdocs/ntnujava/ejsuser/14019/users/sgeducation/lookang/MagneticField2Wire_pkg/MagneticField2Wire.propertiesFull screen applet or Problem viewing java?Add to exception site list
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