he simulation has added a new feature, test charge polarity to simulate -e

enjoy!

[img]http://2.bp.blogspot.com/-xrSAvoJhsp8/UgwhpzU1ZpI/AAAAAAAAZvY/aKOkJKd5V6M/s640/charges_2013-08-14_1706up.PNG[/img]
an electron test mass moving through the point of minimum potential between Q2 = +3C and Q1= +1C. the theoretical path is one that the test charge electron moves and oscillates in the curved path as shown. The slight loop shape in due to the unbalanced forces at a far distance that deviates the direction of the electron to go slightly towards to the Q1 in this case. 

[img]http://3.bp.blogspot.com/-95nMHHkH8qQ/Ugwjoac1HTI/AAAAAAAAZv8/xp3GTiuzjW0/s640/soilution_ajc_2013-08-15_0840.png[/img]
solution to  2012 A level H2 P2 Q7d(ii) by jeffrey toh. and his team. thanks for sharing!