Click [b]+/-[/b] to add or remove charge. You can drag the charge to different position. However, it is fixed at the same position.
Click [b]Test[/b] will generate [u]test charge[/u] which will be accelerated by the E field generated from the above charges.
You can enter electric potential as a function of x,y then click [b]U(x,y)[/b] button to set up Electric field.