Electric force on a test charge q due to field producing charge Q.
Version 2 Updates:
Either charge can be positive or negative (N.B. technically test charges are only positive).
Scale slider can be used to keep electric force vector within confines of screen.
Either charge q or Q can be moved.
Designed for use in "modeling instruction". Students can plot electric force versus "q" or "Q" or "r" and show that the slope must be a result of a product of the remaining constants
and k = 9.0x10^9 Nm^2/C^2
