While investigating Coulomb's Law, I created a graph of F-1/2 vs. r to display results. The gradient of this graph is much steeper than it should be. What factors could affect the electrostatic force to make this happen? Any suggestions would be greatly appreciated! (The experiment was to vary the distance between a charged sphere and earthed plate, and measure the force between them using a force meter. Note: The graph does not display exactly r on the x-axis, but something slightly more complex. However, it represents the same thing so for this question I just left it as r for simplicity).