Chapter 2: Force of a Point Charge

Interactive 1

The forces of attraction/repulsion between two point charges  and  (charges whose size is much smaller than the distance between them) are given by Coulomb’s law:

where  m/F in SI units, and R is the distance between the two charges. Here,  is the force exerted on , and  is the force acting on . The unit vector  points from charge 2 toward charge 1. Accordingly, .

In the arrangement below,  (C) is the source-charge. Its position and value are fixed.

 is the test-charge whose force  we “measure”. The magnitude of the test-charge is fixed at  (C); however, you can change its sign by clicking on the “sign” window.

Click on the test-charge and pull it to an arbitrary location within the shaded area to observe the change in the magnitude and direction of the force (the blue arrow).

 
Interactive Applet

Force of a Point Charge