This Demonstration shows the action of Coulomb's law in the field of several fixed-point charges. You can select the positions and charges of the source particles. When you click the "find initial electric force" button, the initial force on the mobile test charge (black disk) is shown as a vector representing its initial motion. After you vary the charges of any of the objects, click the "reset" button and click "find initial electric force." The results are expressed in terms of the Coulomb's law constant.