Field lines are useful for visualizing vector fields, which are otherwise hard to depict. In this model we consider the electrostatic field of a system of point charges.

The DRAW THE LINE button launches the motion of a test charge q along a force line of the field; the moving charge leaves a trace, making the line visible. To obtain a new line, drag the point q. By clicking on the arrows on both sides of numeric values of the fixed charges, you can change these values and thus, the structure of the field.