18.1.3—An electric field by means of field lines

Electric field lines point in the direction of force on a positive test charge; closer lines indicate stronger field qualitatively.

Lines start on positive charge and end on negative charge or infinity, do not cross, and are perpendicular to conductor surfaces in electrostatic equilibrium.

Parallel equally spaced lines represent a uniform field between large oppositely charged plates.

A negative charge moves opposite to the arrow direction, and field-line density is a model rather than an exact numerical scale.