We can’t see the electric field. Consequently, we need pictorial tools to help us visualize it in a region of space. One method, introduced in Chapter 22, is to picture the electric field by drawing electric field vectors at various points in space. Another way to picture the field is to draw electric field lines. As FIGURE 23.7 shows,

• Electric field lines are continuous curves tangent to the electric field vectors.

• Closely spaced field lines indicate a greater field strength; widely spaced field lines indicate a smaller field strength.

• Electric field lines start on positive charges and end on negative charges.

• Electric field lines never cross.

The third bullet point follows from the fact that electric fields are created by charge. However, we will have to modify this idea in Chapter 30 when we find another way to create an electric field.

FIGURE 23.7 Electric field lines.

FIGURE 23.8 shows three electric fields represented by electric field lines. Notice that the electric field of a dipole points in the direction of $\vec{p}$ (right to left) on both sides of the dipole, but points opposite to $\vec{p}$ (left to right) in the bisecting plane.

FIGURE 23.8 The electric field lines of (a) a positive point charge, (b) a negative point charge, and (c) a dipole.

Neither field-vector diagrams nor field-line diagrams are perfect pictorial representations of an electric field. The field vectors are somewhat harder to draw, and they show the field at only a few points, but they do clearly indicate the direction and strength of the electric field at those points. Field-line diagrams perhaps look more elegant, and they’re sometimes easier to sketch, but there’s no formula for knowing where to draw the lines. We’ll use both field-vector diagrams and field-line diagrams, depending on the circumstances.