Electric Field Lines

The algebraic equations of Coulomb's Law fully describe an electric field, but they don't help you visualize it. For this, we can use geometric drawings of things called field lines. Field lines begin at positive charges and end at negative charges, although sometimes a line extends off to infinity.

The picture below, which emphasizes the continuous nature of a field lines, can be augmented by coloring the line to illustrate that the field strength along a line is not constant. Field lines can be curved, and at any point on the field line, the direction of the field is tangent to the curve. A second common representation of a field line is a stick with an arrowhead. The length of the stick is a measure of the strength of the field at its ``tail'' end, and the direction of the field is along the arrow.

When the objects' charges do not add to a total charge of zero, then some of the field lines will extend to infinity. When you look at a drawing of field lines, do not think that the absence of a field line means that there is no field. They are more analogous to a fluid that extends everywhere in space than to a trajectory of a particle.

Here is a program that simulates the electric field of a positively charged point particle. Move the mouse cursor around to see how the length and direction of the field arrow change. Also click the right mouse button to draw field lines, and then hit E, backspace, or delete to clear the field lines. (Mac users: clover-click instead of right-click.)

This next example is the same, except the charge is negative.

What do you think the electric field would look like if there were two charges of opposite sign some distance apart? For one thing, most of the field lines are now curved. Try it:

Field lines show the general trend of the electric field in some region of space, rather than what it is doing at each and every point. Also, keep in mind that in order to find the sign of the charge, you need to observe the direction of the field near a charge. Mathematical equations do a better job of describing the field at any location and provide a quantitative result, but the concept of the field is easier to grasp when it is visualized. You should become familiar with both geometric and algebraic representations of the electric field.

Now work with your classmates on some more complicated examples.

There is also an animated example of field lines that you can watch.