Electric Field Lines

Electric lines of force, also called electric field lines, are imaginary lines that help us visualize and understand the electric field.

Definition:

A line of force in an electric field is the direction along which a free test charge would move.

For example, near a positive point charge, a free test charge is repelled away along a radial straight line. So the lines of force are radial, directed away from the charge (see drawing on the left).

One can also say that a line of force in an electric field is an imaginary line such that the tangent drawn to it at any particular point gives the direction of the field at that point. For a set of straight field lines like the ones in the picture, this does not give any additional information, but we will see that field lines can also be curved.

We can also get the magnitude of the field from lines of force: their number per unit cross-sectional area perpendicular to the lines is proportional to the field. So, for example, if there is a crowding or concentration of the lines in any area of space, the field there is stronger. The opposite is also true: if there are few field lines in an area, then the field is weak.