The direction of electric intensity in an electric field can also be indicated by drawing lines. These lines are known as electric lines of force. They originate from a positive charge and terminate at a negative charge and are such that the tangents drawn at various points on a line give the direction of electric intensity at those points.
Fig. 15.8 shows an electric line of force. Tangents drawn at three points A, B, C on this line have been shown by the sign of arrows which indicate direction of electric intensity at these in the direction of these tangents. Fig. 15.9 shows lines of force of some common electric fields.
Fig. 15.9 (a) shows the lines of force due to a positive charge. By electrostatic induction this positive charge induces negative charge on the surrounding body; the lines of force terminate at these charges. The induced negative charge has not been shown in the figure. Similarly, fig. 15.9 (b) also does not show the oppositely induced charge.
The number of lines of forces is also related with the strength of the field. The lines are close together in the region where the field is strong and are far apart in the weaker regions of the field. Thus, just by seeing the lines of force, we can get information about the magnitude and direction of electric intensity.
Fig. 15. 9 – Lines of force of different electric fields.