Figure 3 shows the symmetric electric lines for two charges A and B of equal magnitude but of opposite sign. This charge configuration is called an electric dipole. In this case, the number of lines that begin at the positive charge must equal the number terminating at the negative.

d

- q + q

Figure 3

The electric dipole moment (p) for an equal and opposite electric charges (q) is equal to the multiple of the charge by the distance between them and directed toward the positive charge, as shown in the Figure 3.

p = q d …………………………….. (2-7)

The electric dipole potential (V)between two point charges – Q and + Q separated by a distance (d) at a point P as shown in the Figure 4 below is:

V = Kq(1/r_b – 1/r_a) …………… (2-8)

Figure 4