Figure 16.1: Electric field lines for an electric dipole. The dipole moment vectorppoints to the left in this
case. (©GNU-FDL, Wikimedia Commons [11].)
- At any point along a field line, the electric field vectorEis tangent to the field line.
- We cannot possibly drawallfield lines (because they fill all space), so we draw only a few. The
number of field lines you draw is somewhat arbitrary — we just draw enough to visualize the field
without making the diagram too crowded. - The number of field lines terminating on a charge should be proportional to the charge.
- The closer together the field lines are, the stronger the electric field.
16.3 The Electric Dipole
As an example, consider Fig. 16.1, which shows two charges of equal magnitude and opposite sign, separated
by a fixed distance; such an arrangement is called anelectric dipole.
An electric dipole may be characterized by a quantity called thedipole moment. The dipole momentpof
an electric dipole is defined as
pDqd; (16.3)whereqis the magnitude of either of the charges in the dipole, anddis a vector whose length is equal to
the distance between the charges, and which points from the negative charge to the positive charge (opposite
the direction of the electric field line between the charges). The dipole moment essentially measures how
electrically “polarized” a pair of charges is, with larger values when more charge is separated by a greater
distance. Electric dipole moment is measured in units of coulomb-meters (C m).
16.4 Electric Flux
Electric fluxmay be thought of as being proportional to the total number of electric field lines passing through
a given area. Given an areaAembedded in an electric fieldE, the electric fluxˆEpassing through planeS
of areaAis equal to the product ofEand the component ofAperpendicular to the field:
ˆEDEnOADEAcos: (16.4)