- 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 closer together the field lines are, the stronger the magnetic field.
- Unlike electric field lines (which terminate on electric charges), magnetic field linesneverterminate.
They form closed loops, or sometimes may form a pattern that continues indefinitely without repeating
or terminating.
31.4 The Magnetic Dipole
As an example, consider Fig. 31.1, which shows the magnetic field due to a bar magnet; such an arrangement
of two magnetic poles separated by a fixed distance is called anmagnetic dipole.
Figure 31.1: Dipole magnetic field due to a bar magnet. (©GNU-FDL, Wikimedia Commons [11].)A magnetic dipole may be characterized by a quantity called themagnetic (dipole) moment. The magnetic
momentmof a magnetic dipole is defined as
mDqd; (31.3)whereqis pole strength of either end of the dipole, anddis a vector whose length is equal to the distance
between the poles, and which points from theSpole to theNpole (opposite the direction of the magnetic
field line between the poles). The dipole moment essentially measures how magnetically “polarized” a dipole
is, with larger values when more pole strength is separated by a greater distance. Magnetic dipole moment is
measured in units of A m^2.
31.5 Magnetic Flux
Magnetic fluxmay be thought of as being proportional to the total number of magnetic field lines passing
through a given area. Given an areaAembedded in a magnetic fieldB, the electric fluxˆBpassing through
Ais equal to the product ofBand the component ofAperpendicular to the field:
ˆBDBADBAcos: (31.4)