Polarization, for Better or Worse
In a previous article we discussed the phenomenon of induction,
that is, the effect of an electric field on a conductor (see Mr.
Static in the May/June issue). If a conductor is placed in an
electric field, charges will move within the conductor until the
interior field is zero. If the conductor is grounded, the free
induced charge disappears. If the ground connection is broken
and the conductor is removed from the field, the conductor will
retain a net charge. It has been charged by induction.
Now suppose that the material placed in the electric field is
an insulator. This being the case, the above processes cannot
take place because of the absence of mobile charge carriers. But
the field may disturb the otherwise symmetrical distribution of
positive and negative charges in the molecular structure of the
insulator. Where this slight relative shift of electrons and nuclei
is created in an electric field we have an effect called polarization.
But before we begin explaining what polarization is, we should
touch briefly upon what it means to compliance engineers and other
electronics industry professionals. And its meaning is twofold.
For polarization can be not only beneficial, making it possible
to increase the capacitance of capacitors, but also detrimental,
causing plateout to occur on all types of surfaces in an electric
field, for instance on wafers in cleanrooms.
Figure 1. Atom in field-free region
with nucleus (A) and symmetrical electron cloud (B).
Figure 2. Atom in electric field
(E) with nucleus (A) and asymmetrical electron cloud (B).
Figure 3. Electrical dipole.
Figure 5. Simplified dipole string.
Figure 1 shows an atom in a field-free region. The time-mean
distribution of the atom's charges is symmetrical, so that there
is no external field. The atom is neutral. If an electric field
E is applied (Figure 2), the symmetry will be disturbed. The electrons,
or rather the center of distribution of the electrons, will be
displaced in the opposite direction of the field. For some materials,
the nucleus may shift its position in the direction of the field.
The situation shown in Figure 2 may be represented by a negative
charge and a positive charge separated by a distance dependent
on the field strength (see Figure 3). This is called an electrical
dipole, and such dipoles are formed throughout the insulator,
hence the name polarization.
An insulator in which dipoles may be formed is often called a
dielectric. The dipoles line up end to end along the field lines.
If the field is rectilinear, we can imagine a situation like the
one shown in Figure 4. The internal positive and negative charges
cancel each other, and the dipole string acts like one long dipole
(see Figure 5). So what will these polarization dipoles do to
the field inside the dielectric? Let's answer this question in
A conductor "A" placed in an electric field with the strength
E0 is shown in Figure 6. The field binds (in this case)
a negative charge on the left side (i.e., the bound induced charge)
and frees an equally large positive charge on the right side (i.e.,
the free induced charge). Thus, the total field inside the conductor
is zero. The free charge may be removed if the conductor is grounded.
(An explanation of this situation was given in the article on
induction in CE's May/June issue).
Figure 6. Conductor in electric field
Figure 7. Dielectric in electric field
Figure 8. A parallel-plate capacitor
connected to an electrometer.
Figure 9. Polarization plateout.
If, however, the body A is a dielectric, the situation is different
(see Figure 7). The external field E0causes polarization,
that is, it forms dipole strings. These strings will be stacked
on top of each other, creating a dipole field Ep in
the opposite direction of and superimposed on E0. The
is smaller than E0, and it turns out that
is a constant characteristic for the dielectric in question.
The value r
is called the relative permittivity or dielectric
constant. Many commonly used dielectrics have r
values from 2 to 7. The charges of the dipoles are
called polarized charges. In contrast to induced charges, polarized
charges cannot be removed from the dielectric. The situation shown
in Figure 7 is the simplest possible, with the external field
being homogeneous and the field lines being perpendicular to the
sides of the dielectric. If the field lines are not perpendicular
to the sides of the dielectric, E0 and E will not be
parallel. In such a case, a "refraction" happens at the interface,
and we have a parallel to Snell's law of optical refraction, where
the optical refractive indices are substituted by the relative
Let's consider two practical effects of polarization.
Effect on Capacitance. Figure 8a shows a parallel-plate
capacitor connected to an electrometer. The assumption is that
the capacitance of the electrometer is negligible compared with
that of the parallel-plate capacitor. There is air (or vacuum)
between the capacitor plates. The system is charged with a charge
q, and a voltage Vv is displayed
on the electrometer. When the space between the capacitor plates
is filled with a dielectric (Figure 8b) the voltage drops to Vd.
As previously explained, the field strength in the dielectric
will be r
times smaller than it was in air, and because the
voltage difference across the capacitor is the field strength
times the plate spacing, s, we have the following:
And, since the charge q is the same in the two situations,
Cd = r
The capacitance thus increased by the factor r
when the interspace was filled by the dielectric.
Using a dielectric in a capacitor has another advantagean increase
in the breakdown voltage. This occurs because the breakdown field
strength of a dielectric is usually considerably higher than that
Polarization Plateout. Figure 9 shows an airborne, insulative
(dielectric) particle P in an inhomogeneous field E. The field
polarizes the particle. The positive and negative polarized charges
have the same numerical value, but because the field strength
is higher at the positive end than at the negative one, the positive
force F+ will be stronger than the negative
force F. The result is a net force
F = F+ F
in the direction of increasing field strength. The uncharged
particle tends to move in an inhomogeneous field and eventually
lands or plates out on the first solid or liquid surface intersecting
the field lines. (It should be noted that if the particle P is
conductive, induction will make it behave in a similar way.)
Suppose we have a positively charged surface, for instance, a
sheet of plastic. The sheet will obviously attract negatively
charged, airborne particles and reject the positive ones, but
what may be just as relevant is that it definitely will also make
the neutral particles move, not necessarily toward the charged
surface, but always in the direction of increasing field strength.
An important example of this is the occurrence of static charges
in cleanrooms. Although the air is clean, there are always some
airborne particles around. For example, if a wafer carrier has
a charge, it may cause, by polarization plateout, some of the
particles to land on the wafer surface with very unwanted results,
at worst a ruined wafer.
Let's finish this by looking at another well-known example of
polarization plateoutthe field in front of a monitor or TV screen.
The field (created by the electrodes in the tube) is strongest
at the surface of the screen, so the particles plate out there.
Now if a viewer faces the front of the screen, he or she will
be virtually at ground potential, so the field lines will converge
toward his or her face, especially around the nose and chin, and
possibly around the ears. These areas are now the primary sites
of plateout. It has been demonstrated that the plateout rate of
airborne particles to the viewer's face is much higher in the
situation just discussed than when the viewer is in a field-free
region. It has also been suggested that this effect could be the
cause of skin diseases and other ailments contracted in the presence
of airborne allergens. This claim, however, does not seem to have
been scientifically proven.
The purpose of this article has been to give an idea of some
of the basic features of the phenomenon of polarization. It should
be stressed, however, that the presentation is far from complete.
A thorough treatment would have resulted in a much longer paper
with an exercise in atomic physics and higher mathematics that
probably would have scared some readers away.
Niels Jonassen, MS, DSc, is retired from the Technical University
of Denmark, where he has conducted classes on static electricity,
ions, and indoor climate. After retiring, he divided his time among the
laboratory, his home, and Thailand, writing on static
electricity topics and pursuing cooking classes. He passed away in 2006.
to July/August Table of Contents