 |
| Virtually all materials, including water and dirt particles in the air,can be triboelectrically charged. How much charge is gen, where that charge goes, and how quickly, are functions of the materials' electrical characteristics. |
| |
| Insulative Materials |
| A material that prevents or limits the flow of electrons across its surface or through its volume
is called an insulator. Insulators have an extremely high electrical resistance, generally greater than 1 x 10E12 ohms/sq (surface resistivity) and 1 x 10E11 ohm-cm (volume
resistivity). A considerable amount of charge can be generated on the surface of an insulator.
Because an insulative material does not readily allow the flow of electrons, both positive and
negative charges can reside on insulative surface at the same time, although at different
locations. The excess electrons at the negatively charged spot might be sufficient to satisfy
the absence of electrons at the positively charged spot. |
| |
| Conductive Materials |
| A conductive material, because it has low electrical resistance, allows electrons to flow easily
across its surface or through its volume. Conductive materials have low electrical
resistance, generally less than 1 x 10E5 ohms/sq (surface resistivity) and 1 x 10E4 ohm-cm
(volume resistivity). When a conductive material becomes charged, the charge (i.e., the
deficiency or excess of electrons) will be uniformly distributed across the surface of the
material. If the second conductor is
attached to an earth grounding point, the electrons will flow to ground and the excess charge
on the conductor will be "neutralized." |
|
| Static Dissipative Materials |
| Static dissipative materials have an electrical resistance between insulative and conductive
materials (1 x 10E5 - 1 x 10E12 ohms/sq (surface resistivity) and 1 x 10E4 - 1 x 10E11 ohm-
cm (volume resistivity). There can be electron flow across or through the dissipative material,
but it is controlled by the surface resistance or volume resistance of the material.
As with the other two types of materials, charge can be generated triboelectrically on a static
dissipative material. However, like the conductive material, the static dissipative material will
allow the transfer of charge to ground or other conductive objects. The transfer of charge from
a static dissipative material will generally take longer than from a conductive material of
equivalent size. Charge transfers from static dissipative materials are significantly faster than
from insulators, and slower than from conductors. |
| |
| Electrostatic Fields |
| Charged materials also have an electrostatic field and lines of force associated with them. Conductive objects brought into the vicinity of this electric field will be polarized by a process
known as induction. A negative electric field will repel electrons on the surface of the
conducting item that is exposed tothe field. A positive electric field will attract electrons to
near the surface thus leaving other areas positively charged. No change in the actual charge
on the item will occur in polarization. If, however, the item is conductive or dissipative and is
touched to ground while polarized, charge will flow from or to ground to compensate for the
charge imbalance. If the electrostatic field is removed and the ground contact disconnected,
the charge will be trapped on the item. If a nonconductive object is brought into the electric field, the electrical dipoles will tend to align with the field creating apparent surface charges. A
nonconductor cannot be charged by induction. |
|
