Static Electricity 2000 Edition

D.1.19 Current (I).
A measure of the rate of transport of elec-
tric charge past a specified point or across a specified surface.
The symbol I is generally used for constant currents and i for
time-variable currents. The unit of current is the ampere. One
ampere equals 1 coulomb per second.
D.1.20 Density, Bulk.
The mass per unit volume of a dust pile
or a dust deposit.
D.1.21 Dielectric Breakdown.
A voltage-dependent failure
mechanism that occurs when a potential difference is applied
across a dielectric region that exceeds the region’s inherent
breakdown characteristics.
D.1.22 Dielectric Constant.
The ratio of the permittivity of a
material to the permittivity of a vacuum that indicates a mate-
rial’s ability, relative to a vacuum, to store electrical energy or
charge, when the material is placed in an electric field. Typical
dielectric constants and dielectric strengths are shown in
Table D.1.22.
A dielectric is not necessarily an insulator. For example,
water, which has a high dielectric constant, is not a very good
insulator. The measure of a good dielectric is its polarizability
rather than its conductivity.
D.1.23 Dielectric Strength.
The maximum electrical poten-
tial gradient (electric field) that a material can withstand with-
out rupture, usually specified in volts per millimeter of
thickness; also known as electric strength or breakdown
strength. See Table B.2 for typical data. The maximum dielec-
tric field strength in atmospheric air is around 3 
× 10
6
V/m.
Table D.1.12 Electrostatic Voltages (kV) Resulting from 
Triboelectric Charging at Two Levels of Relative Humidity
Situation
RH 10–20%
RH 65–90%
Walking across carpet
35
1.5
Walking over vinyl 
floor
12
0.25
Working at bench
6
0.1
Vinyl envelopes for 
work instructions
7
0.6
Poly bag picked up 
from bench
20
1.2
Work chair padded 
with polyurethane 
foam
18
1.5

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APPENDIX D
77
–53
2000 Edition
This figure implies the existence of maximum value of surface
charge density. Oscillating electric fields produce a signifi-
cantly greater stress on materials than do time-independent
fields. For this reason, insulation systems based on DC ratings
will fail quickly when used with AC power units.