Direct Energy, 2018a

1 INTRODUCTION 19
Parallel Plate Capacitor
l
w
Partial Turn Inductor
l
w
d thick
d thick
Figure 1.2: Geometry of a parallel plate capacitor and partial turn inductor.
Permeability μ is a measure of the ability of a material to store energy
in the magnetic eld due to currentsdistributed throughout the material.
Materials can also be described by their relative permeability μ r , a unitless
measure.
μ r = μ μ 0
(1.17)
While permeability describes a material, inductance describes a device. The
magnetic ux density in a material is a scaled version of the magnetic eld
intensity.
−→ B = μ
−→ H (1.18)
Often insulatorshave permeabilitiesclose to μ 0 while conductorsused to
make permanent magnets have signicantly larger permeabilities. The right
part of Fig. 1.2 shows a partial turn coil in a vacuum with length l, thickness
d thick , and width w. The inductance and permeability of thisdevice are
related by [11, p. 311]
L = μd thickl
w . (1.19)
Permittivity ɛ is a measure of the ability of a material to store energy
as an electric eld due to charge separation distributed throughout the
material. Materials can also be described by their relative permittivity ɛ r ,
a unitless measure.
ɛ r = ɛ
(1.20)
ɛ 0
The displacement ux density in a material is a scaled version of the electric
eld intensity.
−→ −→ D = ɛE (1.21)
Some insulatorshave a permittivity hundredsof timeslarger than the permittivity
of free space. Permittivity is a measure of ability to store energy
in a material while capacitance is a measure of the ability to store energy
in in a device.