Geoscience Reference
In-Depth Information
5.2
Electricity and magnetism
its electric
field is symmetrical about the pole with the
equipotential surfaces, i.e. surfaces created by joining points
having the same electrical potential, forming concentric
portionally with distance from the pole. The gravitational
potential energy of a body increases when it is moved
against the in
uence of gravity, for example when lifted
from the
floor to a high shelf. Similarly, a charged particle
may increase its electrical potential energy when moved
An electric current is a flow of electrically charged par-
ticles: an analogy can be drawn with the
flow of water, i.e.
the movement of H
2
O molecules. The circuit through
which the electric current
flows is represented by a
'
system through which
fluid water
ows. We
use this analogy and the concept of an
plumbing
'
'
throughout our description of the properties and behav-
iour of electricity.
Electromagnetism is the interaction between electricity
and magnetism, an in-depth understanding of which
requires advanced mathematical skills. Fortunately, the
principles of electromagnetism as applied to geophysical
surveying can be adequately understood from a purely
qualitative description.
'
electrical
uid
a)
kg
Gravity
5.2.1
Fundamentals of electricity
-
+
An electric current is made up of moving charges. In solids
it is loosely bound outer-shell electrons that move, a process
known as electronic conduction. In liquids and gases it is
electrons and/or ions (positively and negatively charged)
that move; this process is known as ionic conduction.
A dielectric or displacement current may also be created
by distortion of an atomic nucleus and surrounding elec-
trons. Normally the electrons are symmetrically distributed
about the nuclei of the atoms forming a substance. If the
atoms acquire a consistent asymmetrical form there must
be a net movement of charge through the substance, i.e. an
electric current. In most geophysical situations the contri-
bution of displacement currents are negligible, but they are
important contributors to the geophysical response of radar
and radio frequency EM methods (see online
Appendix 5
)
.
Negative electric
charge
Positive electric
charge
b)
Cation
-
-
-
-
-
+
+
+
+
+
+
Anion/electron
-
-
+
-
-
Polarised
body
(stationary)
c)
Electrical polarisation
-
+
H
O
Electrons
H
5.2.1.1
Electric fields
Electrically charged bodies have an electric
d)
field associated
with them and are therefore affected by other electric
fields,
just as bodies with mass (i.e. everything) create and are
affected by gravitational
fields. In the same way that a mass
has a gravity
field everywhere around it, an electric charge
is surrounded by an electric
field. In both cases the
fields.
act on distant objects; masses always attract, but electrical
charges may be positive or negative, so the forces may be
either attractive or repulsive: like charges repel each other
and unlike charges attract.
Electric
fields are de
ned in terms of electrical potential.
For an isolated compact electrical charge, known as a pole,
-
+
Field lines
Equipotential surfaces
Figure 5.2
Electric charges and their
field of an
isolated electric pole. Note the similarity to the gravity (potential)
fields. (a) The electric
field of a mass. (b) Polarisation of a body in an external electric
field
due to movement of electrical charges within the body. (c) The
polarised H
2
O molecule. (d) The
field of an electric dipole.
Search WWH ::
Custom Search