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We've heard that space plasma is "neutral." Doesn't that
mean it can't conduct currents?
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Short Answer
No.
Plasma is quasi-neutral, meaning it has approximately
the same number of positive charges (protons or +ions) in a given
volume as it has unbound negative charges (electrons) in the same
volume. Since its charges are free to move independently, unlike in
neutral atoms, plasma is referred to as quasi-neutral
to reduce confusion.
However,
quasi-neutral and nonconductive are not
synonymous. Neutrality has to do with the relative proportion
of positive and negative charges in a given volume. Conductivity
has to do with the freedom of the charge carriers in a medium and the
ease with which an electric current can flow through it.
In
a plasma, the charge carriers are able to move freely and thus the
conductivity of plasmas can be extremely high.
Observations
When
it comes to a discussion of plasmas, the language of high school
electrostatics can create confusion. To properly differentiate the
behaviors of gases and plasmas within the cosmos, it's essential that
we first clarify the language used to describe electric charge and
electric currents in plasmas and other materials. It's particularly
important to understand that, unlike most solids and gases that we
are familiar with, a neutral plasma can conduct electricity.
Since
conductivity and relative charge are independent variables, there can
be four scenarios where they co-exist:
Objects that:
- are
neutral and don't conduct a current;
-
are
neutral but do conduct a current;
-
are
non-neutral but don't conduct a current;
-
are
non-neutral and do conduct a current;
Within
the context of high school electrostatics, one might use the example
of a wooden pith ball hanging on a string from the ceiling. If we add
several electrons to the pith ball (causing it to become charged),
then it's no longer electrically neutral. That is to say, it
has more charged particles of one sign (negative charges) than
particles of the opposite sign (positive charges). Additionally,
since it is made of wood, it is an insulator and an electric current
cannot flow through it. A pith ball, therefore, is non-neutral
and does not conduct electricity.
However,
if an object with a sufficient positive charge were brought into
close proximity, a discharge (spark) might occur in the space between
the oppositely charged objects. Such a discharge may redistribute
charges between the objects, sufficient to return them to a
state close to charge neutrality (approximately equal proportions of
positive and negative charges in and on each object). This is the
traditional context in which non-neutrality is discussed in high
school physics classes.
A
plasma may be considered "quasi-neutral" if it
possesses an equivalent number of (positively charged) ions and
(negatively charged) electrons. Plasma is also oftentimes described
as an "ionized gas", which is to say that some number of
its ions and/or outer valence electrons are freed from bondage
(dissociated). This freedom of the charge carriers makes plasma
highly conductive. Therefore plasma is quasi-neutral
and does conduct an electric current.
A
plasma can exist in different degrees of ionization. For example, we
can clearly observe the Sun's corona. At the temperatures achieved in
the corona, we know that the plasma there will be 100% dissociated.
In other words, there are no "gases" (composed of neutral
atoms) within the Sun's corona.
Within
the interplanetary medium (the plasma contained within the
heliosphere) the degree of dissociation is still around 100%, but the
density is very low. Nevertheless, where it exists, the plasma's
temperature is very high (~100,000K) and the conductivity is
still very high (on the order of metals).
In
the interstellar medium (the plasma pervading the space between
neighboring stars), there are different regions whose degree of
dissociation varies from less than 1% to nearly 70%. Even "gases"
in which the degree of ionization is as low as 0.00001%, may still be
influenced so much by the tiny ionized proportion that the partially
ionized gas as a whole still behaves as a plasma and may still be
significantly electrically conductive.
Now,
suppose that the excitation (from heat, current, etc.) is removed
from the plasma and the positive and negative charges get back
together (they "recombine" into atoms). We would then have
a non-ionized cloud of gas rather than a plasma. This cloud of gas
is also well described by the word neutral and it does not
conduct an electric current.
A
non-neutral plasma, on the other hand, is one that contains an
unequal number of ions and electrons. But, it is still a plasma
because it is still completely or partially ionized. Therefore, it's
non-neutral but does conduct electricity.
We
should now understand, from the above examples, that conductivity is
related to the freedom of the charge carriers and not to the relative
proportions of positive and negative charges in a given volume.
-
Atmospheric
gases usually act as an insulator, that is to say they are
neutral and non-conductive.
-
A
pith ball is a piece of wood (also an insulator) to which
excess charges can be added, making it non-neutral but still
non-conductive.
-
A
non-neutral plasma is an "ionized gas" with an
uneven number of positive and negative charges that are free to move
independently, thus it is non-neutral and does conduct.
-
A
typical plasma has approximately equal numbers of positive
and negative charges and those charges are likewise free to move,
thus it is quasi-neutral and does conduct.
The
outer layer(s) of the sun are composed largely of plasma.
Interplanetary space is filled with low-density plasma. Interstellar
and intergalactic space is filled with extremely low-density plasma.
In fact, up to
99.999%
of the visible universe is composed of matter in the plasma state.
To
argue that the quasi-neutrality of space plasma precludes that plasma
from conducting currents would be erroneous. In light of this
revelation, we suggest that astronomers and astrophysicists should be
re-evaluate many current quandaries and prior assumptions in light of
over a century of low-density plasma discharge experiments by such
pioneering individuals as Kristian Birkeland, Irving Langmuir and
Hannes Alfvén (among others).
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