The word ion (in Greek, ion)
means wanderer. It denotes an entity, a particle, that will
move under the action of an electric field. So, in principle,
valence electrons in metals or holes in semiconductors could
be considered ions. But in practice, the name ion is reserved
for two species: electrolytic ions and gaseous ions.
If you have an aqueous solution of silver nitrate,
the AgNO3 is dissociated as
Ag+ + NO3
Ag+ is called a silver ion and NO3
a nitrate ion.
If an electric field is now applied to the liquid,
the positive silver ions will move in the direction of the
field toward the negative cathode, where they each will
receive an electron, become neutralized, and plate out onto
the electrode. This is the basis for electroplating.
A somewhat similar process takes place at the anodebut
we are not going to discuss electrochemistry in detail.
Rather, I will point out just a few facts about electrolytic
ions. The silver and nitrate ions, as well as other electrolytic
ions, have well-defined properties. All silver ions are
identical, at least chemically speaking, and they never
change their properties no matter what you do to them, as
long as they remain ions.
If a given ion is exposed to an electric field with
the strength E, it will move with a constant velocity v
where k is a constant representing the mobility of the
ion. Again, a silver ion always has one positive charge
and always the same mobility, at least when you consider
a given isotope of silver. The same constancy is true for
any other electrolytic ion.
Although ions may be formed in most gases, we will
restrict ourselves here to discussion of those types of
ions that may be formed and found in atmospheric air, the
so-called air ions or atmospheric ions.
|Figure 1. How air ions are formed.
The formation of an air ion starts with an electron
being knocked off a neutral air molecule, as shown in Figure
1. The now positive molecule (oxygen or nitrogen) will rapidly
attract a number of polar molecules (1015), mostly
water, and this cluster is called a positive air ion. The
electron will probably attach to an oxygen molecule (nitrogen
has no affinity for electrons), and this negative molecule
will attract a number of water molecules (maybe 810),
forming a cluster called a negative air ion. It is important
to note that ions are always formed in pairs, and always
the same number of positive and negative ions.
It takes a certain energy, about 34 eV (~5.4 x1018
J) to knock off the initial electron. This energy may be
delivered by shortwave electromagnetic radiation (x-rays
or gamma rays), or more often from a colliding particle.
Most of the ionization in the lower atmosphere is
caused by airborne radioactive substances, primarily radon
and its short-lived daughters. In most places of the world,
ions are formed at a rate of 510 pairs per cm3
per second at sea level. With increasing altitude, cosmic
radiation causes the ion production rate to increase. In
areas with high radon exhalation from the soil (or building
materials), the rate may be much higher.
It is primarily alpha-active materials that are responsible
for the ionization. Each alpha particle (for instance, from
a decaying radon atom) will, over its range of some centimeters,
create approximately 150,000200,000 ion pairs.
Although ionization from radioactive sources (often
a polonium isotope) is used for technical purposes, and
for certain applications it is to be preferred for any other
method, the most common artificial method of producing ions
is by field ionization.
It's somewhat ironic to realize that this method
presupposes an ongoing, however weak, natural ionization.
If a sufficiently strong electric field is establishedfor
instance, between an electrode at a potential of some kilovolts
and a groundthe electrons being freed by natural ionization
may be accelerated to such velocities that they themselves
can cause ionization, again creating pairs of (positive
and negative) ions. It should be stressed that it does not
take a high voltage, but high field strength, to cause ionization.
The breakdown field strength, as it is called, is
somewhere around 3 MV/m between plane electrodes (in air
at atmospheric pressure). If you have two metal plates at
a distance of 1 cm, you need a voltage difference of about
30,000 V for ionization to take place in the space between
the plates. If, however, one of the plates is replaced by
a sharp metal point or a thin wire, the necessary voltage
may be only a few kV. The explanation is that for a given
voltage difference, the field strength in front of a point
is much higher than between plane electrodes. Thus although
the breakdown field strength is higher in front of a point,
ionization is still established at lower voltages using
Now let's imagine an electrode, say a sharp metal
point, kept at a positive potential of some kV with respect
to ground, which may be represented by the walls of the
room, as shown in Figure 2. In a small volume, perhaps a
few cubic millimeters around the tip of the electrode, ion
pairs are formed. The negative ions are attracted to the
electrode, where they give off their charge and cease to
exist as ions. The negative charge from the ions runs through
the electrode to the voltage supply, making it look as though
the electrode delivers a positive current to the air. The
positive ions, formed in front of the electrode, are repelled
by the electrode and move away. All in all, it appears that
positive ions are emitted from the positive electrode.
|Figure 2. Field inonization, caused by an electric
field between an electrode and ground.
But this conclusion is completely wrong. The positive
ions have never been in contact with the electrode. The
electrode, often called an emitter, doesn't emit anything.
Rather, it collects things (specifically, negative ions).
Sadly, it's probably too late to change this linguistic
Ions don't live forever. They may recombine with
oppositely charged ions or, more likely, combine with aerosol
particles in the air. The charged particles, sometimes called
large ions, will also move in an electric field, although
much more slowly than the air ions do.
This is the principle for the first technical electrostatic
invention, the electro filter, without which we would have
no means of effectively cleaning the smoke from coal- or
oil-fired power plants and many other industrial installations.
Ions may also plate out onto surfaces, either by
diffusion or aided by an electric field. And this is the
basis for another important technical use of ionization.
Let's assume we have a charged insulator and we want
to remove the charge.
Well, let's face it. It can't be done. There's no
way by which a charge can be removed from an insulator.
But don't panic. The charge in itself doesn't do any harm.
It's the field from the charge we have to worry about. And
the field may be used to neutralize itself.
If the charged insulator is exposed to an atmosphere
containing ions of polarity opposite that of the charge,
the field will attract ions, which will move toward the
body and neutralize the charge. At least that's what appears
But a more strict formulation would be that the original
(excess) charge is still there, and so is its field. The
oppositely charged ions, attracted from the air, will deposit
around the original charge, but not annihilate it. The resulting
field, the sum of the fields from the opposite charges,
will be zero, or at least very close to zero.
The use of air ionization for abating static electric
effects is a slow method, compared to methods like the grounding
of conductors or surface treatment with topical antistats.
But it should be stressed that when we are talking about
charged insulators, exposure to ionized air is the only
method to remove the effects of the charge.
Soon after the discovery of atmospheric ions about
a century ago, it was suggested that the ions might have
an effect on people breathing the air containing the ions.
Among the effects suggested was that air with an excess
of negative ions would feel fresh, while an excess of positive
ions would make the air stuffy.
This popular but still undemonstrated belief will
be the subject of a subsequent column on static electricity
Niels Jonassen, MS, DSc, retired from the Technical University of Denmark, where he conducted classes on static electricity. After retiring, he divided his time among the laboratory, his home, and Thailand, writing on static electricity topics and pursuing cooking classes. He passed away in 2006. .
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