Describe the nature of a plasma.
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From: http://www.nasaexplores.com/show_58_teacher_st.php?id=030410101246There are three classic states of matter: solid, liquid, and gas; however, plasma is considered by some scientists to be the fourth state of matter. The plasma state is not related to blood plasma, the most common usage of the word; rather, the term has been used in physics since the 1920s to represent an ionized gas. Space plasma physics became an important scientific discipline in the early 1950s with the discovery of the Van Allen radiation belts. Lightning is commonly seen as a form of plasma.
Plasmas, the "4th state of matter", are the dominant form of matter in the universe. As material is heated from solid, to liquid, to gas phase, and then even hotter, it begins to become ionized .... that is, one or more of the electrons on an atom become liberated. A plasma can be described as a collection of ionized particles, which interact collectively by long-range electromagnetic forces associated with their charges and motion. When the inter-particle binding energy is small compared to the average kinetic energy of the particles, then a plasma exhibits its most complex collective behaviors.
Fortunately for human beings and other living things, plasmas are not routinely encountered on the surface of the Earth. However, we do see natural plasmas in the form of lightning and auroras, and man-made ones most commonly in the form of fluorescent light bulbs. A little farther away from home, the Earth's magnetosphere contains a plasma which is populated by particles of the solar wind interacting with ones from the Earth's upper atmosphere. The solar corona is a good example of a hot plasma. Each planet with a magnetic field tends to have its own plasma, such as around Jupiter, or even the moon Io. As comets come into the inner solar system, a plasma tail often develops (in addition to non-charged dusty tail), and was easily visible on the recent Hyakutake and Hale-Bopp comets, for example. Beyond the solar system, in interstellar space, ions and electrons (at very low densities) are found everywhere. Beautiful plasma nebula can be photographed from earth, excited by light from nearby stars. Generally, every energetic object in the universe has a plasma associated with it. We can detect plasmas by observing their electromagnetic radiation (light, x-rays, radio waves, etc), or in some cases, by observing the interaction of their ions and electrons with other objects.
Matter changes state as it is exposed to different physical conditions. Ice is a solid with hydrogen (H2) and oxygen (O) molecules arranged in regular patterns, but if the ice melts, the H2O enters a new state: liquid water. As the water molecules are warmed, they separate further to form steam, which is a gas. In these classic states, the positive charge of each atomic nucleus equals the total charge of all the electrons orbiting around it so that the net charge is zero. Each entire atom is electrically neutral.
When more heat is applied, the steam may be ionized: an electron will gain enough energy to escape its atom. This atom is left one electron short and now has a net positive charge; now it is called an ion. In a sufficiently heated gas, ionization happens many times, creating clouds of free electrons and ions; however, not all the atoms are necessarily ionized, and some may remain completely intact with no net charge. This ionized gas mixture, consisting of ions, electrons, and neutral atoms, is called plasma. A plasma must have sufficient numbers of charged particles so that the gas, as a whole, exhibits a collective response to electric and magnetic fields. Plasma density, therefore, refers to the density of the charged particles.
Although plasma includes electrons and ions and conducts electricity, it is macroscopically neutral: in measurable quantities, the number of electrons and ions are equal. The charged particles are affected by electric and magnetic fields applied to the plasma, and the motions of the particles in the plasma generate fields and electric currents from within. This complex set of interactions makes plasma a unique, fascinating, and complex state of matter.
Plasma is found in both ordinary and exotic places. When an electric current is passed through neon gas, it produces both plasma and light. Lightning is a massive electrical discharge in the atmosphere that creates a jagged column of plasma. Part of a comet's streaming tail is plasma from gas ionized by sunlight and other unknown processes. The Sun is a 1.5-million-kilometer ball of plasma, heated by nuclear fusion.
Scientists study plasma for practical purposes. In an effort to harness fusion energy on Earth, physicists are studying devices that create and confine very hot plasmas in magnetic fields. In space, plasma processes are largely responsible for shielding Earth from cosmic radiation, and much of the Sun's influence on Earth occurs by energy transfer through the ionized layers of the upper atmosphere.
Image courtesy of http://FusEdWeb.llnl.gov/CPEP/
In the lower atmosphere where we live, any atom that loses an electron (say, by being hit by a fast cosmic ray particle) soon recaptures it or one like it. The situation is quite different at high temperatures, such as exist on the Sun. The hotter the gas, the faster its atoms and molecules move, and at very high temperatures, the collisions between such fast-moving atoms are violent enough to rip off electrons. In the Sun's atmosphere, a large fraction of the atoms at any time is "ionized" by such collisions, and the gas acts as a plasma.
Unlike cool gases (e.g. air at room temperature), plasmas conduct electricity and are strongly affected by magnetic fields. The fluorescent lamp, widely used in the home and at work, contains a rarefied inert gas with a fraction of a percent mercury vapor, which produces a plasma when heated and agitated by electricity, from the power line to which the lamp is connected. The power line makes one end electrically positive, the other negative (see drawing below) causing (+) ions to be accelerated towards the (-) end, and (-) electrons to the (+) end. The accelerated particles gain energy, collide with atoms, eject additional electrons and thus maintain the plasma, even if some other particles re-combine. The collisions also cause mercury atoms to emit light, and in fact, this source of light is more efficient than conventional lightbulbs. Neon signs and streetlights operate on a similar principle, and some plasma devices are (or were) used in electronics.
[In case you ask: when the fluorescent lamp is first turned on, the gas is cold, but a few free ions and electrons are always present, due to cosmic rays and natural radioactivity. The filaments at the ends also release electrons. Collisions quickly multiply their number.
A plasma is a very hot gas, hot enough to conduct electricity. It better not touch the walls around it! If it does, it will pass some of its heat to those walls, and one of two things may happen. If it is dense and contains a lot of heat, it might melt the walls, and if it is rarefied and contains just a little heat, it will lose its heat to the walls, cool down and stop being a plasma.
So how can you contain a plasma? Two ways have been tried.
With a rarefied plasma, one can continually supply fresh energy and keep heating it up. This is what happens with a fluorescent lightbulb, which contains plasma. The wires leading to the lightbulb continually supply fresh electric energy, so the plasma is continually renovated, which allows it to continue carrying an electric current.
The gas inside the lightbulb also absorbs energy from the plasma, and emits it as light, which turns out to be a very efficient way of producing light--more efficient than that of lightbulbs whose light comes from a hot wire.
The other way is to use a strong magnetic force to keep the plasma confined: this is made possible by the electrical properties of a plasma, which determine the way it interacts with magnetism. This is the way favored by researchers trying to confine a dense plasma. The plasma is inside some container, but the magnetic "field" (region of magnetic forces) keeps it from touching the walls. The trouble is that this sort of situation can be very unstable--the plasma also creates its own magnetic field, which modifies the one keeping it trapped, and so far, just when things get interesting, it is likely to shake itself loose, slip away to the walls, cool down and be gone.
A lot of clever physics has gone
into this in the last 50 years, and
scientists have managed to trap denser
and denser plasmas, but even now they
are a bit short of the temperature,
density and trapping time which would
allow them to convert hydrogen to helium
(nuclear fusion) and create energy that
way. The Sun does release fusion energy,
but it traps the plasma in its core by
its enormous gravity, which we cannot
duplicate in the lab.
As noted, the Sun consists of plasma. Another important plasma in nature is the ionosphere, starting about 70-80 km above ground. Here electrons are torn off atoms by sunlight of short wavelengths, ranging from the ultra-violet to X-rays: they do not recombine too readily because the atmosphere becomes increasingly rarefied at high altitudes and collisions are not frequent. The lowest part of the ionosphere, the "D layer" at 70-90 km, still has enough collisions to cause it to disappear after sunset. Then the remaining ions and electrons recombine, while in the absence of sunlight new ones are no longer produced. However, that layer is re-established at sunrise. Above 200 km, collisions are so infrequent that the ionosphere persists day and night.
The topside ionosphere extends many thousands of km into space and merges with the magnetosphere, whose plasmas are generally more rarefied but also much hotter. The ions and electrons of the magnetospheric plasma come in part from the ionosphere below, in part from the solar wind (next paragraph), and many details of their entry and heating are still unclear.
Finally, there exists the interplanetary plasma--the solar wind. The outermost layer of the Sun, the corona, is so hot that not only are all its atoms ionized, but those which have started off with many electrons have several of them (sometimes all of them) torn off, including deeper-lying electrons which are more strongly attached. For instance, characteristic light has been detected in the corona from iron which has lost 13 electrons.
This extreme temperature also prevents the plasma of the corona from being held captive by the Sun's gravity, and instead it flows out in all directions, filling the solar system far beyond the most distant known planets. Through the solar wind the Sun shapes the Earth's distant magnetic field, and the wind's fast flow (~400 km/s) supplies the energy which ultimately powers the polar aurora, the radiation belts and magnetic storm phenomena.
Ions and electrons in space are usually intimately mixed, in a "soup" containing equal amounts of positive and negative charges. Such a mixture is known as a plasma (the same term has a different meaning in medicine; see the history of plasma). In many respects it behaves like a gas, but when electric and magnetic forces are present, additional properties come to light, quite unlike those of ordinary gases.
The ionosphere above our heads is a plasma. Unlike air, it conducts electricity, and in fact, the ionosphere in the polar regions carries large electric currents, as is discussed in a later section. The electric conductivity of the ionosphere, unlike that of metals or seawater, is very much influenced by the Earth's magnetic field. This is a rather special plasma, because the ionosphere also contains a fairly high number of neutral atmospheric molecules, with which the ions and electrons constantly collide.
In contrast, collisions are extremely rare in the solar wind. If this were an ordinary gas, or if the Earth lacked a magnetic field, the solar wind would have penetrated all the way to the top of the atmosphere and would then have flowed around the Earth, the way water flows around a rock in a stream. Something like that in fact happens at the planet Venus, which seems to have no magnetic field of its own. At Earth, however, a strong magnetic field confronts the solar wind, forming a much bigger obstacle than the Earth itself. Because the solar wind is a plasma, it is forced to detour around the Earth's field, creating a large shielded cavity around the Earth--the magnetosphere.
The explanation of space phenomena thus requires a good understanding of plasma physics. Unfortunately, no laboratory can duplicate the large dimensions and the very low particle collision rates found in space plasmas. The behavior of such plasmas can be sometimes simulated by computers, but ultimately, to figure what actually happens, one needs to send instruments into space and study their observations.
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Content provided by
Dr. David P.
Co-author: Dr. Mauricio Peredo
Georgia Perimeter College
Page created by Pamela J.W. Gore
Georgia Perimeter College,
Page created May 19, 2007
Modified June 25, 2007