What Is A Plasma?

The fact that blood plasma exists in Medicine, should not be confused with the fact that Plasmas exist, that are defined in Physics, and which all matter can be converted to. In short, a Plasma is what becomes of a gas, when its temperature is too hot, for it to be a gas.

The long form of the answer is a bit more complex. In Elementary School, Students are taught that there exist three familiar phases of a given substance: Solid, Liquid and Gas. But according to slightly more advanced knowledge in Physics, there is no real guarantee, that there will always be these three phases. A gas first results when the thermal agitation between molecules becomes stronger – i.e. their temperature hotter – than the force that holds individual molecules together. At that point, the molecules separate and a gas results, the physical behavior of which is approximately what one would obtain, if a swarm of particles was to exist through collisions but through few other interactions.

Similarly, Liquids will form, when the molecules are forced from occupying fixed positions, but when they still don’t expand.

Well, as the degree of thermal agitation (of a Gas) is increased further, first, molecules become separated into atoms, and then, the electrons get separated from their nuclei, as a result of ordinary collisions with other atoms. This results in the negative particles – electrons – following different trajectories than the positive particles – the nuclei. And the result of that is that the collective behavior of the fluid changes, from that of a gas.

When a charged particle crosses the lines of force, of a magnetic field, a force is generated which is perpendicular to both the velocity vector and the magnetic field vector. As a result, the particles can travel without restriction along the lines of magnetic force, but their motion at right angles to it is deflected, and becomes helical. Not only that, but the direction in which the paths of the particles becomes curved, is opposite for the negative and positive particles.

For this reason, Plasmas can be confined by magnetic fields, except along the lines of the magnetic field. Increasing the strength of an applied field will also cause a Plasma to become compressed, as these helices become narrower.

A good natural example of this type of Plasma, is what becomes of the substance of the Sun. Its temperatures are easily hot enough to cause the transition from Gas to Plasma, especially since the temperature inside the Sun is much higher, than the temperatures which are observed at its surface. At 5000K, gasses are still possible. But at hundreds of thousands Kelvin, or at a Million degrees Kelvin, the bulk of the Sun’s substance becomes a Plasma.

Now, if the reader is a skeptic, who has trouble believing that ‘other phases’ can exist, than Solid, Liquid and Gas, there is an example that takes place at lower temperatures, and that involves Oxygen, namely, O2. We’re aware of gaseous O2 as well as liquid O2 that gets used in rocketry. But as the O2 is cooled further, to 54.36K at 1 atmosphere, it solidifies. Thus, it has already demonstrated the 3 phases which we’re taught about in Elementary School. But, if we cool already-solid O2 below an even lower temperature, 43.8K at 1 atmosphere, its phase changes again, into yet another phase, which is also a solid one. It’s currently understood that solid O2 has 6 phases. (:1)

At the same time, many fluids are known to exhibit Supercritical Behavior, which is most commonly, a behavior of a fluid which is normally differentiated between Liquid and Gaseous, losing this differentiation, due to its critical pressure being exceeded, but at temperatures at which fluids are commonly boiled. This has nothing to do with Plasmas, but without any distinction between Liquid and Gaseous, a substance which is ordinarily though to have three phases – such as water – ends up demonstrating only two: Fluid and Non-Fluid.

So there is no ultimate reason for which matter needs to be in one out of three phases.

(Updated 10/14/2018, 10h25 … )

(As of 10/13/2018 : )

Also, failures of Science to control Fusion, are not due, to a failure to contain Plasmas with magnetic fields. They are failures to keep doing so, at temperatures and pressures – i.e., field intensities – needed to achieve fusion, i.e., at millions of degrees Kelvin. The reason for this is the fact that Plasmas generate a spontaneous field, in reaction to the applied field, such that the spontaneous field can become more intense than the applied field. Under fusion conditions, this makes the Plasmas uncontrollable, and their spontaneous field forms ‘kinks’ in the (self-)contained plasma, the geometry of which breaches the intended apparatus. Such a breach has only minor consequences to the apparatus and the people near it, but also causes a failure to harness fusion.

When experiments to control fusion were still ongoing, most of the focus was on active electronics, that would sense the onset of these kinks, and the purpose of which was to counteract them – by changing the applied field quickly – before they would become pronounced. But precisely because not all feedback-systems are stable, those efforts failed.

Further, so-called ‘low-temperature plasmas’ also exist, the ionic nature of which is not directly due to thermal agitation, but which is due to some other form of excitation, such as just, an intense, applied electrostatic field. This type of plasma is also called an ‘arc’, or an ‘electric spark’, and even though its temperatures may be lower than those of fusion plasmas, this form of plasma still exhibits the essential property, that magnetic fields will deflect it. Further, I think that the Earth’s radiation belts are also examples of low-temperature plasmas.


Further examples of one substance occupying more than 3 phases exist in the element sulfur (in its Chemically pure form). Under ambient conditions, sulfur forms ringed molecules of S8, that give a solid, yellow crystalline phase. But if we try experiments which I remember clearly out of my childhood, to melt sulfur will typically result in an orange liquid forming briefly, which then quickly forms a brown polymer. Next, allowing the polymer to cool off fails to undo this second phase-change, until leaving the polymer at room temperature for more than a minute, causes it to revert to the yellow crystals one started with.

These apparent phases of sulfur could be argued, because they form when the rings of S8, that have successfully become a liquid, break open, and join with other chain-fragments, forming the longer chains that tend to define a polymer. So somebody could claim that this is a Chemical change and not a Physical change. But unfortunately, Chemistry and Physics are ultimately linked. They are both descriptions of matter near the atomic scale, and in the case of sulfur, descriptions involving only one element.

And logically, if we were to heat sulfur sufficiently, in the absence of oxygen, chain-fragments would also break away, to form a compressible, though horrid gas-like phase.

This failure of pure sulfur to form a stable liquid affects how it needs to be mined. One process which I’m familiar with is excavation of deposits at the surface of the Earth. A different process I’m familiar with involves injecting steam into sub-surface sulfur deposits, that causes the sulfur to melt. Because simple valves can control the pressure of steam very accurately, simple valves at the surface can also control its temperature very accurately, such that the sulfur remains liquid long enough to flow out of the mining site.


(Update 10/14/2018, 10h25 : )

Even though other fluids can exhibit a supercritical point, the most-studied case is water. And one idea which I mentioned, was that if the pressure of some substance is supercritical, it will really only exhibit fluid and non-fluid phases.

There is a problem when trying to characterize water in this way. It has the unusual property of expanding when it freezes. What this also means is that extreme pressure will prevent water from freezing, or from staying solid. This is the main reason for which ice-skates work: Their knife-edge will put high pressure on the ice, so that the ice briefly melts beneath the skates. And this is why skates exhibit such a low coefficient of friction. It’s an everyday phenomenon.

But this can also cause water to fail, at entering a solid phase, when under extreme pressures, which are then also supercritical. So then, it would really only demonstrate one phase, not two.

If the subject was Supercritical Methanol, this oddity would vanish.



Technically, it would be incorrect to refer to all the solid phases of Oxygen, as ‘forms of O2‘. The reason for this is the fact that this designation implies the existence of actual O2 molecules, which will only exist if the solid is ‘a molecular solid‘. Because some solid forms of Oxygen are covalent solids, they should just be referred to, as ‘additional forms of Oxygen‘. And I see that the metallic solids are classified unto themselves.



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3 thoughts on “What Is A Plasma?”

  1. Good one about interference patterns.

    Reminds me of the total solar eclipse back in Hubli, India (Longitude (East): 75.23, Latitude: 15.33) back in 1980.

    We wanted to record the interference shadow bands, but they were fast and our exposures abysmally incorrect, we had to do with just observing them.

  2. Back in high school, 1969, I tried to be smart, and told my physics teacher when he was covering states of matter, that there were 4 states of matter.

    Not well received, as you can imagine.

    He said give me an example of plasma: that got me, but I rescued myself by saying the sun and the stars and the fluorescent tube light in the classroom.

    Now at 68, and after my recent visit to CERN Geneva, that still brings a smile on my face.


    1. Fortunately for me, I did not make the experience much, where Science Teachers were factually wrong about something. What I do recall on one occasion though, is that as a High-School Student, I had given a verbal answer, that ordinary light, scattered off a surface, was continually forming interference patterns, of constructive and destructive interference, but that these interference patterns were changing too quickly, to be measured or otherwise observed. I was saying that examples produced with lasers, were only different because in those cases, the interference patterns can be observed. The teacher told me that this was wrong.

      But as it happens, I later learned in life, that the teacher’s answer to that question was actually correct. It’s more correct to say that interference patterns don’t form, when they can’t be observed.

      To make this question a bit less ambiguous, the term which gets used today, is to say that ‘A stable interference pattern forms,’ rather than just to suggest that an unobservable one could also be forming.

      Also, I got into some arguments with Physics teachers, because I had the rather simple notion that light consisted of waves, while the teachers actually knew, that light consists of photons, and that the relationship between photons and waves has issues.


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