Shulou(Shulou.com)11/24 Report--

Let's take a look at a funny thing first.

Its main body is an airtight glass ball filled with some kind of low-pressure inert gas, and the center of the ball is a high-voltage electrode. After electrification, the internal gas produces discharge sparks due to high voltage, forming strips of colored light, forming a "spherical lightning".

When you put your hand on the ball, the colored lightning will concentrate towards the position of the hand. What's more interesting is that it can also light up the nearby fluorescent lights!

Lightning? Does this have anything to do with lightning on earth?

Yes, not only lightning, but also auroras are closely related to it.

Lightning, like a king with super power, though only for a moment, has made people shudder.

The aurora looks gentle and gentle, but it is so rare, except in those specific places.

However, they correspond to the same form of matter. Not only them, but the solar wind that distorts the earth's magnetic field.

There are also the same kind of luminous substances in the neon lights of those night markets.

Of course, it also includes the funny thing in front of it, which is called "plasma ball".

Yes, they are all plasmas, or Plasma.

So, what is plasma?

In addition to solids, liquids and gases, matter has a fourth state, which is plasma.

The reason why a substance changes from solid to liquid and from liquid to gas is caused by the increase of temperature. Similarly, the gas to the plasma is also caused by the increase in temperature.

As the temperature increases, the kinetic energy of molecules, atoms and electrons increases. When the temperature rises to a certain extent, the kinetic energy of the electron is so large that it is out of control, it wants to be free, and neither the molecule nor the atom can bear it, so they have to let the electron go. As a result, part of the material structure collapses and becomes a material form composed of molecules, free electrons and positive ions.

From the above statement, we can see that plasma is actually a kind of high temperature gas. It's just that unlike ordinary gases, it contains free electrons and positive ions.

Then some people will ask: is it true that as long as a gas with electrons and positive ions is a plasma?

No! Only when the concentration of charged particles is high, can it be regarded as plasma. For example, a common question: is flame plasma? The correct answer is: the general flame is only a high temperature gas, not plasma, but if the concentration of charged particles in the flame is high, the flame is plasma.

However, how high is the concentration of free electrons before plasma?

To answer this question, we have to start with the macroscopic electrical neutrality of the plasma.

Although the plasma contains positive and negative charges, it is generally electrically neutral. This determines that there must be enough charge in the plasma. Because only in this way can we ensure that even if there is a concentration of charge anywhere, we can immediately mobilize the opposite charge from somewhere else to balance it, so as to keep it electrically neutral.

It is impossible to have an electric field inside an electrically neutral plasma. In fact, this point can also be explained from the properties of the conductor: the conductor always has enough free charge to shield the external field. For plasma, the electrons in its body are free, so plasma is a very good conductor. According to the electrostatic balance condition of the conductor, it is impossible to have an electric field inside it.

Therefore, macroscopic electrical neutrality is the most important characteristic of plasma.

Suppose a positively charged body is placed in the plasma, and since the plasma is a good conductor, in order to keep the internal electric field at zero, there must be enough electrons running around the charged body-just like the electrostatic induction of the conductor.

But unlike electrostatic induction, because the thermal motion is too strong, these electrons are not on a thin layer like the electrostatic balance of ordinary conductors, and the charge layer they form in the plasma has a certain thickness.

In this charge layer, the plasma material is affected by the electric field. But outside this range, the electric field is shielded, which is the shielding effect of the plasma, which protects the plasma from the influence of the external electric field and maintains the electrical neutrality.

The thickness of this electron layer is called shielding distance. German physicist Debye pointed out that the shielding distance is satisfied.

In the formula, the number density of electrons is the temperature of isoelectrons. It can be seen that the shielding distance is different at different electron concentrations and temperatures.

What is the use of this shielding distance?

Obviously, if the space range of an ionized gas is much larger than the shielding distance, then most of the ionized gas can effectively maintain the macroscopic electrical neutrality.

Based on this point, Debye proposed the determination condition of plasma: the ionized gas whose size is much larger than the shielding distance can be regarded as plasma.

With this formula, the debate about whether it is plasma or not can stop! Everything is subject to the results of the calculation. Why is the flame of a candle not plasma? Because the concentration of its charged particles is not high enough, the shielding distance is too large to exceed the size of the flame itself.

Now that we know what plasma is, let's talk about some of its characteristics.

First, let's look at the temperature of the plasma.

The temperature of the plasma is high and bottomed, such as the plasma material in the fusion reaction, its temperature is as high as hundreds of millions of K; the low temperature, such as the plasma material in the MHD generator, its temperature is usually in thousands to tens of thousands of K. But compared to the daily temperature, these are very high.

The temperature of electrons in neon or argon plasma used as neon lamps is as high as 20,000 degrees Celsius. This seems to be contrary to life experience: 20,000 degrees? Can the lamp still exist and is it cold to the touch? Why?

To put it simply, because this temperature is only the temperature of electrons in the plasma, the temperature of cations and molecules is much lower than this! So the plasma in the lamp is not very "hot", and it is because of this that the lamp containing the plasma will not be burned out.

Why are there two temperatures?

Because the mass of the electron is very small, its collision with cations and molecules is like a ping-pong shot put, which is completely elastic and there is no energy exchange between them, so the electron cannot reach thermal equilibrium with the whole plasma.

Let's look at the interaction between plasma and magnetic field.

Because it contains a large number of free electrons and cations, plasma is a very good conductor. As mentioned earlier, there is no electric field inside the plasma, but there can be a magnetic field.

However, the magnetic field should be limited, because the "electric field is zero" has a strict constraint on the magnetic field, that is: the magnetic field can not change!

Why? Because according to the law of electromagnetic induction, the changing magnetic field will produce an electric field, since the electric field is not allowed to exist, the only way is to keep the magnetic field unchanged!

Therefore, if there is no magnetic field in the plasma, there will always be no magnetic field; if there is a magnetic field in the plasma, the same magnetic field will always be maintained. This is the magnetic field freezing in the plasma.

You may wonder: how is the effect of this freeze achieved?

There is a large amount of free charge in the plasma. as soon as the magnetic field in the plasma changes, the induced current is generated immediately. according to Lenz's law, the magnetic field of the induced current always counteracts the change of the original magnetic field. That's how the magnetic field freezes.

So what are the consequences of magnetic field freezing?

We know that the distribution of magnetic field in space is described by magnetic induction lines. Since the magnetic field in the plasma is constant, it means that the magnetic induction line in the plasma is unchanged! So when the plasma moves in a magnetic field, it moves along with the magnetic induction lines in the body, as shown in the following image.

When a plasma without a magnetic field enters the magnetic field, because it always keeps the internal magnetic field at zero, it will squeeze the magnetic induction line, as shown in the following image.

What the solar wind emits is a large number of charged particles, that is, plasmas, which cause the earth's magnetic field to deform when they blow toward the earth. On the side near the sun, the earth's magnetic field is compressed, while on the side away from the sun, the earth's magnetic field extends for hundreds of thousands of kilometers.

In fact, the characteristics of plasma are much more than these, it involves too many things. Because of this, after the 1920s, plasma physics has become an independent discipline to study the formation, properties and motion laws of plasma.

Generally speaking, there are three research methods of plasma physics.

The first is to study the motion law of charged particles, which is actually involved in high school physics-"the motion of charged particles in electromagnetic fields".

The second is magnetohydrodynamics, which studies plasma as a whole, which is a macroscopic theory similar to thermodynamics.

The third is to establish the microscopic theory of plasma according to the statistical method, just like the gas kinetic theory.

Here is a simple mention of the magnetohydrodynamic method.

Since the plasma contains a large number of charged particles, when these particles move in the magnetic field, of course, they will be affected by the magnetic field, the sum of which is the effect of the magnetic field on the plasma.

Usually, in order to study the motion law of plasma in magnetic field, people regard plasma as a kind of fluid and establish magnetohydrodynamic equations. However, because it is too complex, it will not be listed here. If you are interested, you can refer to the relevant materials.

On the one hand, the continuity equation, momentum equation (Newton equation) and adiabatic equation of fluid are obtained according to mass conservation, momentum conservation and energy conservation, respectively. At the same time, since the theory of electromagnetic field is involved, it is necessary to combine the Maxwell equations. In this way, the equations of magnetohydrodynamics are obtained.

So, what kind of plasma is there in nature?

The easiest thing to think of is the ionosphere over the earth. Due to radiation from the sun and cosmic rays, the whole earth's atmosphere more than 60 kilometers above the earth is in a state of partial or complete ionization, forming an ionization region. It can refract, reflect and scatter radio waves, which is very important for radio communication, broadcasting, radio navigation, radar positioning and so on.

Then there is lightning, because air molecules are ionized to form plasma, which has good electrical conductivity, so it leads to a violent discharge phenomenon.

As for the aurora, it is also a large-scale discharge phenomenon. When the hot charged particle stream of the solar wind blows toward the earth, the charged particles are captured by the magnetic field and collide with atmospheric molecules to cause luminescence, forming a brilliant aurora phenomenon.

In addition, there are all kinds of man-made plasma.

If you give an example, the most typical is the matter in thermonuclear fusion, whose temperature is as high as hundreds of millions of degrees, and the matter is ionized and becomes a plasma. Therefore, the study of plasma has become the key issue of controlled thermonuclear fusion.

And then there is the magical plasma ball mentioned at the beginning of this article, which also works by using the plasma material, the discharge gas. It originated from the "inert gas discharge tube" first invented by Nikolai Tesla, and was later transformed into the present form by Bill Parker, a student from the Massachusetts Institute of Technology, so it is also called "Tesla Ball".

In addition, plasma is widely used in various lights, such as neon or argon plasma is often used in neon lights. In addition, some flames can also be regarded as plasma.

It is worth pointing out that there is also an artificial substance called quark-gluon plasma. Scientists created the substance using the Relativistic heavy Ion Collider (RHIC). It is a completely new form of matter that widely existed within a few millionths of a second after the birth of the universe.

What else? Think about it, where else is plasma in the universe?

The largest plasma in the solar system is, of course, the sun, because it itself is constantly engaged in thermonuclear fusion, its internal temperature is as high as 15 million degrees, and matter is ionized. Even the material it ejects is plasma, a high-speed stream of charged particles, the solar wind.

A little further, aren't all the stars in the universe suns? Yes, they are also made of plasma material. In fact, this is true not only of stars, but also of those giant nebulae and most interstellar matter.

In fact, more than 99.9% of the visible mass in the universe is plasma!

Yes, plasma is the protagonist of visible matter in our universe. The three common states of solid-liquid gas are actually very few, and they only exist in planets and some interstellar gas and dust.

reference

Zheng Chunkai, Plasma Physics, Beijing, Peking University Press, 2009.

Zhang Sanhui, University Physics-electromagnetism, 3rd Edition, Beijing, Tsinghua University Press, 2008.

This article is from the official account of Wechat: University Physics (ID:wuliboke), by Xue Debao.

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