There are three states of matter according to our experience of the world – solid, liquid and gas. Solid, most dense state turns to liquid and liquid to gas on heating, but what happens when we heat gas too? It turns into plasma, which is also called the fourth state of matter.

Plasma-The Fourth State of Matter
Photo by Zoltan Tasi on Unsplash

What is Plasma – The Fourth State of Matter

Plasma is simply an ionic form of gases which contains ions and free electrons. Its existence was first identified by Sir William Crookes in 1879 as the fourth state of matter and later, in 1928, Irving Langmuir introduced the term plasma. Plasma can occur naturally as well as artificially. The most common example of plasma are Sun and other stars. Other examples include solar wind, lightning, ionosphere, polar winds, plasma displays, arc discharge and other discharges.

Gas turns into plasma by the application of heat. When gas is heated, gaseous molecules acquire thermal kinetic energy. When this kinetic energy becomes sufficient to overcome the molecular binding energy, molecular gas gradually dissociates into atomic gas. Upon further heating the atomic gas, electrons of gaseous molecules get excited. If the supplied thermal energy on heating gas is equal to the ionization energy, gaseous atoms get ionized. These ionized gases are called plasma if certain properties are satisfied.

As there is always some degree of ionization in a gas, the gas with ionized particles is not necessarily a plasma. Plasma is a quasineutral gas of charged and neutral species exhibiting collective behavior.  In a gas, as the temperature is increased, the ionization degree is low. Then on further increasing the temperature, there is a sudden rise in ionization degree along with its ionization energy, and the gas goes to the plasma state. The plasma can be fully ionized or partially ionized.

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What makes ionized gas a plasma?

There are several properties which makes an ionized gas a plasma. They are as follows.

1. Macroscopic Neutrality

A plasma is a macroscopically neutral gas. It does contain both charged and neutral species but the charge cancel out in a macroscopic view. In the absence of external disturbances, the charged particles in plasma are distributed in such a way so as to decrease the existing potentials to maintain macroscopic neutrality (charge neutrality) and exhibit collective behavior.

This means that space charge of the positive ions and negative electrons are equal to ensure charge neutrality, however this neutrality takes place only when sufficiently large volume of plasma is considered for sufficiently large interval of time. Therefore, the plasma is considered as a quasineutral gas with the densities of negative and positive charges almost equal.

2. Debye Shielding

A plasma has a fundamental characteristics of shielding out the electric potentials applied to it and tend to remain in a quasineutrality state. This phenomenon of shielding is called Debye shielding. The region where this shielding takes place is called Debye sphere. When an external potential is introduced in a plasma, the charged particles in the vicinity are attracted to the potentials forming a layer within. The distance over which the quasineutrality is violated is called Debye length which is the radius of the Debye sphere.

The shielding of the external potential is a consequence of the collective behavior of the charged particles comprising the plasma. In order to screen the effect of external potential, the large number of the charged particles interacts collectively. This breaks the charge neutrality within the Debye sphere, whereas outside the sphere the plasma retains its quasineutrality state.

3. Plasma Frequency

When the plasma in equilibrium condition is instantaneously disturbed, the resulting internal space charge fields give rise to collective particle motions that tend to restore the original charge neutrality.

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This means that when charged particles, say electrons in the plasma are displaced from a uniform background of ions, the resulting electric field will act in such a direction so as to pull the displaced electrons back to its original position and to restore charge neutrality of the system. However, the electrons will overshoot the equilibrium position because of its inertia and oscillate with characteristic natural frequency known as plasma frequency.

This collective motion for electron is faster than that for massive ions and the electron oscillates with the restoring force provided by the ion-electron coulomb force. The collective oscillations gradually damped due to further collision of the charged particle with the neutrals. For an ionized gas to exhibit plasma characteristics, the collision frequency of charged particle with neutral must be less than that of plasma frequency.

4. Sheath

Plasma interactions with wall is eminent in all practical plasma devices where it is confined within the chamber of the wall. Since, electrons in plasma being lighter have greater mobility than ions and thus move faster towards the wall leaving the plasma with net positive charge.

As these electrons hit the wall, it acquires negative potential. The negative wall then attracts positive ions from the plasma resulting in the formation of a thin positive space charge layer at vicinity of wall in the order of Debye length called plasma sheath.

The advantage of the sheath is to electrostatically confine more mobile species like electrons forming a potential barrier whose height is adjusted itself such that the flux of electrons which have enough energy to overcome this barrier and going towards the wall is equally balanced by the flux of ions reaching the wall.

These properties can be used to distinguish ionized gas from plasma, the fourth state of matter.

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