There are three states of matter according to our experience of the world – solid, liquid, and gas. The 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.
What is Plasma – The Fourth State of Matter
Plasma is simply an ionic form of gas that 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 is 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 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.
What makes ionized gas a plasma?
There are several properties that make an ionized gas 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 cancels out in a macroscopic view. In the absence of external disturbances, the charged particles in plasma are distributed in such a way as to decrease the existing potential to maintain macroscopic neutrality (charge neutrality) and exhibit collective behavior.
This means that the space charge of the positive ions and negative electrons are equal to ensure charge neutrality, however, this neutrality takes place only when a sufficiently large volume of plasma is considered for a sufficiently large interval of time. Therefore, the plasma is considered a quasineutral gas with the densities of negative and positive charges almost equal.
2. Debye Shielding
Plasma has a fundamental characteristic of shielding out the electric potentials applied to it and tends to remain in a quasineutrality state. This phenomenon of shielding is called Debye shielding. The region where this shielding takes place is called the 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 the external potential, a large number of the charged particles interact 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.
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 their original position and to restore charge neutrality of the system. However, the electrons will overshoot the equilibrium position because of their inertia and oscillate with a characteristic natural frequency known as plasma frequency.
This collective motion for electrons 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 particles with neutral must be less than that of plasma frequency.
Plasma interactions with the wall are eminent in all practical plasma devices where it is confined within the chamber of the wall. Since electrons in the plasma are lighter have greater mobility than ions and thus move faster towards the wall leaving the plasma with a 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 in the vicinity of the 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 that have enough energy to overcome this barrier and go 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.