Plasma (physics)

Plasma is often called the fourth state of matter. It exists alongside solids, liquids, and gases.

A plasma forms when a gas undergoes ionization. This means the gas gains a significant amount of charged particles. These particles include positive ions and negative electrons. Because these particles carry a charge, plasma is highly electrically conductive. In fact, for many purposes, its conductivity is treated as infinite. This makes plasma very different from a standard gas. While a gas is mostly an insulator, plasma responds strongly to electromagnetic fields.

To understand how plasma works, we must look at the movement of its particles. In a normal gas, particles mostly interact through short-range, two-particle collisions. In a plasma, however, the charged particles experience long-range interactions. This leads to collective motion, where the particles move together in complex ways. This motion is governed by electromagnetic fields. Any movement of a charged particle creates an electric current. This movement then affects the fields, which in turn affects the other particles. This cycle creates waves and other collective phenomena.

Scientists categorize plasmas into different types based on their properties. A plasma is called "partially ionized" if it still contains some neutral particles. Common examples of this include neon signs and lightning.

An "ideal plasma" is defined by specific scientific criteria. First, it must follow the plasma approximation, where the number of charge carriers is high. Second, it must be quasineutral. This means the density of positive and negative charges is roughly equal over large volumes. Third, it can be "collisionless" if electrostatic interactions dominate over ordinary gas kinetics.

There are also more specialized forms of plasma. A "non-neutral plasma" has a significant excess of one type of charge. This can happen in charged particle beams or electron clouds. Another type is "dusty plasma," which is often found in space. These plasmas contain tiny, highly charged particles of dust. In a laboratory setting, scientists sometimes refer to these as complex plasmas.

Our understanding of plasma grew through the work of several important scientists. In 1816, Michael Faraday hypothesized the existence of "radiant matter." He believed it was a state far beyond simple vaporization. In 1879, Sir William Crookes first identified this matter in a laboratory. He used Faraday's term, "radiant matter," to describe his findings. Later, in the 1920s, Irving Langmuir began systematic studies of the substance. Langmuir introduced the term "plasma" in 1928. He chose this name by analogy to blood plasma. He noted how blood plasma carries cells, much like how plasma carries charged particles.

Plasma is incredibly important in the universe. While it is rare to encounter on Earth, it is everywhere else. It is estimated that 99.9% of all ordinary matter in the universe is plasma. Stars are almost entirely composed of massive balls of plasma. Plasma also dominates the vast spaces between galaxies, known as the intergalactic medium. On Earth, we use plasma in modern technology. Plasma televisions use it to create images. It is also used in plasma etching to shape tiny components.

Finally, the behavior of plasma is closely tied to temperature and magnetism. High temperatures are usually required to sustain the ionization process. In many technological plasmas, electrons reach thousands of Kelvin while ions stay cooler. Plasma can also be "magnetized." This happens when a magnetic field is strong enough to influence particle motion. When a plasma is magnetized, its properties change depending on the direction of the field. This makes plasma an essential subject for studying both Earth's atmosphere and the deepest parts of space.