What's a Plasma?

Plasma is the fourth state of matter. Like the other states of matter: solid, liquid and gaseous, it has unique properties. Just as most substances become solid if cooled sufficiently, any substance will become a plasma if heated enough. In a plasma the electrons are stripped from the atoms, creating a substance that resembles a gas but that conducts electricity. Over 99% of the universe exists in the plasma state. The stars, in particular, are giant balls of plasma. The earth is an unusual place where matter exists in the other three forms. Plasmas occur naturally on the earth in flames, electrical discharges, lightning bolts, the upper atmosphere (ionosphere), and the aurora borealis (northern lights).

Plasmas represent a fundamental field of study with many astrophysical and terrestrial applications. One important application is the potential use of plasma to produce electricity from controlled nuclear fusion.
As mentioned above, in order to create a plasma a gas must be heated to very high temperatures. An example of a household plasma is a fluorescent light bulb. Electric current is used to strip the atoms of the gas inside the tube of some of their electrons and to heat the plasma. The resulting temperature of the plasma inside approaches 10,000 C. Yet, when you touch the bulb it feels relatively cool. This is because there is relatively little gas inside the fluorescent bulb, so it has a low heat capacity, despite its high temperature. As an example, consider a pot of boiling water. The water and the steam are both at temperatures of about 100  C. Yet you can put your hand in the steam safely, whereas you wouldn't even consider plunging your hand into the water. The steam is much less dense than the water, and has a lower heat capacity.

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