Thursday, May 5, 2011
Plasma: The Fourth State of Matter
This week's episode of The Joy of Science was about States of Matter. Most people are familiar with three: solid, liquid, and gas. But there is a fourth state of matter: plasma.
Plasma is a gas-like field made up of charged atomic particles – negative electrons and positive ions (atoms which have lost some of their electrons, and so have an excess of positrons). As they move, these particles generate electricity and magnetic fields. Plasma requires low pressure and extremely high temperatures. On Earth, plasma only occurs naturally in the form of lightning, polar auroras, and extremely hot flames. However, plasma is actually the most common state of matter in the universe, since it makes up stars and other celestial bodies, as well as the space in between.
Plasma was first identified in 1879 by Sir William Crookes, who called it "radiant matter." It can be created artificially by running an alternating electric current through certain types of gas in vacuum tubes. This “knocks electrons” off the atoms inside.
In this experiment, we decided to use a plasma globe to observe some properties of plasma. A plasma globe is a type of lamp that you can buy in a novelty shop, museum gift shop, or through a science supply house. Inside the glass bulb of the plasma globe is a Tesla coil. This creates a plasma field of electrically charged particles, which look like small tendrils of lightning.
When we turned the plasma globe on and touched the glass, the tendrils of lightning concentrated at the spot that was touched. Touching it in more than one place at the same time created several points of concentration. Our plasma globe also had a setting that made it react to sound waves. We put mp3 speakers next to it and watched it flick on and off in relation to the music. Interestingly, it was more affected by frequency (how high or low the note was) than to volume. It reacted very strongly to particular notes and not at all to others. This reminded us of seeing the band ArcAttack at Maker Faire NY. ArcAttack creates music using giant Tesla coil-driven plasma arc speakers.
The first experiment we did was to hold different kinds of unplugged light bulbs near the globe. As described on the Plasma Ball experiments page on the Wonders of Science website from the University of Wisconsin, some electrons from inside the globe travel through the glass to the light bulbs.
Inside the bulbs are gas molecules. In fluourescent bulbs, molecules of mercury vapor become excited by the energy of the charged particles bombarding them from the plasma field. Electrons in the mercury atoms make a quantum jump to a higher energy level (or shell) around the atom's nucleus. When they return to their previous energy level, the extra energy is given off in the form of light. When they are plugged in, fluourescent tubes also operate by creating plasma fields out of the mercury gas.
We got good results with fluorescent and neon lights, above and in the videos below. We also tested Halogen and Xenon bulbs, but were unsuccessful.
Our second experiment was more elaborate. First, we balanced a penny on the top of the globe. Next, we took another penny, and close to the penny balanced on the dome, but not touching it. As in the first experiment, some electrons from the plasma field traveled through glass and were carried by the penny on top as an electrical current. The penny was able to carry a current because they are made out of a conductive material, copper. Holding a second penny above the first drew the electricity throught the air, creating a tiny spark. You can just barely see the spark in the photo below; in the video you can see and hear the tiny crackle as the pennies spark.
We also tried sending sparks to our fingers. We found that, if we weren’t grounded, we could send a spark from the penny to one of our fingers without feeling a shock!
Stay tuned for more exciting plasma experiments in the second part of our report on States of Matter!