What part of a plasma ball is plasma?

Answer 1

A plasma ball has a high voltage in the centre. Because the walls of the ball are insulating the voltage is an alternating current, AC voltage. The AC voltage will drive a current through an insulator which is called a displacement current. If an electron happens by chance to appear in the globe it will be accelerated by the AC field. Because the gas pressure in the globe is below normal the electron can travel far enough and gain enough speed that when it does hit a molecule of gas such as a nitrogen or oxygen molecule it knocks off another electron. Now two electrons head off and so on. We have an avalanche of electrons which partially ionize the gas. Some of the electrons hit a molecule and do not ionize it but just excite it. So that we get light. If there are enough electrons and ions in the plasma ball then a plasma can form. The electrons in the plasma are heated by the AC field and the electron temperature can reach over 10,000 degrees. At this temperature the electrons is so hot that they ionize the gas and cause a lot of light to be emitted. But because the electrons are light they do not transfer much energy to the gas or the ions and so the gas and ions are cold. It is called a non-equilibrium plasma. When you touch the wall you reduce the AC resistance path to ground and the AC current flows through the plasma, through the wall and through you to ground. But the current is so small that you do not feel it. Where does the first electron come from - well there is a small chance that a cosmic ray goes through the ball or a gamma particle from radio-active decay appears, but it only takes one electron to start the plasma.