During one of our Tuesday evening meetings we experimented with creating plasma.
How do you make plasma? Well, usually air insulates electric charges, so it requires high voltage to break apart the molecules. In a partial vacuum, however, this breakdown potential may decrease enough that two uninsulated surfaces with different potentials might induce the electrical breakdown of the surrounding gas. For example, the exposed tips of two big screws, such as we have in our experiment.
When there are molecules and free electrons floating around inside a chamber like ours, there is the chance of collision between an electron and a molecule, which could ionize the molecule. Electric fields need either very high temperature or very low pressure to break down a gas so we used an old pump to suck as much air as we could from a sealed PVC tube. Of course it wasn’t completely sealed – it’s impossible to create plasma in a pure vacuum because plasma is defined as ionized gas. As you’ll see in the video, we control the amount of gas in the chamber by fidgeting with the release valve. After a few trials we discovered the amount of air pressure required to ignite the plasma then we decreased pressure to maintain the pretty purple lights.
Once upon a time in Tübingen a gentleman named Friedrich Paschen wrote a physics paper about the potential difference for sparking air at various pressures. The formula he magicked from the ether became known as “Paschen’s Law” and it’s an equation that gives the voltage necessary to start a discharge between two electrodes in a gas as a function of pressure and gap length.
If you are curious, follow this link to a web site where you can calculate the breakdown of your own gas!