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Q & A: Plasma ball capacitive experiments

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Most recent answer: 10/22/2007
Q:
how come when I put a film (plastic,non conductive)canister full of Water ontop of my plasma ball and touch the top of the plastic lid of the canister the plasma ball still responds as if i were touching the Plasma ball with my hand? maybe the Radio frequency waves produced by the tesla coil are traped by water? amplified? Also, i wrapped a coil of 6 volt wire around the plastic container and did the same thing as before ( the wire was insulated and it was not touching the water or the plasma ball) and the same effect was amplified enough to discharge and melt my home made grounding rod. I dared not touch it with my hand! so tell me Watts going on?
- matthew
Renner Middle School, Plano, Tx, USA
A:
Hi Matthew,

Welcome to the fun and exciting world of experimental physics! We are glad you are curious about your plasma ball -- how it works and how it responds to stuff placed next to it or touching it. We have lots of answers to questions about plasma balls in of our questions and answers site.

The short answer to why your hand attracts the plasma discharge when you touch the ball is that your hand makes the second lead of a capacitor. Charge can flow in your body to your fingertips and build up on the glass, attracting opposite charges on the other side.

The same thing happens with the canister full of water, more or less. If the water has salt in it, charges can flow in it and build up on one side. Even if the charges cannot flow easily, the water will polarize -- every H2O molecule has one end with a positive charge and one end thatís negative. The water molecules can turn so that they on average point their positive ends more often in one direction than the other. The electric field of the polarized water molecules attracts opposite charges on the surface of the plastic, which polarizes too, leaving a charge on the outside surface of the plastic, which attracts charges on the glass, which polarizes the glass. Itís called "capacitive coupling".

Coiling a wire around the canister magnifies the effect. Without the wire, the canister was probably only in close contact with the plasma ballís surface only at one point. Adding the wire vastly increases the capacitance of the canister arrangement because charge can now flow along the wire and be attracted to polarized water all the way around the can instead of just on one edge or point. On the other end of the wire, the electric field is large because of the small, sharp tip of the wire (it behaves somewhat like a lightning rod) -- in general the sharper a piece of metal is, the stronger the electric field will be at the tip when it is charged up.

I can not tell from what you wrote where the grounding rod comes in. If your grounding rod was a wire attached to the earth somehow, then you are taking advantage of the very large capacitance of the earth (the canister of water is very small by comparison). The glass around the plasma ball still cannot conduct electricity. It can only polarize a small charge at a time, so I am surprised you melted something with the plasma ball -- it could be a safety hazard to operate it in this way. (Please do not open up the power supply in the base -- that can be dangerous!).

Tom

(published on 10/22/2007)

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