In the following video, the positive and negative terminals of a high voltage source are connected to two slightly separated metal plates. In-between the plates is a ball coated in conductive paint and suspended by wire.


When the source is turned on and the ball is touched to one of the plates, it becomes statically charged. This repels it from the plate and attracts it towards the other. When it touches the other plate, the charge is transferred and the ball becomes oppositely charged. Then the process is reversed.

However, it would seem that if the ball was connected by a wire to earth, it would be unable to carry the charge as any net accumulation of electrons would sink to earth (like an anti-static device). Is that the right understanding?

Then a follow up question: If that is the right understanding, does that mean a small current flows between the terminal and earth through the ball? That would seem counter intuitive, as you always need a circuit for current to flow.



1 Answer 1


Let's imagine the scenario with one charged plate and the other at Earth potential. Let's also ignore effects due to the air.

You don't need any net charge on the ball for attraction to happen, you just need there to be more charge on one side than the other.

If the ball were grounded, then once it hit (or got close to) the charged plate, you'd have a short-circuit to ground, discharging the plate.

Depending on how the plate is powered, several things could then happen. If the plate is powered by a low-impedance source capable of supplying high current, you may get lots of sparks, arcing and fire. If the plate were powered by a high-impedance source (say a van de Graaf generator), then the plate would just discharge and the ball would fall back. The plate could then re-charge and the cycle would continue.

  • $\begingroup$ So even if the power source was isolated from earth (i.e. a fully charged high voltage capacitor with no other connection), a current would flow from the capacitor to earth through the ball, despite this not forming a "circuit". Would this effect happen only once, or multiple times? Thank you so much for your answer! $\endgroup$ Commented Sep 28, 2018 at 13:39

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