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If I took a Tesla coil into space, and then turned it on near, but not touching metal objects- would charge jump to the metal objects? It's my understanding I wouldn't see a flash, since the lightning comes from ionizing the air, but would the charges still move?

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    $\begingroup$ Yes, no sparks. No much wow and wiiii in space. But yeah! Charges will still move, that don't depends on the external medium. $\endgroup$ – raul Oct 23 '15 at 5:49
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The conventional definition of lightning is a current though a plasma (not necessarily through air as lightning happens on other planets) so in a vacuum there cannot be any lightning. However charge does still flow between electrodes in a vacuum, and from personal experience I know that we can get something very like lightning in the right circumstances.

In a vacuum charge can leave a surface due to field emission. Basically the field gets so strong that the electrons at the surface can get enough energy to overcome the metal work function by moving only a small distance and they tunnel out of the metal. However this is a rather diffuse flow and wouldn't generate any light so it wouldn't resemble lightning.

I did claim you can get an effect like lightning, and this can happen if there is a rough spot on the electrode surface that concentrates the electron flow. If the electron flow gets concentrated enough it can vaporise the electrode at the high point then you get an arc through the vapour. In the old days when we used giant Van de Graaff generators as particle accelerators this used to happen on a regular basis leaving a scar on the dome that then needed to be polished away.

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  • $\begingroup$ Hey! Some of us still use Van de Graaff generators! Our High Voltage Engineering (Robert Van de Graaff's company) EN Tandem is 53 years old and runs every day for experiments. We are very surprised if we get a tank spark - those should not happen on a regular basis. (Note ours has been upgraded to a Pelletron chain, which eliminates the main sparking cause of dust from the belt). $\endgroup$ – Jon Custer Oct 23 '15 at 14:08
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    $\begingroup$ I would like to add that there is vacuum switching gear, which is declared up to 160kV (the number a few years ago, so it could be more today). These are used in the electric power system to separate parts of the network. They do arc while separating the contacts, the arcs are created from the contact material. The crux is, with enough voltage the contact material will vaporize and start flowing from one electrode to the other, creating a conducting channel which becomes visible as an arc (when current starts flowing). $\endgroup$ – WalyKu Oct 23 '15 at 14:18
  • $\begingroup$ @JonCuster: I'm remembering this stuff from about 1983 (good grief, 32 years ago!) and I've probably remembered the arcing as being more of a problem than it was. $\endgroup$ – John Rennie Oct 23 '15 at 17:55
  • $\begingroup$ @JohnRennie - no problem - the technology continues to improve. Earlier this year we opened the Tandem tank for the first time in 5 years to do maintenance. That would have been unheard of 30 years ago... $\endgroup$ – Jon Custer Oct 23 '15 at 18:21

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