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Name: Kevin
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I know that lightning colour depends on atmospheric conditions, but if electrical discharge were to occur in a perfect vacuum, what colour would it be ?

Hi, Kevin.

Actually, for an electrical discharge to occur, some material has to be present, so it could not happen in a 'perfect' vacuum. What usually happens is that the electrical field builds up until molecules of the electrodes start to move from one electrode to the other. This generates a plasma (ionized gas) that conducts the arc. This in turn heats the electrodes up and carries more material with it, which intensifies the arc, and the cycle repeats until a steady state is reached. So the color of the arc would depend on the electrode material.

David Brandt

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