Eli5: How high voltage (lightning, HV charged metal sphere) discharge to earth without earthing, while battery requires both terminal to complete circuit?
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What you have been taught in school is correct about a steady state circuit. All the currents are direct currents and does not change over time. In that case the charge goes from the positive pole of a battery and have to return to the negative pole in a continuous fashion. The chemistry in the battery keeps the voltage up while current is flowing.
However in a Van de Graf generator which you describe as a HV charged metal sphere, there is an internal mechanical band to move the charge from the base to the top. Instead of a steady state as in a battery the voltage between the top and the grounded base continues to increase, until finally it is released back to ground through a spark. The same thing happens with a lightning strike.
But it does not always have to return to ground, or the base rather. If you are grounded you have the same electrical potential as the ground. If you then insulate yourself from the ground you still have the same amount of charge in you and therefore the same electrical potential. The voltage between you and the ground is zero, which means that the voltage between you and the top of the generator is very high. If you get close to the sphere then there will be a spark and current is transferred. However you can not hold much charge in your body so this will be a small spark until you are charged up at the same electrical potential as the sphere. This is not a steady state as you have been taught in school.
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