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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Because you have been taught the untrue statement that “electricity follows the path of least resistance”. The correct statement is “electricity will have the highest current through the path of least resistance” (you should actually replace resistance with impedance, but that is a story for another time). This is just saying Ohms law is true (Current = Volts/Resistance).
In actual practice, current flows along almost every path in inverse proportion to its resistance. Current does NOT require a metal wire to flow, however in most situations the resistance outside of the wire is so high that a negligible current is flowing. If you have a very high voltage then even through a high resistance path it is possible for a non-negligible current to flow (once again Ohm’s law).
Air normally has a VERY high resistance. However, when air is exposed to very high voltages it can be ionized (electrons freed) and become quite conductive. This is what happens in lightning. The high voltage ionizes a path through the air which acts like a low resistance wire allowing massive current to flow.
This is also how your fluorescent light works. The tube is filled with a low pressure gas and mercury. The resistance across the tube is extremely high. They heat up filaments to force free electrons into the gas to improve the resistance a little, and then hit the lamp with a high enough voltage to create an ionized path across the tube. The resistance suddenly drops, and the ballast must quickly reduce the voltage to prevent too large of a current to flow and trip your circuit breaker (Ohm’s law again!)
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