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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When the voltage is enough, current flow through the air. It’s about 3000 V/m, give or take, for air.
So if you had a battery with a voltage of 30 kV and you held the electrodes 1 cm apart, the air would break down and current flow from one terminal to the other.
The same happens with lightning, just at much higher voltage. At a cloud ceiling of 100m you need 300 million Volts to break down the air. Which is what a typical lightning bolt actually has. A lightning bolt is what happens when the breakdown voltage between the cloud amd ground is exeeded, at that point the air (in a thin channel) suddenly becomes conductive and shorts to ground, and at 300 milliom volta, that short circuit to earth leads to a **massive** rush of current of 30 000 Amps which turns the air into a 28 thousand °C super bright lightning, which is the flash you see.
Lightning actually happens _during_ earthing. What happens is the clouds are very highly charged, the ground is very not charged, and the air between the clouds and the ground provides some amount of resistance. At a certain point the difference between the charge in the clouds and the 0 charge at the ground gets so high that it can overcome the resistance of the air and all the charge blasts to earth and lightning happens. The circuit is temporarily complete. Batteries don’t have nearly that amount of charge so you need stuff with way less resistance, like wires, to discharge. They also store their charge potential chemically so it’s only theoretically electricity until you complete that circuit. But conceivably if you managed to find 2 chemicals and metals with enough potential they could discharge from the + side to the – side through the air just like clouds (+) discharge to the ground (-) through the air.
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.
Electric discharge through the air is called “arcing”. The static electricity you feel when you touch a doorknob is the same type of action as a lightning strike.
Air is an insulator – it has a high resistance to electric current. But that doesn’t mean that it completely blocks current. As the electric field in the air between the charged objects increases (voltage increases), it begins to ionize the air itself, ripping free electrons. This creates a path of least resistance, which allows the discharge of current.
Given a high enough voltage, electricity can discharge across any insulator.
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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