Tried googling this, but I must be using the wrong terms. I'm just wondering if anyone can explain the electrical theory behind how current discharges into the air, and where it goes, since there isn't a complete circuit.
The main example I'm thinking of is a tesla coil. It builds up a high voltage and low current in that torrid, but then the sparks just shoot off into the air. With no apparent completed circuit.
I could understand if the sparks all struck ground, completing the circuit, and making a path for the current, but if it just shoots off into the air, then where do all those little electrons go?
Air is a conductor if enough energy is available to ionize the air molecules. This creates both negative and positive charges, which drift, creating a current flow.
The breakdown field strength for dry air is, IIRC 3MV/m. In practice sharp points allow
even a few hundred volts to create field-strengths exceeding that, and certainly by 1kV
you have to avoid sharp points or you just get strong corona discharge around them.
Corona discharge is the principal mechanism for low current flow into air, the high
electric field strength causes local breakdown in the air and the ions move towards the
point or away depending on sign. Its commonly seen when removing nylon clothing in
the dark when static build-up discharges.
Higher powers available lead to actual sparks which have a well defined channel of
ionized air that carries a far higher current density (heating the air enough to make it
a blue-white hot plasma (10 to 20kK or thereabouts I think). By comparison corona
discharge is a much colder phenomenon, giving off very faint light (recombination of ion pairs?).
The behaviour of DC and high-frequency AC discharges is rather different, but its
still ionised air molecules and free electrons as charge carriers.