How does the ionocraft's electrical circuit work without a ground?

[ANarwhal]

I am currently interested in ionocrafts (http://en.wikipedia.org/wiki/Ionocraft) and the physics that guide them.

Something I do not understand, is according to the build instructions (http://jnaudin.free.fr/html/lftbld.htm) the positive wire is connected to the thin copper wire on the top with the ground being connected to the aluminium surface below. I do not understand how the high voltage current runs across the wire without a ground to enter into at some stage (I didn't think just putting a ground near the wire would be sufficient)? If this were the case wouldn't the electricity take the shortest path and just jump to the ground directly below the wire almost instantly when entering the wire above the ground?

 

[K^2]

Have you ever seen two wires arc when very high voltage is applied between them? Something similar happens here. The difference is that when electricity arcs, the electric field is high enough to ionize air everywhere between the two wires. Here, the geometry of the construction makes sure that the electric field is high enough to ionize air near the copper wire, but not all the way to the foil ground. What that means is that the copper wire will strip off electrons from just the nearby air molecules. These ionized molecules get attracted to negatively-charged ground terminal (foil), generating a downwards flow of air, and therefore thrust. The ions come in contact with foil, pick up electrons, and become neutral.

So the current does flow between +ve and -ve terminals, but it's the positive ions that carry the current, and not the electrons. So the current ends up being significantly smaller than it would be if the voltage was high enough to completely arc.

 

[ANarwhal]

Something I don't understand is if the electric wire is stripping electrons off the air and into the wire (which doesn't have a connected ground unless I am mistaken) and the ground gives the air back its electrons, how do the electrons go from being collected on the wire to the ground?

Also if it is stripping electrons off the air does this mean its current grows stronger the further down the wire is?

 

[K^2]

Electrons must flow from the ground, not to the ground. And they don't have to be the same electrons leaving negative terminal that reach the positive.

Yes, the current in the wire will be different everywhere, and there will be an associated voltage gradient due to that. But these voltage variations are small compared to overall applied voltage.

 

[ANarwhal]

I see, that makes more sense. So this is still basically a circuit using the air as a way to transfer the electricity?

If that is the case, could the whole craft be treated as a big capacitor circuit (http://electronics.howstuffworks.com/capacitor1.htm) to save energy?

 

[K^2]

How does that save energy? You can think of it as a capacitor in parallel with resistor with very high resistance, but I'm not sure how it helps you.

And yes, it's a closed circuit with air carrying electricity.

 

[ANarwhal]

Let's say I had a certain amount of energy going from the ground to the positive terminal, at the start of the current flow it would have x amount of energy (before resistance). Once the energy almost reaches the positive terminal, how much energy would it have lost?

What is the voltage resistance of ionised air?

 

[K^2]

You lose 100% of it, but you get thrust in return. The power spent is P=VI. The thrust you get is F=Id/k, where d is distance between wire and foil and k is about 2E-4 m²/Vs.

Resistance will depend a lot on how much air you can ionized, so it will depend on wire diameter, applied voltage, perimeter of the ionocraft, and distance between wire and foil.By the way, if you haven't figured it out yet, building one of these can be quite dangerous. The corona voltage is around 30kV. The currents that normally flow through ionocraft are not lethal, but a voltage supply that generates 30kV might pump lethal currents if it shorts through your body. And at 30kV, you don't need to even touch a wire. Being within an inch of it will be sufficient for current to arc across.

 

[ANarwhal]

I am aware of the dangers, I've never actually built one and don't really intend on doing so, I am just interested in them from a theoretical perspective.

So just to clear things up, let's say this ionocraft is a circuit like so:
+ --- Ionocraft --- -

If I stuck lightbulbs (for arguments sake let's say one that operates at 30kV) in the circuit:
+ --- bulb --- Ionocraft --- bulb --- -
Would both of these lightbulbs light up? Or only 1?

 

[K^2]

Both or neither, of course. The current going in has to be same as current going out. Just because you are using air as conductor, this does not change.

 

[ANarwhal]

Wouldn't this imply that all the power is not lost in the production of thrust if a bulb can still be lit after the thrust has been produced on the same circuit?