Why does a tesla coil make sparks if it doesn't have another wire to spark to?

[jamessinghal]

I have been looking into tesla coils, and my understanding is that they create a high voltage on the wire, but I am trying to figure out how they make sparks in the air, if they aren't sparking to anything.

[German_EE]

The air breaks down and becomes conductive, the 'live' end of the Tesla coil then shorts to ground through the air.

[buck converter]

I just have to mention it, GreatScott uploaded video about tesla coils TODAY!
https://youtu.be/LbTyEratSTI

[Terrius]

There is a fairly interesting image with a caption on the Tesla coil Wikipedia page that basically answers the question.

"The high electric field causes the air around the high voltage terminal to ionize and conduct electricity, allowing electricity to leak into the air in colorful corona discharges, brush discharges and streamer arcs."
https://en.wikipedia.org/wiki/Tesla_coil

It seems then that it ionizes the air around it, and then follows a path of of that ionization to ground, very similarly to how lightning travels through the air, but on a much smaller scale.

I'm certain there is some very in-depth physics that can explain it better than Wikipedia, but the Tesla coil article on Wikipedia is actually a very interesting read, in my opinion.

[IanB]

I have been looking into tesla coils, and my understanding is that they create a high voltage on the wire, but I am trying to figure out how they make sparks in the air, if they aren't sparking to anything.

The bottom of the Tesla coil is grounded to earth, and the top generates a high voltage relative to the ground (earth) potential. The sparks are trying to complete the circuit by getting back to earth, and they do this by seeking a path through the air, overcoming its insulating properties due to the very high voltage.

[CatalinaWOW]

Perhaps a more interesting question is why these sparks often don't follow the obvious shortest path to ground.  I know there are several factors including the magnetic fields generated by the coil itself, but I can't provide a simple and intuitive answer.  Always one of the clues to me that I don't understand what is going on well enough.

[IanB]

I suspect there is more repulsion than attraction going on.

When a lot of charge gets concentrated at the top of the coil the mutual repulsion of like charges causes the charge to try and spread out radially in the surrounding air away from the point where it was most concentrated. Once the charge has spread out and become more diffused into a lower energy state, it can then find its way back to Earth in a slower and more leisurely fashion.

[CatalinaWOW]

Sounds right, but implies that sparks would fly even if the ground were infinitely far away.  That return to ground is less important than simple field/capacitance effects.  Field gradient becoming the factor of interest, and sparks occurring whenever the gradient is greater than the dielectric breakdown of air.

[T3sl4co1l]

Think of it this way:

The air around the coil itself has capacitance.

Therefore, the high voltage is attempting to charge that capacitance.

When the voltage is low, it is charged by induction (proximity to the coil).

However, when the voltage exceeds the breakdown field, the air becomes conductive and electrons and ions surge into that space until the electric field strength is below breakdown.  Once this is done, the capacitance seen by the coil is larger, because it is conducting into a larger volume than before.

The equivalent circuit is a resistor in series with a capacitor.  The capacitor represents the amount of capacitance gained by the increase in size or extent; the resistor represents the voltage drop across the spark.

This therefore loads the coil, reducing its resonant frequency and Q somewhat.

The Q factor of a spark discharge is evidently fairly high, so that a coil driven at high power does not detune very much in the process.

A large top load (high initial capacitance) helps, by "swamping" the change in capacitance.  (That is, if we make C larger to begin with, then if the spark causes some increase \$\Delta C\$, it is a smaller percentage change in C.)  Also, by being physically larger, the top load acts to couple to the spark, i.e., the spark's capacitance to its surroundings is partly taken up by the top load itself, and so the spark-to-ground capacitance is smaller.

At very high voltages, streamers leave the vicinity of the top load, or strike ground, and both C and Q can go to crap again; it's a matter of degree with respect to power level, and the type of design used (a fixed-frequency SSTC won't tolerate detuning, but a PLL tracking, or self oscillating, type will).

Tim

[madsbarnkob]

The main reason that sparks can fly out in air and even go past grounded objects is for the simple reason that it is a high frequency AC signal, a lot of the common rules of current always seeking the path of lowest resistance does not apply here.

I can not explain this 100%, because it requires a deep knowledge of both the electrical, the high frequency and to a much wider extend the plasma physics behind a streamer path being fed from a oscillator. The streamer represents a load that is so great that it can pull the primary/secondary resonant circuit completely out of tune, if you have a heavy ground strike, but when they just fly into air there is not the same hard discharge conditions from the secondary capacitance.

I have built many Tesla coils, the largest producing 4 meter of sparks, but I am still very very far from being able to completely understand and explain what goes on in spark formation. Recent years with QCWDRSSTCs have shown that with bus voltage modulation there is suddenly a very high degree of spark formation/growth control, but that is just another step in a long range of developments in the digital Tesla coil era since the first OLTC/BRISG coils seen in the start of the 2000's.