# Electric arc in 0 gravity? Curved or straight?

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1. Jan 21, 2016

### kevin_tee

I observe that electric arc forms a curve. I think that this is because electric arc is extremely hot and hot air goes up because it has less density. So if electric arc occur at 0 gravity would it be straight?
from wikipedia https://en.wikipedia.org/wiki/Electric_arc

Thank you.

2. Jan 21, 2016

### Hesch

No, it isn't.

When the arc passes through an airgap, it needs free charged particles ( ions/electrons ) to be created along the path. Having one particle created, the particle will be accelerated along the electric field, achieving velocity/energy. And if the achieved energy matches the energy needed to create a new free charged particle, at that instance the first particle hits a molecule, this will happen. So in turn, an avalanche/electric arc will be created.

So creation of an arc depends on:

1. Electric field strength ( not too high, not too low )
2. Density of gas ( distance between molecules, gas pressure )
3. Sort of gas ( oxygen, hydrogen )
Now, say the electric field strength is too high, the arc must choose a path where along the field strength is weakened. At the same time it must follow the direction of the electric field. So if the difference in voltage is kept constant between "+" and "-" in the below figure, the arc must follow a longer/curved path. In this way, the field strength will be weakened. So if the applied voltage is close to the minimum voltage needed, the arc will be straight. With more voltage applied, the arc will be curved.

Some diagrams have been made, regarding gas, pressure, field strength. I think they are called something like "von Paschen curves" ( not sure ).

3. Jan 21, 2016

### Hesch

Well, here they are.
https://de.wikipedia.org/wiki/Paschen-Gesetz

( Sorry, could only find them in german )

4. Jan 22, 2016

### Staff: Mentor

It does not have to happen, and there is no maximal energy. "Field strength too high" does not exist, a higher field strength makes arc production easier.

5. Jan 22, 2016

### Bernie G

The effect of the earth's magnetic field on the arc could be tested by changing the orientation of the arc 90 or 180 degrees.

6. Jan 22, 2016

### ZapperZ

Staff Emeritus
There is a flaw in your reasoning, because electrical breakdown also occurs in vacuum (i.e. in 10^-9 Torr or less). Here, there is very little "gas" and there is no such thing as "convection" (i.e. no viscous flow). The "arc" can take many shapes and forms.

Zz.

7. Jan 22, 2016

### OCR

8. Jan 22, 2016

### kevin_tee

I think my question not well explain, The plasma arc I was talking about is plasma arc create by breakdown of air at atmospheric pressure, I observe that when high voltage arc across two electrodes, it form an electric arc that curve. Photo from wikipedia, as you can see it curve.

From what I see is that the electric arc curve upward, so it might do something with gravity.
More photo (google image serch: microwave oven transformer plasma)
http://cdn.instructables.com/FD5/CBF3/FQMXESGP/FD5CBF3FQMXESGP.MEDIUM.jpg

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9. Jan 22, 2016

### OCR

A big one...

Last edited: Jan 22, 2016
10. Jan 22, 2016

### kevin_tee

Yeah big one, but why?

11. Jan 22, 2016

### billslugg

There is a 500,000 volt potential behind that very long arc. The switch you see opening here is "disconnect". It is normally opened only with no power to it. It has no arc quenching devices. In this case, a substation breaker probably failed in the closed position and they were forced to use the disconnect to cut the power. The hot ionized arc channel floated upwards until the potential could no longer sustain the arc and it then stopped. So, yes, under gravity, arcs generally curve upward. There are other forces involved, including the fact that a conductor repels any other current carrying conductor in which the current is travelling in the opposite direction. It will attract a segment in which the current is travelling in the same direction. All forces considered, a tortuous, sinuous shape develops.

12. Jan 22, 2016

### kevin_tee

Can you please explain more of that, I don't really understand and it is kind of interesting.

13. Jan 22, 2016

### OCR

14. Jan 22, 2016

### OCR

15. Jan 23, 2016

### Hesch

Thank you.

The first paragraph under "Paschen curves" indicates that there is a field strength gap wherein the arc prefer to pass.

I think that if e.g. an electron passes an atom with too high velocity / too much energy, the electron will simply pass the atom without creating new free electrons. The energy must somehow match the limits of an energy gap.

That's why an arc searches for a path, where along the energy achieved between two atoms matches a certain (gapped) energy, depending on the sort of gas.

I agree that arcs ( e.g. Jacob's ladder ) not forms perfect curves. That is due to coincidences, ( which atom will be the next to be hit ). It's like birds flying in a flock: If the first bird turns, all birds will follow.

16. Jan 23, 2016

### 256bits

Interesting stuff.

One also has glow discharge for example in a neon lamp. Paschen curves should be applicable for low pressure minimum voltage in these cases also where there is not an actual arc. The whole plasma will glow from electrons hitting the molecules and cause an avalanche of electron flow, "igniting" the whole plasma, with light given off as the molecules return to lower energy state from excited.
https://en.wikipedia.org/wiki/Glow_discharge

In an electrical discharge there are positive and negative species.
One aspect from the wiki is that,
for the discharge to commence, and
at the moment of discharge.

If the voltage across the gap is greater than that dictated by the Paschen curve, one would conclude that the above quotes cease to apply.

17. Jan 23, 2016

### Staff: Mentor

I don't see how you could interpret the article like that.
For distances that are too small (compared to the mean free path length), you need an even stronger field to get enough ionization. That's what the article says: a higher field strength helps. So does a distance of the order of the mean free path length. There is no "field strength gap".

18. Jan 23, 2016

### Hesch

At a constant voltage, when the distance is too small, the field strength will bee too high. Hence the arc must choose a detour ( curved path ).

Field strength = ΔV / distance.

The discussion is not whether a higher voltage will create an arc or not, but whether the path of the arc will be straight or curved.

19. Jan 23, 2016

### Staff: Mentor

No, the distance will be too short.
Keep the distance and increase the voltage and you get the arc even at a smaller distance.
There are exotic cases where a curved arc is favored (electrodes and gaps of micrometers at atmospheric pressure), but those are not due to the field strength, but due to the longer distance, giving a higher chance of ionization.

20. Jan 23, 2016

### Hesch

That doesn't make sense to me.

Is the distance kept, or will it get even smaller ?

I say that if you have a constant distance between two electrodes, and that you at some voltage have a (nearly) straight arc between the electrodes, then if you increase the voltage, the arc will be curved, thereby increasing the length of the arc, so that the field strength = ΔV / (length of arc) ≈ constant.

21. Jan 23, 2016

### Staff: Mentor

The distance is set, I don't understand what "will it get even smaller" means.
That is wrong, and I don't see where you would get that from.

22. Jan 23, 2016

### Hesch

In #19 you wrote:

Keep the distance and increase the voltage and you get the arc even at a smaller distance.

"That is wrong" is not a reasonable argument. How do you think the arc will be ignited?

I get it from the first paragraph under "Paschen curves", link: https://en.wikipedia.org/wiki/Paschen's_law

23. Jan 23, 2016

### Staff: Mentor

That is correct. What is unclear about it?
You can have an arc at a given distance, then you reduce the distance, and the arc disappears. Now increase the voltage, and you get an arc again.
See the previous posts, see the Wikipedia article, see the references given there. You make an unsourced claim. You have to find a reference for it.
Which is in clear contradiction to what you claim here.

24. Jan 23, 2016

### Hesch

It says:

Early vacuum experimenters found a rather surprising behavior. An arc would sometimes take place in a long irregular path rather than at the minimum distance between the electrodes.

Like e.g. "Jacob's ladder" or lightning in a thunderstorm.

Last edited: Jan 23, 2016
25. Jan 23, 2016

### Staff: Mentor

For reasons I explained here. Distance too short.

Lightning in a thunderstorm is an entirely different topic, and has different mechanisms.