So my brother was telling me how he has a 250v air ionizer, yet it makes a 30,000 volt spark gap. He believes that how far a spark gap goes is dependant on current the same as it is voltage because "you need electrons to pass through air" etc. This started while we were talking about how capacitors charge (another story) but basically I need some clearing up because he messed with my head.

 

Welcome to AAC!

Your brother is wrong. The spark comes first, current follows.
https://en.wikipedia.org/wiki/Spark_gap

 

Welcome to AAC!

Your brother is wrong. The spark comes first, current follows.
https://en.wikipedia.org/wiki/Spark_gap

Thanks and thanks, but I'll have trouble convincing him regardless lol.

 

Voltage jumps gaps...
One might be "convinced" by showing them the "dielectric" rating of any insulating material..
The specifications are typically listed as "volts per mil" or similar with NO mention of a current requirement/specification...
Example 1500V/mil meaning 1mil of thickness of that material is capable of insulating from 1500V.. regardless of the available current of said power source..

 

He believes that how far a spark gap goes is dependant on current the same as it is voltage because "you need electrons to pass through air"

But all you need are enough electrons to ionize the gap, which depends upon the voltage.
And it requires only a very small current to maintain the arc after that.
So the arc length is basically independent of the current above a low minimum sustaining current, once the voltage starts it.

 

I think the issue is more emotional than technical.

In that case, one amp of current is one coulomb (of electrons) per second. One coulomb is ~6.242×10^18 electrons. So if the current was only 1uA, you'd still have 6.242x10^12 (or 6,242,000,000,000 or 6.242 trillion) electrons moving thru every second. I think you have enough moving thru the gap.

So really, the current is irrelevant. You do need the electrical pressure (voltage) to ionize the air:

 

Thanks and thanks, but I'll have trouble convincing him regardless lol.

I would've put it as; "the spark IS the current".

Breakdown is essentially exceeding the dielectric strength of whatever is in the spark gap (eg air), A wider gap has greater dielectric strength and takes more voltage to break it down.

 

I think the issue is more emotional than technical.

Huh? I don't see much emotional content in the physics of sparks.

At high enough voltage, a narrow path of air can ionize and become dramatically more conductive, thereby passing current. Formation of the path requires a high voltage because air is normally not very conductive or easily ionized. The amount of current that flows then depends on other factors such as surface area, charge imbalance and so on.

 

One question would be, what is the minimum current that can sustain an arc once it is started?

 

One question would be, what is the minimum current that can sustain an arc once it is started?

Ask a welder. I think the short version is "a lot", because the ions dissipate continuously. They don't just sit there like a wire.

 

Ask a welder. I think the short version is "a lot", because the ions dissipate continuously. They don't just sit there like a wire.

True. So the question is, what is the minimum current to generate the necessary amount of ions for a continuous arc?
Likely not a simple question to answer.

 

Ask a welder. I think the short version is "a lot", because the ions dissipate continuously. They don't just sit there like a wire.

The low resistance of the arc usually shorts a current limited source to the point the arc dissipates, the power supply recovers and the process begin again until something burns. This supply is only about 10mA at 90kV continuous but the peak current at the moment of discharge is much higher due to filter capacitor storage.

Safety grounding rod not fully raised.

It's also a DC vs AC question with DC being the winner in sustaining the arc.

 

A quick question about current and arc. To reduce the current, I will put a HV resistor before the tip/sphere. This reduces the Voltage at the tip/sphere whatever.

So I can not reduce the current and leave the voltage the same.

Does it make sense?

 

A quick question about current and arc. To reduce the current, I will put a HV resistor before the tip/sphere. This reduces the Voltage at the tip/sphere whatever.

So I can not reduce the current and leave the voltage the same.

Does it make sense?

It would reduce the voltage if the current stayed unchanged.
But an arc is very low impedance so its voltage is fairly constant with a change in current, thus adding a resistor will reduce the current but leave the voltage largely unchanged (as long as the arc is still maintained).

 

A Neon lamp might be a good example. Generally there is a current limiting resistor in the circuit. You can actually place a steady C voltage across the lamp that won't cause it to glow. Then you can raise it briefly and it will continue to glow. You can lower it briefly and it will stay off.
that's essentially how Plasma displays work.

The arc is a low impedance path when made. It's a high impedance path when not. It jumps the gap at a particular voltage. It generally won;t extinguish at the same voltage though.

 

I need to experiment to have a clear answer. I.e. I put 30kV at a sphere which is 1cm away from another earthed sphere. I have an arc.

I put a 100Mohm resistor (HV one) at the sphere with the HV. I do not have an arc I believe.

The air gap is a resistor I believe. A large one when I only have ionization, maybe at the range of 10^10ohm, and a much smaller one when I have an arc. Actually you can measure the current. You will count μAs when you have ionization and mAs when you have an arc.

I will try to make some measurements and come back. Maybe I am wrong

Sorry if I confused you

regards

Doros