Sparks

[5614]

This came up in a thread a while back but we never go to the end of it... what order does a spark happen in, taking into account dielectric or electric breakdown of air and movement of electrons?

 

Does the air ionise allowing electricity to flow... or... does the electricity flowing cause the air to ionise?

 

Now I know that some of those ionised particles will react, so ozone and nitrates are formed, but do all the ions react or do some remain as ions and what happens to them?

 

Assuming there is standard air conditions, how do you calculate the required voltage or current to make a spark of length x?

[Douglas]

This came up in a thread a while back but we never go to the end of it... what order does a spark happen in' date=' taking into account dielectric or electric breakdown of air and movement of electrons?

 

[b']Does the air ionise allowing electricity to flow... or... does the electricity flowing cause the air to ionise?[/b]

 

Now I know that some of those ionised particles will react, so ozone and nitrates are formed, but do all the ions react or do some remain as ions and what happens to them?

 

Assuming there is standard air conditions, how do you calculate the required voltage or current to make a spark of length x?

The air ionizes causing electricity to flow.

 

I'm not sure you could calculate it, but you could do it empirically.

 

I used to build "bug zappers" for my friends. I used discarded 10Kv oil burner transformers. I had to experiment with the grid spacing so I wouldn't get a spark under all the conditions of the air.

[Lance]

I disagree. Although ionization CAN happen first it does not always. It completely depends on the conditions. If you have two spherical electrodes the air will ionize the same time the spark forms because the spheres reduce corona keeping the air from becoming ionized. In triggered spark gaps a higher voltage spark will ionize the air allowing a lower voltage discharge across the gap.

 

Also, a spark IS ionized air (plasma).

[5614]

This is where we got last time; the two views, and they're both taken up by different people!!!

 

OK, so if it depends, two scenarios:

 

1) Lightning,

 

2) Two normal wires (or Mg ribbon!) placed near each other with 30V across them.

 

What order does ionisation/current flow happen in?

 

Also from before:

 

Now I know that some of those ionised particles will react, so ozone and nitrates are formed, but [b']do all the ions react or do some remain as ions and what happens to them?[/b]

[Lance]

30V will NOT spark through the air without preionization. However, Assuming you can supply enough current, if the wires come close enough together to touch the tips will heat up enough to ionize the air. The wires can then be pulled back creating an arc between the wires. This works much better with carbon though.

[5614]

OK, well I got some Mg ribbon to spark with 30V, I found I had to put it really close to start off with, like fractions of a millimeter, and then I could pull it apart really quickly to several mm and it'd be quite cool! And because it was ribbon it wasn't just like a wire, there was a thick spark!

 

OK, so then with lightning what happens with ionisation/current?

 

And the quote in post #4!

 

(It has got to be mating season for animals, yesterday there were two squirrels chasing each other for ages, now it's the same but with 2 pigeons!!!!(this is in my garden.))

[swansont]

Assuming there is standard air conditions' date=' [b']how do you calculate the required voltage or current to make a spark of length x?[/b]

 

 

This thread, post 17.

[5614]

In the site:

http://home.earthlink.net/~jimlux/hv/paschen.htm

that you linked to, there's a table about 1/2 way down with 'Minimum sparking potential for various gases' however it shows air as having a value of:

327Vs min

and

0.567 pd at Vs min (torr cm)

 

Right, what does that quite mean??

 

With the 327Vs what's the 'min' for?

 

And what's 'pd at vs min'?

(I mean, I read that pd is pressure x diameter, but I don't quite get it,)

[YT2095]

PD is usualy Potential Difference actualy

[YT2095]

in this instance sure, but in the same field it has 2 meanings

no wonder ppl shy away from Science, too many Abreviations!

 

you can actualy cheat with your 30v spark, have you tried bare wires across a candle flame yet? it`s quite interesting, esp with a meter in line, the flame will actualy Conduct

[Douglas]

This is where we got last time; the two views' date=' and they're both taken up by different people!!!

 

OK, so if it depends, two scenarios:

 

1) Lightning,

 

2) Two normal wires (or Mg ribbon!) placed near each other with 30V across them.

 

What order does ionisation/current flow happen in?

 

Also from before:[/quote']I'll stick with my original answer. In both cases, the air ionizes then current flows.

 

If you take the case of an arc welder, I believe you have to actually touch the object to generate a spark, then pull the welding rod back.