# Minimum amount of charge required?

Discussion in 'Electronics Homework Help' started by error404, Sep 2, 2012.

1. ### error404

3
0
Sep 2, 2012
Hello,

I've been looking online for an answer to this question but didn't find any yet. hope you guys can help.

If I had 2 point charges, X and Y spaced by 1 Meter of air. what are the minimum values for X and Y that could form a spark between the 2 charges?
I saw online that the breakdown value for air was 30 Kv/cm or something like that but i have no clue how to convert this value into charges in coulombs!

Is there any formula that can predict the values for X and Y in such a case?

Thanks,
Error404

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
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Jan 21, 2010
There are just so many variables.

For example, air pressure, temperature, humidity, composition of atmosphere.. they all make a difference.

Then you have the nature of the charges. Are they charged spheres, or point charges. The smaller the object with the charge, the more readily it ionises nearby atoms.

This is 206, I suggest you 303 to google.

3. ### wingnut

242
9
Aug 9, 2012
My best shot is this

V= kQ1/r

For two point charges just add them
Vt = kX/1 + kY/1 remembering the sign of each charge

If breakdown in air is as you say 30kV/cm that means Vt= 30kV x 100 as there are 100cm in a m. Vt = 30 000 00 V

Therefore 3000000 = kX/1 + kY/1 where k = 9 x 10^9

The above may be wrong but is my best shot

4. ### error404

3
0
Sep 2, 2012
Great info, thanks mate.
can i know the name of this formula u just used?

Error404

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Since you're not interested in practicalities, I can only assume this is homework.

3,635
5
Apr 7, 2012
Cute

13,903
1,971
Sep 5, 2009

Dave

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
404 is a return code, as are 206 and 303

200!

9. ### Harald KappModeratorModerator

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Nov 17, 2011
I'll have to remember that

10. ### error404

3
0
Sep 2, 2012
Mr.Steve
These variables actually contribute to a change in the answer to my question, that's why I was looking for some rule to take as much variables as possible into account. (does such a formula exist?)
They are point charges, as I have assumed in my question.
I am not looking for a sharply correct value, but the closest indication of how much charge is required.

Sorry to disappoint you, but this is not a homework

Error404

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Well, if it's not a homework question then they're not point charges.

Why not try describing your actual problem rather than making up a theoretical question?

Point charges tend to leak away by themselves. Especially if they are truly mathematical points.

See here.

12. ### Harald KappModeratorModerator

11,825
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Nov 17, 2011
Whatever:
Asking for the absolute value of the charges makes no sense. Both charges can be arbitrarily high (or low) without any spark as long as the potential difference (also know to insiders as "voltage") is below the breakdown voltage of the medium.
What you're looking for is the voltage or potential difference and therefore the difference in charge between points X and Y.