Breakdown potential.....

[MMK]

 

Is it true that 10000 Volts can jump through air and travel 1 cm?

 

If the two electrodes are spaced 1 cm from each other with the above mentioned potential difference........ And how breakdown potential fill in here? What is Breakdown Potential? Plz explain it to me.......

[okr491]

" The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductible. "

http://en.wikipedia.org/wiki/Breakdown_voltage

 

Under normal conditions, air is an insulator, but if you apply enough voltage, it will conduct the electricity.

 

" The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm. "

http://en.wikipedia.org/wiki/High_voltage#Sparks_in_air

- cited from A. H. Howatson, "An Introduction to Gas Discharges", Pergamom Press, Oxford, 1965, no ISBN - page 67

 

Some other sources state the necessary voltage as 19.5 kV (19,500 V) at a distance of 1cm.

http://ethesis.nitrkl.ac.in/2875/1/Full_Thesis_Print_04.07.2011.pdf

[swansont]

http://en.wikipedia.org/wiki/Paschen's_law

(sorry, you'll have to copy/paste to get the link to work)

 

For air at STP, the voltage needed to arc a 1 meter gap is about 3.4 MV. The intensity of the electric field for this gap is therefore 3.4 MV/m. The electric field needed to arc across the minimum voltage gap is much greater than that necessary to arc a gap of one meter. For a 7.5 µm gap the arc voltage is 327 V which is 43 MV/m.

 

If the breakdown is several MV/m, a 1cm gap will require several tens of kV. (at STP)

 

(edit: xpost with okr491, and we have some bugs to work out with the link protocols)

[Enthalpy]

The potential increases with distance, but more slowly than the distance, ie not linearly. This holds for all insulators. So a "breakdown field" holds for a (very) limited range only. It also decreases with the area. The breakdown field seems more related with the insulator's volume.

 

The shape of the electrodes is paramount, as these concentrate the field in their vicinity, and breakdown anywhere on the path is likely to propagate over the whole distance - except if the field is much weaker farther away, in which case the discharge can be local, as in corona effect.

 

A lower pressure, say in an airplane, lowers the potential a lot. Used in fluorescent lamps, which can be metre long.

 

So 20kV over 1cm with spherical electrodes (the only reproducible ones) can easily convert in 10kV with sharper ones.

[MMK]

What if one electrode is pointed and the other one is a plate? Then how will the maths go in this situation??

[Enthalpy]

Field concentration at the pointed electrode only. It's similar to two pointed electrodes at twice the distance, except that the positive and the negative electrodes have different roles.

 

A negative very sharp electrode produces a local corona discharge that doesn't generalize over the full path, while a positive sharp makes most often a full spark. Or it's the opposite if I mixed it up...