December 05, 2021, 08:26:23 AM
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Topic: discharging arc - voltage drop  (Read 592 times)

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Offline xshadow

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discharging arc - voltage drop
« on: October 30, 2021, 02:24:48 PM »
Hi.
Studying discharging(electric) arc I don't undesrtand some phrases :

" when ionization energy of plasm constituents is higher  the  arc voltage drop  increases"

" arc has much more electron density (current) than glow discharge and so the voltage is smaller"


Remember some physics formulas :   V= IR
So if the curreint is higher (like in arc),  the voltage also should be higher!! I'm missing something...mhh ???

Don't undesrtand...

Offline Borek

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Re: discharging arc - voltage drop
« Reply #1 on: October 30, 2021, 04:48:30 PM »
Remember some physics formulas :   V= IR
So if the curreint is higher (like in arc),  the voltage also should be higher!!

Assuming constant R - which is not true in arc discharges.
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Offline xshadow

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Re: discharging arc - voltage drop
« Reply #2 on: October 30, 2021, 04:59:20 PM »
Remember some physics formulas :   V= IR
So if the curreint is higher (like in arc),  the voltage also should be higher!!

Assuming constant R - which is not true in arc discharges.

why is R not constant?

R what is it?

The resistance of the dielctric/gas pahse?
I could think that a gas-phase (between the two electrodes) with more ions  should conduct much easier ....is it correct?

thanks

Offline Borek

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Re: discharging arc - voltage drop
« Reply #3 on: October 31, 2021, 03:43:16 AM »
why is R not constant?

Quote
I could think that a gas-phase (between the two electrodes) with more ions  should conduct much easier ....is it correct?

In terms of Ohm's law: the more the ions the lower the resistance. But: higher current means more heating, more collisions, and more ions, so the resistance changes during arcing.
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Offline xshadow

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Re: discharging arc - voltage drop
« Reply #4 on: October 31, 2021, 08:27:00 AM »
why is R not constant?

Quote
I could think that a gas-phase (between the two electrodes) with more ions  should conduct much easier ....is it correct?

In terms of Ohm's law: the more the ions the lower the resistance. But: higher current means more heating, more collisions, and more ions, so the resistance changes during arcing.
Thanks Borek : )

Because I've read  that:

 in Glow discharge we have high voltage and low current
 in electric arc we have instead low voltage and high current

So the low voltage in electric arc is due to the high-ionized gas in the medium (plasma)  (the arc works at atmosferic pressure) --> low resistenze R  -->  it's required only a low voltage applied to electrode in order to get an high current


WHile I know that  in glow discharge  we have low current (due to few ionized species in the "medium" cause low gas pressure) but high applied voltage  ...because we have few charge carriers  (low pressure than arc)

Is it correct?
thanks :)


Offline marquis

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Re: discharging arc - voltage drop
« Reply #5 on: December 01, 2021, 10:38:06 AM »
There are cases where ohms law doesn't hold true.  These are cases of " negative resistance", sometimes also called dynamic negative resistance. There are "S" type and "N" type negative resistance.  An arc can be one of these cases, depending on how it's set up.  Check and see if the arc discharge You are seeing fits into the category of negative resistance or not.   Good luck.

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