Chemical Forums

Specialty Chemistry Forums => Other Sciences Question Forum => Topic started by: xshadow on October 30, 2021, 02:24:48 PM

Title: discharging arc - voltage drop
Post by: xshadow 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...
Title: Re: discharging arc - voltage drop
Post by: Borek 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.
Title: Re: discharging arc - voltage drop
Post by: xshadow 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
Title: Re: discharging arc - voltage drop
Post by: Borek 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.
Title: Re: discharging arc - voltage drop
Post by: xshadow 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 :)