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Understanding corona discharge

by TheLil'Turkey
Tags: corona, discharge
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TheLil'Turkey
#1
Feb16-12, 12:20 AM
P: 58
I understand corona discharge in electrostatic situations, but in circuits without capacitors/gaps, I don't understand why it can ever happen (since as far as I know, the electric field is virtually 0 in the air around such a circuit). So why is there corona discharge around high-voltage power lines when there are more charged particles in the surrounding air than normal (like after a nearby lightning strike)? There's something I'm missing here and I have no idea what it is.
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Antiphon
#2
Feb16-12, 12:34 AM
P: 1,781
What you are missing is that everything in the world that conducts is a capacitor and a potential spark gap.
Naty1
#3
Feb16-12, 09:20 AM
P: 5,632
Try the introduction here:
http://en.wikipedia.org/wiki/Corona_discharge

At high enough voltages [ potentials] air ionizes [particles separate and display their charged nature] and can carry a current.....current is the flow of these charged particles....
that's lightning, for example.

TheLil'Turkey
#4
Feb16-12, 10:36 AM
P: 58
Understanding corona discharge

Quote Quote by Naty1 View Post
Try the introduction here:
http://en.wikipedia.org/wiki/Corona_discharge

At high enough voltages [ potentials] air ionizes [particles separate and display their charged nature] and can carry a current.....current is the flow of these charged particles....
that's lightning, for example.
This article doesn't even try to explain why the electric field outside a circuit is non-zero, let alone why it can become so high in some situations that corona discharge occurs. Can someone who understands this please explain it or provide an appropriate link?
nasu
#5
Feb16-12, 12:12 PM
P: 1,970
Quote Quote by TheLil'Turkey View Post
I understand corona discharge in electrostatic situations, but in circuits without capacitors/gaps, I don't understand why it can ever happen (since as far as I know, the electric field is virtually 0 in the air around such a circuit).
What makes you think this? There is a non-zero field bot inside and outside the current carrying conductors.
TheLil'Turkey
#6
Feb16-12, 12:56 PM
P: 58
Quote Quote by nasu View Post
What makes you think this? There is a non-zero field bot inside and outside the current carrying conductors.
Gauss's law (to answer the bolded). It seems to me that this law implies that corona discharge around power lines should be impossible.

I understand that there's a non-radial electric field inside current carrying conductors; if there wasn't, there'd be no current (unless they were superconductors).
nasu
#7
Feb16-12, 06:43 PM
P: 1,970
Quote Quote by TheLil'Turkey View Post
Gauss's law (to answer the bolded).
For what surface?

The charge density on current carrying conductors is not uniform in general.
For example, you can get some idea about the charge and field distribution here:
http://www.google.com/url?sa=t&rct=j...MOTIBVJP4s2JfQ

Some people have even visualized the field outside the conductors:
http://electretscientific.com/author...0n1pp19-21.pdf

Both papers are freely accessible.
Dotini
#8
Feb17-12, 09:27 AM
PF Gold
P: 516
Quote Quote by TheLil'Turkey View Post
This article doesn't even try to explain why the electric field outside a circuit is non-zero, let alone why it can become so high in some situations that corona discharge occurs. Can someone who understands this please explain it or provide an appropriate link?
Corona discharge off grass, trees, and other objects with sharp points on the ground beneath thunderstorms is the source of positive charge carried upward from the earth to the atmosphere.

There is a natural electric circuit between the negatively charged earth and the positively charged atmosphere. Thunderstorms act as batteries to keep the earth charged negatively and the atmosphere charged positively. Atmospheric electrical currents flow downward in fine weather and upward in thunderstorms. Thunderstorms deliver charge to the earth by lightning, rain and corona discharges.

"There is an electrical balance to the atmosphere.

The electrical resistivity of the atmosphere decreases with height. From the point of view of atmospheric electricity, the resistivity is sufficiently low at an altitude of about 30 miles that the voltage does not vary much above that point. The region beginning at about 30 miles and extending upward is called the electrosphere. The voltage between the earth and the electrosphere in regions of fine weather is about 300,000 volts. To maintain this voltage the earth has a negative charge of about a million coulombs on its surface and an equal net positive charge is distributed throughout the atmosphere. Measurements have shown that the negative charge on earth remains roughly constant with time. At first glance, this fact is difficult to understand since the charge on earth is continuously leaking off into the conducting atmosphere. In fact, calculations show that if the earth's charge were not being continuously re-supplied, the charge on earth would disappear in less than an hour.

The earth is recharged by thunderstorms. Thunderstorms deliver a net negative charge to earth as a result of the sum of the effects of the following processes:
(1) negative charge carried from cloud to earth by lightning,
(2) positive charge carried from cloud to ground by rain and
(3) positive charge carried upward (the equivalent of negative charge carried downward) through the air beneath and above a thunderstorm, the source of the positive charge being corona discharge off grass, trees and other objects with sharp points on the ground beneath thunderstorms. The total current flowing beneath all thunderstorms in progress throughout the world at any given time is thought to be about 2000 amps, and is in such a direction as to charge the earth negatively. An equal and opposite current flows in regions of fine weather. The result is that the net negative charge on earth and the equal and opposite net positive charge in the atmosphere remain approximately constant."


Source: "All About Lightning", Martin A Uman, Dover, 1986

Respectfully submitted,
Steve


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