Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding corona discharge

  1. Feb 16, 2012 #1
    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.
     
  2. jcsd
  3. Feb 16, 2012 #2
    What you are missing is that everything in the world that conducts is a capacitor and a potential spark gap.
     
  4. Feb 16, 2012 #3
    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.
     
  5. Feb 16, 2012 #4
    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?
     
  6. Feb 16, 2012 #5
    What makes you think this? There is a non-zero field bot inside and outside the current carrying conductors.
     
  7. Feb 16, 2012 #6
    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).
     
  8. Feb 16, 2012 #7
  9. Feb 17, 2012 #8

    Dotini

    User Avatar
    Gold Member

    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook