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

Why hasn't lightning charged the earth?

  1. Jun 28, 2011 #1
    The earth has been struck by lightning for 4.5 billion years.Then why is it that the earth has acquired no [or negligible] charge. I Have also heard that their is a permanent potential difference between the earth's atmosphere and surface.What is the cause of this?Why doesn't lightning neutralize the potential difference between the atmosphere and surface?
     
  2. jcsd
  3. Jun 28, 2011 #2
    Think about this one.

    Where does the charge come from in the first place to create the lightning?

    Lightning is a sign that charge separation is being discharged.
     
  4. Jun 28, 2011 #3

    Dotini

    User Avatar
    Gold Member

    There is an electrical balance between the earth and the atmosphere. Lightning is constantly charging the earth, and the earth is constantly discharging 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
     
  5. Jun 28, 2011 #4
    thank you!But work has to be done to restore the potential difference,right?Most systems at equilibrium attain a state of least potential energy.Why would the earth do extra work to restore the potential difference?Wouldn't it be favorable[energy wise] to have zero potential?
     
  6. Jul 1, 2011 #5
    Think about the lightning observed during a volcanic eruption-- There's a flow of particles and (molten) droplets, charge separation, a fine display...

    Also, IIRC, from a TV documentary on lightning, where they hung a cable across a deep valley to study the clouds passing within, lightning may be positive or negative...

    Here's food for thought:
    http://en.wikipedia.org/wiki/Kelvin_water_dropper
     
  7. Jul 2, 2011 #6

    Drakkith

    User Avatar

    Staff: Mentor

    The Earth has no choice in the matter, lightning is simply a byproduct of the current situation it is in. It boils down to the effect of heat transfer from the earth. The sun heats the earth, which causes things like water evaporation to happen. As a consequence of this, lightning occurs due to charge separation. In the far future after the sun dies and no more light is heating the earth, we will not have lightning anymore.
     
  8. Jul 2, 2011 #7

    Astronuc

    User Avatar

    Staff: Mentor

    Lightning is the mechanism by which the Earth reduces a potential difference that is established with a charge separation. The sun imparts energy to the earth, and that energy drives the weather system. As far as we know, there is an electron for every proton in the part of the universe.
     
  9. Jul 2, 2011 #8

    Dotini

    User Avatar
    Gold Member

    In addition to its role in regulating Earth's electrical balance, thundercloud lightning may also be responsible for gamma rays, particle beams and antimatter. Exactly why and how is poorly understood.
    http://www.nasa.gov/mission_pages/GLAST/news/fermi-thunderstorms.html

    @ Nik: Positive lightning is fairly rare, but of great interest, since their peak current and and total charge transfer can much larger than the more common negative flash. Positive ground flashes are probably initiated from the upper positive charge in the thundercloud when that cloud charge is horizontally separated from the negative charge beneath it, and may actually be closer to terrestrial features such as mountainsides or tall buildings.

    Positive lightning is generally thought to be the energy source for upper atmospheric sprites, although I have heard of disagreement.

    http://www.spacedaily.com/news/earth-02x.html
    "We discovered that seven to ten times as many negative cloud to ground strokes produce sprite halos as do positive cloud to ground strokes. That, coupled with the fact that every cloud to ground stroke, positive or negative, tries to produce a sprite or sprite halo, indicates that the amount of energy being deposited in the mesosphere by these sprite processes and related processes exceeds what we thought the sprites did by a factor of 50."

    Bering says that amount of energy is comparable to the amount of energy the sun pumps into that same volume of atmosphere above the thunderstorm in daylight hours.
    (!)

    Respectfully submitted,
    Steve
     
  10. Jul 2, 2011 #9
    Surely it's pretty obvious that in intercloud lightning one cloud must be positive, the other negative?
     
  11. Jul 2, 2011 #10

    Dotini

    User Avatar
    Gold Member

    Positive charges usually collect in the upper regions of a thundercloud, and negative in the lower. There may be a small amount of positive charge at the very bottom.

    Cloud lightning is any lightning that does not connect to earth, and constitutes the majority of all lightning. Cloud discharges can be subdivided into intracloud, intercloud, and cloud-to-air flashes. These are not usually studied because they are not of great practical interest - do not cause forest fires, power outages or ill effects of cloud-to-ground lightning - and are not easily studied. They are of interest to aircraft designers. But investigators sometimes differ as there may be a number of phenomena at work here.

    Intracloud lightning discharges typically occur between positive and negative cloud charges, and takes place over about the same duration as cloud-to-ground discharges, about half a second. A typical cloud discharge transfers tens of coulombs of charge over a spatial extent of 5-10km. It's thought to consist of a continuously propagating leader that generates weak return strokes called recoil streamers, with which are associated electric fields termed K-changes, when the leader contacts pockets of charge of space charge opposite its own. The cloud K-changes are similar, but usually of opposite polarity, to the K-changes associated with K-processes that occur in the interval between return strokes in ground discharges.

    In some cases, flashes that are produced primarily within the cloud, and are best characterized as cloud flashes, produce, seemingly as an unimportant by-produce, a channel to ground.

    All the above information comes from Martin Uman's book, "The Lightning Discharge". I apologize if all this is more than you really wanted to know about lightning. Me, I'm buggy over it.

    Respectfully submitted,
    Steve
     
    Last edited: Jul 2, 2011
  12. Jul 2, 2011 #11
    Perhaps there's a different way of looking at it?

    Suppose the lower part of a cloud becomes negatively charged.

    Then by electrostatic induction the ground immediately below the cloud will become positively charged, as will the upper part of the cloud.

    Similarly if there is another cloud, vertically above the first one, the lower pat of this cloud will become negatively charged.

    If the distance between the clouds is less than the height of the first cloud then the upper cloud will discharge before the cloud to ground stroke.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why hasn't lightning charged the earth?
  1. Why is the earth so big? (Replies: 13)

Loading...