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I Does a lightning rod also avoid lightning?

  1. Sep 27, 2016 #1
    Hi.

    I understand a lightning rod reroutes the current of a lightning around an object, e.g. building. The lightning will preferably strike the rod because the electric field is strong at sharp points and the air is more likely to be ionized there.

    But does it also avoid lightning by giving off charge to air before ionization occurs and thereby reducing the charge difference between building and cloud? I know that e.g. placing a needle on a Van-de-Graaff generator makes it lose its charge very quickly (without sparks), so why wouldn't this also happen with lightning rods?
     
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  3. Sep 27, 2016 #2

    vanhees71

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    As you write in your first paragraph, the lightning rod rather "attracts" charges and make the lightning happen, i.e., it discharges the clouds in a controlled way so that it cannot do harm.

    The van-deGraaf example is the same principle. Here you put the needle on the charged object, and the sharp edge creates a very large electric field around it which is easily able to ionice the air around it which enables easily the discharge of the generator. At large enough charges you get even little lightnings, of course much weaker than in a thunderstorm, because there's much less charge involved.
     
  4. Sep 27, 2016 #3
    If I turn on my generator (without needle), turn it off again, wait a couple of seconds and approach it with a grounded sphere, I can see and hear a spark. If I do the same with a needle on top, there is no visible spark (neither when the generator is running nor when I bring the grounded sphere close) and only hear a faint crackle. I presume that after turning it off, it loses a lot of charge through the needle.
    So I wonder if the same happens at a lightning rod. Does it also prevent the build-up of a huge charge difference that might result in a violent lightning with a strong current?
     
  5. Sep 27, 2016 #4

    Dotini

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    In a thunderstorm, the lightning rod helps to carry negative charge from lightning to Earth. Positive charge is carried upward through the air under and above the thunderstorm, the source of the positive charge being corona discharge off grass, trees and other objects with sharp points. In this role of corona discharge, the lightning rod would probably work better with many small points instead of just one.
     
  6. Sep 28, 2016 #5
    Hi,
    What about this picture (found here, slide 8):

    img9.gif

    Isn't this exactly what I mean? And the lightning rod even has many spikes as Dotini mentioned.
     
    Last edited by a moderator: May 4, 2017
  7. Sep 28, 2016 #6

    Dotini

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    That graphic may not be entirely correct, or complete. As I understand it, the electric field between the Earth and sky goes all the way up, past the clouds, to the electrosphere. The thundercloud has its own electric field. Corona discharge from the Earth is balancing the main field, and more sharp points from lightning rods may help that, but it's not easy to quantify. Corona discharge from the lightning rod may have an effect on the electric field contained within the cloud, but that too is not easy to analyze. It's said that raising positive charge is the same as lowering negative charge, which is what the graphic on the right indicates is happening between the cloud and the ground. It may be that a bunch of multi-pointed lightning rods could reduce the general electric field in an area and reduce the chance of lightning, so that is an hypothesis that could be tested. But I'm no expert in weather control, and I'm unaware of any such experiment.
     
    Last edited by a moderator: May 4, 2017
  8. Sep 28, 2016 #7

    davenn

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    Agree with Dotini

    that graphic is totally incorrect ... the lightning rod DOESNT provide a path for show discharge from the cloud
    rather it provides a safer alternative path for the discharge that is going to happen anyway, regardless of if the rod is there or not


    Dave
     
    Last edited by a moderator: May 4, 2017
  9. Sep 28, 2016 #8

    FactChecker

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    There is a lightning rod on top of the Empire State Building that certainly attracts a lot of lightning.
     
  10. Sep 29, 2016 #9

    davenn

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    yes and it very obviously backs up my statement that lightning rods don't slowly discharge the cloud so as to stop the big bolt discharge :smile:


    Dave
     
  11. Sep 30, 2016 #10
    For what it's worth, I learned the slow discharge idea too. After seeing the many cases of lightning hitting the rod, now I am tempted to believe the idea that the rod selectively leads the lightning (safely?) to the ground.
     
  12. Oct 1, 2016 #11

    FactChecker

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    It's possible that a lightning rod does some of both. It could reduce the charge at that immediate neighborhood in the ground (probably not so localized in the cloud) and also lead any remaining lightning charge safely to ground.
     
  13. Oct 1, 2016 #12

    davenn

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    that is very much closer to the situation ... remove the words selectively leads and replace with "conducts"
     
  14. Oct 1, 2016 #13

    davenn

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    rather, it does the opposite. It peaks the ground charge ( normally positive) at it's location making the discharge from the cloud to rod much more likely ( mainly because of the height difference) than from the building or other lower objects


    Dave
     
  15. Oct 1, 2016 #14

    Stephen Tashi

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    I recall reading that an early controversy in the history of lightning rods was whether they should have a sharp point on their end or have blunt end or even a large globe or plate on the skyward end. A sharp point tends to leak charge into the atmosphere. A blunt end tends to accumulate it.

    I'm sure any type of lightning rod is designed to safely conduct a lightning strike to the ground. If we want to attract a strike it would be better to make the rod act like a good capacitor and equip it with a blunt end. But I've never seen any text that says equipping a rod with blunt end will widen the "cone of protection" that the rod provides.

    Do modern lightning rods have sharp ends or blunt ends ?
     
  16. Oct 1, 2016 #15

    davenn

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    ....and globes. It's a real mix. I know several buildings in my city that have globes at the top of the rod. But the majority are blunt ends
    When I worked in the radio telecoms industry, ALL our lightning rods on radio masts were reasonably blunt ended. 6ft long 2" diameter galvanised iron with a sealed off flat end cap.

    here's one of those globe examples close to home ....

    upload_2016-10-1_18-25-59.png




    Dave
     
    Last edited: Oct 21, 2016
  17. Oct 20, 2016 #16
    I just stumbled upon one more source supporting continuous discharge. In the section "Applications of Conductors" it says

    Lightning rods work best when they are most pointed. The large charges created in storm clouds induce an opposite charge on a building that can result in a lightning bolt hitting the building. The induced charge is bled away continually by a lightning rod, preventing the more dramatic lightning strike.

    Well it seems to me that either there are different types of lightning rods that work quite differently or it's not fully understood how they work.
     
  18. Oct 20, 2016 #17

    FactChecker

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    This is not an either-or situation. A lightning rod that continuously discharges can also attract lightning strikes. As your quote states, the magnitude of a lightning strike can be reduced if there has been continuous discharge to "bleed off" the charge a little.

    I think that the shape of the tip may have more to do with the tip not being destroyed than it has to do with changing the amount of continuous discharge.
     
  19. Oct 21, 2016 #18

    davenn

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    Yeah, I also have a lot of problems with those comments too

    I have NEVER seen a pointed tip lightning rod on a building or other structure ... if they are so wonderful at dissipating charge slowly, why don't they get used ?
    As I said in my earlier post, ALL rods I installed on comms towers/poles were blunt ended and the photo I posted clearly shows a globe atop the rod

    I don't totally disagree with the comment ... I just think it probably works better under controlled lab conditions when things are done on a small scale, compared to the large scale of one to several km that a lightning bolt discharges over and the very high voltages involved

    I would consider that a pointed rod would actually encourage the formation of streamers from it reaching up towards the sky where it may eventually meet with a stepped leader and result in the discharge channel


    Maybe ? ... I cant answer that one


    Dave
     
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