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Who is most likely to be hit by lightning

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data

    During a thunderstorm Bill and Corky are caught outside in the middle of an extremely large paddock (with no trees, houses or any structures in sight). Bill, who is extremely tall and flexible, curls up into a perfect 1 metre diameter ball. Corky who is only 1 metre tall and skinny just stands upright. Using physics principles explain who do you think is most likely to be struck bu lightning and why.

    2. Relevant equations

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    3. The attempt at a solution

    I think that Bill is more likely to be hit by lightning. His body will offer less resistance to the electrical current as the cross-sectional area of his body is larger. I was thinking that maybe it would also be influenced by the surface area of each, the one with the larger surface area to volume ratio losing charge faster...but I decided that this probably wouldn't influence the situation much. I'm really unsure about my answer because google is telling me very different things about lightning strikes!
     
  2. jcsd
  3. Jan 19, 2012 #2

    ehild

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    Hi phosgene,

    You certainly saw lightning rods on tops of buildings. Why are they put there? How they look like?

    http://www.electrical-installation.org/enw/images/5/55/Fig_J12_EN.jpg

    Which boy is more similar to such a lightning rod?
    ehild
     
  4. Jan 19, 2012 #3
    Thanks for your reply. Why is the lightning more likely to hit the lightning rod, though? It seems to me as though it's because the rod is a good conductor and because it's closer to the storm cloud than the building is. But in the case of the two people in the question, they're both at the same height...so I guess it's down to who will conduct the current better. I had a feeling that it was the guy who stood straight up, but I couldn't think of a reason to justify this.
     
  5. Jan 19, 2012 #4

    ehild

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    Lightning is kind of electric discharge and such happens at the places where the electric field strength is highest, at the most pointed places of a charged object.
    My favourite static-electricity experiment is that with my cat: Stroking its head on a dry, sunny day makes its fur electrically charged. If I close my finger to the tip of its ear a sparkle jumps over to my fingertip, and the cat makes a completely confused face.

    The lightning rod is not much closer to the cloud then the roof, but it emerges out from the surrounding. The clouds in a thunderstorm get charged and there is a strong electric field around them. So strong that the molecules in the air get ionized, and making the air conductive, let the charge flow in that direction. Meanwhile the charge of the cloud attract opposite charges, repel like ones in a conducting object on the ground below the cloud. The charges accumulate at the pointed parts: the electric field is highest at the surfaces with the highest curvature, and electric discharge starts from a pointed part of the object. The two discharges unite and become a flow of current between the cloud and the ground through the object.

    Corky stands erect and the top of his head gets charged in the electric field of the cloud. His figure is pointed so the electric field is high at the top of his head, a good place to start an electric discharge. Bill also gets charged but the charges distribute evenly on the surface of the sphere he made of himself. All places are alike, the electric field around him is not as high, as at Corkys head.

    When in a thunderstorm, it is better to lay flat on the ground. Never hold a metal object in your hand. Never stay close to a metal object, like your bike.

    ehild
     
    Last edited: Jan 19, 2012
  6. Jan 19, 2012 #5
    Do the charges accumulate in the pointed parts because there is less volume to spread them out in? Other than that, I think I get it now, thank you! :)
     
  7. Jan 19, 2012 #6

    ehild

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    A sloppy explanation is that the charge of the cloud attracts the opposite charges in the rod and they go as close as possible along the rod towards the cloud, accumulating at the top end, while the equal charges move to the ground. So there is a high charge density at the pointed end, yes, you can say, that it is a small place they can occupy.
    The electric field is proportional to the charge density on the surface, so you get a high electric field around the pointed end.
    Think of Coulomb's Law and Gauss' Law. The electric field of a charged metal sphere is kQ/r2 at distance r from its centre. If you have a large and a small metal sphere and give equal charge Q two both, the electric field will be much bigger at the surface of the small sphere than at the surface of the big one. A pointed end is like a sphere of very small radius...

    ehild
     
  8. Jan 19, 2012 #7

    PhanthomJay

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    I believe the best way (latest theory) to protect yourself from lightning when you can't find shelter in a building or auto is to get away from trees, crouch down with heels clicked together, and block your ears from the deafening sound of a nearby strike. Ground currents from the nearby (very close) strike should pass through your feet and not your heart.
     
  9. Jan 19, 2012 #8
    Thanks ehild, I always have trouble understanding electricity, but you've been really helpful. Thanks again!
     
  10. Jan 19, 2012 #9

    ehild

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    You are welcome. And remember what you have to do if thunderstorm catch you in an open field. :smile:


    ehild
     
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