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

Why don't birds sitting on power cables atop transmission posts get electrocuted?

  1. Jan 10, 2004 #1
    This is the same equipotential lines thread that you may have read
    in the Linear and Abstract Algebra.

    I have questions…

    1. Why don't birds sitting on power cables atop transmission posts get electrocuted? Why do boys get electrocuted when the thread of the kite they are flying accidentally touches a power cable?

    2. When you walk across a carpeted floor and then hold the door knob, you are more likely to get an electric shock. Why? Where does the current come from?

    3. What do the lines of force surrounding a point charge represent?

    4. How do I draw equipotential lines surrounding a point charge?

    5. How do I draw the lines of force surrounding a point charge?

    6. When you get caught at the middle of a thunder storm in an open field, you usually take refuge under a tree lying down. How should you lie down so you won't get a shock if ever the tree is hit? =)
  2. jcsd
  3. Jan 10, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    1. Birds do not get electrocuted because they are at the same potential as the line they are sitting on. There is no potential difference to create a current flow.

    2. The kite string completes a circuit from the wire at an elevated electrical potential to ground. The person holding the string becomes part of the circuit.

    3. The variation of the electric field strength?

    4. In 2d, as circles, in 3d as spheres.

    5. DO NOT lay under a tree during an electrical storm. If you are concerned about lighting lay flat on the ground WELL AWAY from any trees. If there is a tree near by, lay or stand, on a tangent to a circle concentric to the tree, do not lay on a radial line. Lighting strikes create a huge potential gradient in the ground surrounding the strike. If you are laying, or standing so all parts of your body are equidistant from the center of the strike (ie on an exponential line) you will be in less danger then if part of your body is closer to the strike.
  4. Jan 10, 2004 #3
    I think related to the potential gradient you mention, the advice I see most often is to *not* lie down but to squat as low as you can with only your feet touching the ground. That's hard for most americans. :smile:
  5. Jan 11, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, you would be better off squatting. The maximum distance between you "points" of contact would determine the potential difference across your body. What ever the gradient in the ground is, the smaller distance you span the safer you are.
  6. Jan 11, 2004 #5
    Reply to birds.

    As to my "history", my 2nd year teacher told me that birds don't get electrocuted because their feet are somewhat insulated... I don't know if he's right.
  7. Jan 11, 2004 #6


    User Avatar
    Science Advisor

    I wonder if you remember that correctly. As said before, birds don't get electrocuted because they don't make a link to a different potential. A man can hang onto a bare wire without getting electrocuted as long as he doesn't make contact with a ground or a different wire.

    I suppose you could say that the air, or whatever is preventing the bird (or man) from contacting a ground is "insulation" but it certainly doesn't have anything to do with special properties of birds' feet.
  8. Jan 19, 2004 #7


    User Avatar
    Homework Helper

    Just something I thought might be found interesting, and that is related to getting electrocuted by power lines:

    the term "ground" has its historical origin in the fact that the low side of high tension (high voltage) systems are traditionally literally buried in the ground so that the literal ground is at electrical gound. This is still common practice for municiple power distribution (Have you ever noticed the cables coming down from the top of the pole that are buried in the ground like an oversized tent stake), so being in contact with the literal ground means being in contact with electrical ground. This ensures that you will be at a large potential diffenernce from the power lines if you are standing on the ground, but for the birds on the wire, they are not touching literal earth ground, so they are not in direct contact with electrical ground and not necesarily at a high potential differnece from the line.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook