Why birds aren't electrocuted?

  1. 1. The problem statement, all variables and given/known data
    Birds are often seen perched on a high-voltage powerline. Why aren't the birds electrocuted?


    2. Relevant equations
    V=IR


    3. The attempt at a solution
    The birds usually have their legs very close together so the potential difference between each leg is small. The resistance of birds are high so I is small according to the equation. But would another factor be that the power line has an insulating material around it which would dramatically decrease V.
     
  2. jcsd
  3. You have the right answer for nearly the right reason.

    Draw the circuit representing the Bird as a resistor (You could measure resistance across your hand to get a totally non-scientific indication of the birds resistance).
    Assume the resistance in the wire is almost nothing.
    And then calculate the relative current in each path.

    You might want to mention what would happen if a big bird (Stork, Eagle) touched two conductors, or the conductor and the pylon (Assuming it's a metal pylon)?
     
  4. In Tango and Cash, Cash says it's because he's not touching the ground :biggrin:
     
  5. Cash is right. Since the birds arent touching the ground, the circuit is incomplete.
     
  6. turbo

    turbo 7,365
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    The power line is at a high voltage potential, but it is just that - a potential. The bird can perch on the line and come to the same potential voltage as the line with no shock. If there is no complete path from that potential to a higher or lower potential or to ground, no current will flow and the bird will not experience a shock. If you could jump up and grab that power line (without touching anything that would provide a path to ground) you could do the same thing that the bird is doing. Voltage does not hurt you absent current flow.
     
  7. PhanthomJay

    PhanthomJay 6,149
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    ".....Birds are often seen perched on a high-voltage powerline...."

    Depends on how high is high. While you often see them on power lines rated below 20kV to ground, you'll never see them on the big lines at voltages over 50kV phase to ground. You may see them on the pylons, or grounding wires, but not on the energized conductor wires. I believe their feathers start to rustle in close proximity to thes wires, due to the high voltage gradient, and they don't dare to try getting energized at those voltages ..I believe they'd draw an arc in so doing.
     
  8. If the potential is high then a small segment of the line (the distance between the feet of the birds) will provide some potential difference. This will mean a current will flow through the bird. Is that correct? Although if we allow V small and R large than V=IR => small I. Is your argument that I in this case is too small to cause the bird harm. Someone dangling on the voltage lines will not cause harm for the same reason although provided their arms are not too widely spaced? Otherwise the potential difference will be larger however R for a human is smaller than a bird.
     
  9. What was wrong with my reason (which was emphasised in the above post)?
     
  10. DaveC426913

    DaveC426913 15,939
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    BTW, whacking a powerline with a 15' metal pool-cleaning net doesn't cause electrocution either. I'm just sayin'.
     
  11. PhanthomJay

    PhanthomJay 6,149
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    Whoaaaa... whacking a power line with a metal net could cause instant death, even if the rod holding the net is non-metal, for it still will have some conducting properties, and allow current to flow through you to the ground. Shoot 170 milliamps through your herat and we'll see you in the next world. Most powerline wires are NOT insulated ...the wire is bare...or the insulation, IF present, is intended only for preventing short circuits due to occasional short duration contacts of small tree branches, and has perhaps only a 600 volt rating. NEVER touch a powerline with ANYTHING.
     
  12. DaveC426913

    DaveC426913 15,939
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    Yeah. I honestly don't know why I'm still alive.

    BTW, I did not mean to suggest in any way that this was an OK thing to do.
     
  13. Don't lie, you know you were advocating for global electrocution. I was JUST on my way outside to play russian roulette with the powerlines.

    Silly dave, I love you.
     
  14. Try it naked, in a thunderstorm and with a golf club. (Its the golf club that's the biggest risk statistically). Of course, nakedness is not a requirement, it would just really freak the neighbours out.
     
  15. PhanthomJay

    PhanthomJay 6,149
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    I don't see anything wrong with it. The current passing through the bird over that short distance of 'voltage drop' with very low potential difference, coupled with the very low resistance of the wire (something like in the order of one-thousandth of an ohm per foot...I'd have to look it up) would be very low....maybe less than a milliamp or so....an imperceptible and non-deadly current (the 'let-go' current of an adult male human..where the muscles 'freeze'...is about 9 milliamps or so, less for a woman or child (5 or 6 milliamps). But don't assume any insulation on the wires..if they were fully insulated for the applicable phase to ground voltage, that would sure change the picture for the better...but most powerlines use bare wires, at least the high voltage ones.
     
    Last edited: Jan 19, 2007
  16. I don't see anything wrong with it either... especially the nakedness... hehehe
     
    Last edited: Jan 20, 2007
  17. DaveC426913

    DaveC426913 15,939
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    "Clothes make the man. Naked people have little or no influence on society."
    - Mark Twain
     
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