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Homework Help: Transmission Lines and Electrical Fields

  1. May 20, 2007 #1
    1. The problem statement, all variables and given/known data

    A family built a cabin directly under a 138kV, 60 Hz alternating voltage power line in the countryside. They assembled a huge coil under the cabin roof intending to tap off electrical energy from the overhead power lines (no wires are attached to the power lines)

    a) How feasible is this technique to supply energy to the cabin?
    b) Would this method (if it worked) reduce the transmitted power to its destination? Is this theft?
    c) Discuss potential hazards the family might face with these conditions

    2. Relevant equations

    None? It seems moreso a communication question than a calculation question.

    3. The attempt at a solution

    a) I am really lost on this question, but here's my thought so far. The power lines have current through them, so it produces an electric field. This will somehow effect the coil they installed by inducing a current in it, and thus allowing them to tap off energy. But wires aren't attached to the power lines, which makes me doubt my thinking.. So as for how feasible the technique is, I would probably say it's not very feasible at all, since even if they were able to tap some energy, it wouldn't be enough to supply their needs. Oh, and I also thought of this too: since it's an AC, it would mean the the field oscillates back and forth as a sine wave, so would the coil would experience even less effects?...

    b) If the technique worked, I would assume that it WOULD reduce the transmitted power. And I did a bit of reading on power theft, and apparently tampering with power lines in any way is considered theft, so I guess that answers the second part of the question

    c) There's this part in our textbook that says that "even very low exposure to EMR has detrimental long-term effects on health. Extremely low-frequency EM fields can disturb the production of the hormone melatonin and might even be a factor in the occurrence of sudden infant death syndrome. Children exposed to this may also be at a higher risk of leukemia. It may also cause increased estrogen levels in adults, which is linked with estrogen-sensitive cancers like breast cancer." So I guess that pretty much answers this question, though I may still research a bit more on it.

    Part c) is the only one I'm confident about at this point, so it'll be great to have some input concerning a) and b), especially a).
  2. jcsd
  3. May 20, 2007 #2


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    There was this Mythbusters episode where Adam and Jamie tested out the above "myth" of stealing electricity. If I remembered correctly, the conclusion was that it is theoretically possible but very much impractical.
  4. May 20, 2007 #3
    b) is definitely a "yes", but I don't know how much power they would be able to steal. My E&M professor said that if you get one of those long cylindrical fluorescent light bulbs, and hold it under those high power lines, it will light up. So you are in fact "stealing" the power to light the bulb.

    I also saw the myth buster episode. I can't quite remember what they did, though. But always take myth busters with a grain of salt. A lot of the things they call "scientific" are anything but...
  5. May 21, 2007 #4
    For part a, voltage is induced in the coil only if there is a change in magnetic flux. (Recall that emf induced is equal to the rate of change of flux). SO in other words, the alternating current in the power lines is why there would be voltage in the coils in the first place.
    So no, AC would not make the current any weaker.
    (Also the size of emf induced would depend on the number of coils, size of coils, its orientation, distance between power lines and the coil: all these would factor into the 'feasibility' I guess - we know this from faraday's law, and ampere's law for straight wire).
    Hopefully the theory is right.
    Last edited: May 21, 2007
  6. May 21, 2007 #5


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    See my corrections in red. Decades of scientific research have failed to find a definite connection between EMR and health risks. The greater risk may be associated with a fallen wire that could burn the cabin down. Or in the receipt of some very annoying electric shocks from the electric field. Or death during the construction of the cabin in such close proximity to the wires.
  7. Jul 5, 2010 #6
    I wonder, why electric fields produced by transmission lines start at some level, increases to some value in a distance r = 10-20 m to wire, afterwards drops down as r gets larger values. any idea ?
  8. Jul 5, 2010 #7


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    Transmission lines consist of more than one wire, so the peak field might not be directly at a given wire, but it is generally in the vicinity. The electric and magnetic fields then drop off rapidly inversely proportional to the square of the distance from the source.
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