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

High voltage power lines

  1. Aug 14, 2009 #1
    I have a physics question that I'm hoping someone has enough free time and is intrigued enough to figure out for me!

    My sister just bought a house which is near some high voltage power lines. I've heard that people living near these types of lines can sometimes induce a current in wires running through their attic or coils of wire they leave outside.

    I want to know how close you would have to be to these lines to power something useful, e.g. compact florescent light bulbs (~10 watts). I have no idea what voltage the lines are running at, but after some quick research, I think assuming somewhere around 33kV - 110kV might be reasonable.

    I know the inverse-square law probably applies here which makes me think that you'd have to be nearly right on top of the lines to do anything useful, but I wanted to ask anyway as I think it's a fun question. Is this calculation possible or would it just be easier to try it out experimentally with some wire and a multimeter?
     
  2. jcsd
  3. Aug 14, 2009 #2

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    The calculation is possible. All you need is some freshman physics for a quick and dirty calculation. You must assume some voltage (usually 200 - 500 kV) and an amount of power, say 10 MW (I'm guessing here). Using P = IV, you can find the current.

    I would not get close to the lines - there are better ways of lighting a fluorescent bulb. I have heard of farmers burying large area loops of wires under the transmission lines and stealing power by induction to heat up their barns. You will have to figure out the magnetic flux through the buried loop from the magnetic field (assume an infinite wire carrying current I), the area of the loop and the number of turns (if more than one.) Using Faraday's Law, you can figure out the induced emf. Note that the magnetic field lines must enter through the loop only in one direction. If the loop is symmetrically placed under the wire, the net flux through it will be very small at any time. This makes finding the flux not impossible but requiring integration or at least a fudge factor to take into account that the magnetic field is not the same over the loop.

    Disclaimer: I have also heard that some farmers who have done this have been prosecuted for defrauding the power companies. Apparently, significant unauthorized power diversion of this kind causes reflections in the transmission lines, the point of origin of which can be pinpointed by the companies. If you land in jail, don't blame me or PF.
     
  4. Aug 14, 2009 #3
    The inverse-square-law applies to point sources; in this case you have a line source and the fall-off becomes inverse-distance.

    Neil
     
  5. Aug 14, 2009 #4
  6. Aug 14, 2009 #5
    There are usually a lot of laws associated with property that high voltage lines run over. All kinds of things about what you can build, changing the elevation, irrigation, size of farm machinery, grounding fencing if metallic... Were you given any of this information when you bought the house? The Title Company should have informed you about your property, those lines, and the rules associated with them.

    No kites please.
     
  7. Aug 15, 2009 #6
    The power lines are either 3-phase (more likely) or single phase (unlikely). There is no net current flowing in either direction, so the field is not a line source, but two or three line sources with opposing currents. It should fall off as the inverse square.
     
  8. Aug 15, 2009 #7
    Interesting how this works out. A loop of wire would act as one half of a current transformer. The voltage around the loop would vary with load. More voltage would be developed during peak hours.

    Added to the transformer action, there is electrostatic potential beween wires, and between a wire and ground.

    Adding to what Bob says, for single phase transmission the two wires act as a dipole of line current so there is a characteristic distance involved--the distance between the two transmission wires. The larger the spacing between the wires, the larger the induced voltage in a loop.
     
  9. Sep 6, 2009 #8
    I was thinking of a single line, intercepting (receiving) EMP power from an airburst nuclear explosion. The analysis that I have seen, treated a single, long, power line and the fall-off of intensity as inverse-distance.

    Do you know of a discussion of this (preferably on-line)? --my E-M classes were way too long ago!

    Thanks, Neil
     
  10. Sep 7, 2009 #9

    Born2bwire

    User Avatar
    Science Advisor
    Gold Member

    I don't know about line sources, but a similar phenomenon is seen with dipole moments. The close proximity of opposing charges means that in the far-field, the field will drop off faster than the rate normally would for one charge. There should be multiple online discussions on this topic: http://en.wikipedia.org/wiki/Electric_dipole_moment
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook