Induced Current in a Loop - Changing B Field

  • Thread starter jds10011
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Apologies if this is not in the proper location/format/etc; first post. In a very short (3 min) segment on MythBusters, they unsuccessfully tried to induce a current in a loop in the same plane as a current carrying wire (AC). I originally assumed this didn't work because the B field was not changing, but as the wire is carrying AC current, I seem to recall that means the B field is in fact changing. Anyone willing to take a stab at explaining why their setup doesn't work?

NB: The experiment, had it worked, might be construed as illegal, so please note that I'm not asking for an explanation of how to correctly steal power from an electric company, but rather what error in physics they have made in their experiment. (Lack of a load, perhaps?)

The segment may be viewed here: Begin at 1:30 if in a hurry.

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With Three Phase transmission, there are three magnetic fields produced, with 120o of phase between and these will approximately cancel at a distance. Mythbusters missed out on that! Tut tut.

As the basics of transformers are established, the system would have to work in principle, of course, but the number of turns needed with an Air Core at a safe distance is big. I imagine some iron in the coil could help a great deal. Enough to measure but not for significant power, I think.

On the same topic, I remember hearing of another 'farmer' (it's always farmers, isn't it) who had a tuned loop antenna on a building next to a high power mf or lf transmitter. This, reputedly, gave him lights in his chicken houses. The story goes that reception was worse for listeners living in his direction. I think that might a bit more believable, bearing in mind the high mutual impedance (coupling) between structures that are significant fractions of a wavelength. (A Yagi antenna works because of it) But, then again, not many people live next door to a big transmitter.

There may be an alternative possibility of using the E field , using long lengths of parallel wires, placed some way apart and under the lines. There's still the problem of the three phases, though.
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Despite this being a bit dodgy, I can't see anything wrong with discussing this sort of problem because these imagined systems are just too much trouble unless you happen to be a farmer (do we have any farmers on board?).

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