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Pardon me if this is a stupid question but my physics courses are from a long long time ago and I was a teenager back then. ;)

Electricity in a wire generates electromagnetism, right? Would it be possible to capture this EM field and use it as a power source or store in an accumulator?

Would that increase electrical consumption?

I thought that it would be great to get power from existing electrical wires (let's say there is a segment without insulation) without consuming any power from the electrical grid. However, if that would be the case then we surely would have seen some gadgets to that effect in the market, right?

I am also looking for a technical/physical/simple math explanation of why it can/cannot be possible to do so.

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Some points you make are correct, some others are not.

  1. A current that runs through a wire creates a magnetic field around that wire. An alternating current in a wire creates an electromagnetic field around that wire.

  2. Yes, you can capture that electromagnetic field. This is how an antenna works: The signal is sent by having an alternating current run up and down the antenna, and the resulting field can then induce a current in another wire (the receiving antenna). You could, theoretically, use that current to light a bulb or some such thing.

  3. So in principal you could use that to receive and store power. I believe that this is what Nikola Tesla had in mind when he wanted to find a system that could transmit energy wireless.

Two things, however. First, the EM field doesn't come for free. You'd still need the power plant to generate the alternating current. Second, this method of transmission is very inefficient. To receive any appreciable amount of power, the initial signal/current has to be ridiculously high. Think about it this way: Your microwave takes current from the wire and converts it into EM radiation to cook your chicken. Now imagine you wanted to send the same power to the microwave via an EM field. That field would need to have at least the same power as the one you put in your chicken, but that'd mean you'd cook everything else around you too :)

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Actually, this "wireless energy transfer" is in common use nowadays - lots of electric toothbrushes, shavers etc use that for charging their batteries - leaving no contacts exposed, charging through plastic case. The idea is "a transformer with its magnetic core cut in two pieces, one with primary, the other with secondary coil." A small gap between the two halves of the core generates some loss but removes need for electric contact (or even touch) between the two coils. –  SF. Mar 25 '13 at 18:00
    
Great answer! What if I place something under a power transmission line (on the ground) and capture the field? Since there is always alternating current there it could work. How about the transmission line from my backyard vs in the fields? I know that in the fields the power is greater so much more EM, but would my backyard power line be somewhat enough? –  Stécy Mar 25 '13 at 18:00
    
Stecy, I would strongly discourage messing around with your backyard power line... –  Andrew Gibson Mar 25 '13 at 18:58
    
Yes, it is theoretically possible to harness energy from the electromagnetic fields under a transmission line, but the amounts would probably be really small. I remember having this as a problem on the exam of a course in electromagnetc field theory. It turned out that it was probably not worth the effort. Also, if you actually did this efficiently, you might be accused of stealing power from the power grid. –  jkej Mar 25 '13 at 19:02
    
@AndrewGibson: yes, sensible advice. –  Stécy Mar 25 '13 at 19:03
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Yes you can transform power from overhead powerlines if you run parallel to them. I directly undernieth them. For example if you were to use an electric fence setup. By using the insulated attachments and you ran a wire back and forwards say a few 100 feet long, this acts as a transformer to the high voltage and current being used in the above HV lines. I would not recommend doing this if you are not familiar with electricity of this nature. I work on low and high voltage and use the proper safety equipment when doing my job. What I have told you above is for educational purposes only.. cheers

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protected by Qmechanic Mar 22 at 1:23

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