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Why is electricity not transmitted wirelessly such that we don't need to span cables on the earth's surface? As in: electricity is transmitted wirelessly from the power plant to the household.

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Paging Mr. Tesla, you have a call on Physics.SE... –  JYelton Jul 12 '11 at 18:18
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These toothbrushes google.com/… are recharged through plastic. I once showed it to my ex-colleague, physics Prof Shiraz Minwalla, and he was as puzzled as I was: how do the charges get to the toothbrush when only (insulating) plastic matter is in contact (which is the case to avoid short circuit in the bathroom)? Haha, energy doesn't need charge transfer on long distances - electromagnetic waves are enough. –  Luboš Motl Jul 12 '11 at 19:47
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See similar gadgets - transmiting at longer separation - at youtube.com/watch?v=-2zHlQqJ3GM and web.mit.edu/newsoffice/2007/wireless-0607.html (MIT projects) - I personally bet that those gadgets will ultimately become frequent in the households. For longer-distance power transmission, it's inefficient - unless you transfer the energy via lasers whose beams go to a clear destination. –  Luboš Motl Jul 12 '11 at 19:50
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And to follow up on Lubos. The sun heats the earth, via electromagnetic energy waves (light). Aside from the minor correction of the solar wind, no charge transfer is involved. –  Omega Centauri Jul 12 '11 at 19:52

6 Answers 6

up vote 10 down vote accepted

Electricity is the flow of electrical charge - generally electrically charged particles called electrons in a wire. It can't flow through air, except in the form of electrically charged particles of air - as in a spark or lightning stroke.

Magnetic fields can travel in air, so you can send electricity by using it to make a magnetic field and then using the magnetic field at the other end to make electricity. This is how a transformer works - but it only works efficiently if the two sets of wire making the magnetic field are very close.

You can use it for sending small amounts of electricity a short distance where a wire (or connector) would be difficult, such as charging an electric toothbrush - but it's not efficent for large amounts or a long distance.

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This confuses electric current with electrical energy. Electrical energy transmitted by power lines does flow through the air, in the form of the electric and magnetic fields surrounding the wires. Electric charge isn't transmitted into objects and then used up by them. In most power transmission, electric charge just wiggles back and forth a slight distance. –  endolith Jul 13 '11 at 15:56
    
NB the Rolls Royce 102EX electric luxury prototype car can charge from cable or by induction, and they claim 90% efficiency on the inductive charging: if that's true, then large amounts can be transmitted efficiently. That's at short distance. As you say, long distance will be inefficient - as Georg says, "diffusion" is the clue. –  EnergyNumbers Jul 28 '11 at 8:59

Power is transmitted wirelessly in many applications, just not where transmission efficiency is high importance. And that is the reason it is not done more: efficiency. At low frequencies, galvanic (that means metal) conduction of electricity is many, many, many orders of magnitude more efficient than, say, air.

At higher frequencies, one can use electricity to create E-M waves which travel well in air (or even space); but the problem is that the medium in which they travel is typically 3D (rather than the functionaly 1D path in an electric circuit), so the stuff spreads out a lot and the receiving station only gets a very small fraction of it (your car or home radio, receiving a signal from a braodcast station, is an example).

Techniques have been and are continually being developed to force power to travel in a 1-D manner, instead of 3D, in air without wave guides. Some successes include phased array RADARs, lasers, microwave antennas for telecommunications, Yagi antennas, and parabolic dish antennas. However, no commercially viable technique has surfaced to move large amounts of electrical power over long distances without long stretches of metal.

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So, why is this answer downvoted? –  Fabian Jul 13 '11 at 19:32
    
Nothing wrong with it, so upvoted. –  Nigel Seel Jul 15 '11 at 10:55
    
Also up voting. –  MGZero Aug 11 '11 at 16:02

It can be done via electromagnic waves. The higher the tansmission frequency the better. Tesla wanted to do it. But is is very inefficient, much of the power will be dissapated where it won't do you any good (such as heating the ground), or be poached by non-paying people who have learned how to harvest it. There is some work on antennas that can be used to obtain energy from the environment (mainly radio waves), but these are for very low power remote devices, they are not suitable for bulk power applications.

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Actually there have been made some wireless chargers where the energy is transferred by using electromagnetic field. Anyway the efficiency of such chargers is much lower than wired chargers due to following reasons:

  1. The antenna (remote charger) doesn't catch the whole electromagnetic field and therefore only a certain ammount of available energy is received

  2. The power decreases with distance.

The first issue could be solved with directional antenna (in this case in my opinion it is much easier to plug the cable directly into the power source than fixing the receiver - remote charger - into the optimal receiving point)

Regarding the decreasing field amplitudes the electromagnetic field consists of far field and near field. the near field dominates near the source and the far field extends from about two wavelengths distance from the antenna to infinity. The near field however decreases faster (1/d^3) whereas the far field decreases with 1/d^2. Due to these facts wireless communications exploit far field and the frequencies are high.

To conclude the answer I would say although it is possible to transfer the energy wirelessly it is not affordable.

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It is very convenient (no open contacts - already great), just rather problematic in many cases. –  sharptooth Aug 12 '11 at 6:56

I have one example of useful power being transmitted over long distances.

When I was a kid I got an electronics kit and one of the circuits you could build was an unpowered radio receiver - the energy of the broadcast signal was enough to drive the earbud. It is also called a crystal set and you can read more on wikipedia.

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Nicola Tesla was actually perusing this wireless energy transfer by means of his telsa coils. He actually designed large EM coils that could be placed near the generating station and smaller coils at subscriber end.

Well, people at that time, especially under the influence of Edison (they were arch enemies), made a mockery of him and the attempt was soon discarded after his death.(One may call this a conspiracy theory)...

But then it is possible....

EDIT1: One viable solution is Tesla Coil.

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any insight why this answer has been marked -1???? –  Vineet Menon Aug 16 '11 at 8:31
    
Well, Tesla may have spent much of his career battling the PR of Edison, but is it really relevant today, unless the "conspiracy" is ongoing? –  James Mar 10 '12 at 12:05
    
but then much of his papers are not available today! Even now I have heard people say that many of his inventions have been made unavailable to public! –  Vineet Menon Mar 11 '12 at 8:01

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