Re: witricity legality?
 

Ah, to be young and still know everything.

Electricity and magnetism are two inseparable components of electromagnetism. A changing magnetic field creates an electric field, and a changing electric field creates a magnetic field. That's how induction coils work to transfer power through empty space.

At a frequency of 10 MHz, it is radio by definition.

Re: witricity legality?
 

did anybody read the massive post i quoted from wiki!?!?!?!?!?!?!



[quote][Ah, to be young and still know everything/QUOTE]
i hear that to much but i do understand electromagentic theory. i did get side tracked see i read the quote from wiki....

Re: witricity legality?
 

Yes, I read it, actually I read it before I posted the first time in this thread.

10Mhz is radio as Alan said, specifically it is in the range referred to as High Frequency Radio, here is some information about the range. http://en.wikipedia.org/wiki/High_frequency

Re: witricity legality?
 

Shadow,
(what is your real name?) As posted above, RF communication is an electromagnetic signal. Two coils spaced apart are capable of coupling energy but it is unlikely that interference or body absorption would not occur. This is how transformers actually function. If you think about current in a coil of wire producing a magnetic field similar to a bar magnet you can imagine the field produced by this device. The FCC limits and licenses devices used to transmit energy under part 15...
TITLE 47--TELECOMMUNICATION CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION PART 15--RADIO FREQUENCY DEVICES--Table of Contents Subpart C--Intentional Radiators Sec. 15.209 Radiated emission limits; general requirements.
The table contained there (http://frwebgate.access.gpo.gov/cgi-...00&TYP E=TEXT) does not copy well but essentially it requires that unlicensed operation must be below 30 microvolts/meter at 30 meters from the radiator. Most hams naturally consider that transmitters in the milliwatt range (much less than one watt) satisfy non-licensed devices. In that the experimental device was described as transmitting 150 watts, in general it would have needed to be licensed even if only an experimental one.

Re: witricity legality?
 

The odd/nifty/novel thing about what's being called WiTricity here is that it's strictly a "near field" thing. There is no traditional radiative field; as the Wikipedia quote says, it involves "evanescant waves" which fall off exponentially with distance. I haven't seen rigorous mathematical details of the setup, but there are references to the receiving coil having to be within a quarter wave of the transmitter. At 10 MHz, 30 meters is one wavelength. It might be far enough for the field strength to be sufficiently low to not need a license.

But evanescent waves are still EM, and they still interact with any conductors within range -- and in doing so, they can scatter the energy so that it does radiate. Shadow has apparently picked up on a comment that the connection between the tuned coils is "mostly magnetic" and thus infers that it isn't radio, but fails to recognize that all the energy that is not transferred between the coils is going to interact with metal, ionic liquids, human flesh, etc. in the same way as any other RF energy.

Re: witricity legality?
 

Unfortunately, building custom connectors are not legal. Our swerve drive is limited to 8 rotations (on each wheel) and we looked into such connectors, but they cost upwards of $400

Re: witricity legality?
 

Depending on how you actually design and implement it, they are legal if they make it through the electrical flowchart of legality.

Re: witricity legality?
 

Alan - This is aimed at the Wikipedia article, not at you.

There ain't no such thing as a special kind of EM "wave" that is any more or less evanescent than any other in a vacuum.

In Earth's atmosphere, at some frequencies, EM fields are somewhat strongly affected by dust, moisture, ions, (buildings,) etc. At other frequencies, not so much.

But, as for any EM field being more more or less "evanescent" than any other field, and/or getting any other special exemption from Maxwell's equations - I don't think so. Over macroscopic distances all EM fields obey those equations.

Blake

Re: witricity legality?
 

ok lets do something easier...


screw witricity lets get the darn robot to go warp 9 how many lines could we cross in hybrid then
and would a warp core be legal on a robot?
seriously i will have to meet someone in atlanta so that they can explain there arguement. i think i just don't understand what is being said. (which you have figured out)

apperantly im wrong so im just gonna shutup and not even try to do anything like this....

by the way its cody

Re: witricity legality?
 

To play the devils advocate I believe that the key word here is macroscopic. I've been doing my best to research what the M.I.T. team has done and it seems that the entire goal of the project is the tap into field before it propagates. Although there is little regarding the topic out in the public domain I believe the concept behind witricity is this. If you look at a graphical representation of an EM wave you will notice that it consists of electrical and magnetic fields at right angles to each other as predicted by Maxwell's equations. If during the first quarter was a device were to tap into the magnetic field converting its energy back into an electrical current (which would have to occur during the first quarter wave as noted by researchers) then the field would not propagate nearly as far simply because much of its energy is being picked up by the second coil.

This may be complete nonsense; I'm only a teenager and will probably be flame, but so be it. In a few weeks I'll be traveling to M.I.T.'s prefrosh weekend so I'll attempt to arrange a meeting with the professor who developed the concept and gain a better understanding in that way.

Re: witricity legality?
 

Exaclty right. That's the way the judges interpreted it in (specific example) Buckeye. I'm sure that everybody's ok with what we did since the interesting concept to get the messy varying system voltage to something useful for consistent display intensity was part of what intrigued judges for the GM Industrial Design award about our display.

-q

Re: witricity legality?
 

Look it up. It's more of a standing wave than a "traveling" one. Here's a reasonably comprehensive explanation:

http://www.andrew.cmu.edu/user/dcpri...nt%20waves.htm

The resonant coil in the 10 MHz system confines the waves in the same way as does total internal reflection in an optical system.

Re: witricity legality?
 

I'll dig into this some more, and I may wind up eating more crow than I care to (I thought the term evanescent was being used in a layman's sense).

Most of the descriptions I read for this power transfer method smell very strongly of BS. I suspect that this is because well-meaning authors are mangling the real physics involved.

For example, in one I see an assertion that a coil fills the space around it with a non-radiative magnetic field. Something has to be wrong with that statement. It is almost certainly is an ill-formed sentence that is either wrong or lacking essential context.

This paper, http://www.sciencemag.org/cgi/rapidp...ref&siteid=sci , written by Marin Soljačić, does not appear to suffer from those oversimplified statements (nor does it refer to evanescent waves, non-radiating magnetic fields, etc.).

Blake

Re: witricity legality?
 

Having read the two above articles I believe I owe quite the apology to shadow. I too found the language of many witricity articles to resonate of pseudoscience and hence was quite dismissive. Perhaps what is most interesting about the above article is that they observed a radiated power of only 5W (concluding remarks above paper) which would make such a device (if properly resonating) fairly safe.

Re: witricity legality?
 

Cody,
Don't stop thinking or learning and don't listen to any of us, find out for yourself.
Thanks to the other contributors of this thread for the article links. The more important of course, is the one by Soljačić. Now that I see the experimental apparatus it is easy to see the mechanics of what is taking place. I disagree with his method, however. He measures his input power at 400 watts (wall) and determines output power by the brillance of the light bulb. All things being equal I call that 15% max. I would much rather see electrical power measurements in terms of current and voltage at both input and output.
The way I see this experiment, a small loop is used to excite a self resonant and unloaded coil at 9.9 MHz. (As pointed out in the article, current at the ends of the coils is zero, but any ham radio antenna enthusiast could have pointed that out, but it has nothing to do with the experiment.) That coil ought to be able to produce a rather intense magneic field. Another self resonant coil in line with the magnetic field ought to also reinforce the first magnetic field and this reinforced field is then able to induce current flow in an adjacent conductor. However, as any of us can realize, any body in or near any of the coils will affect the self resonance of the coils, thereby affecting the coupling efficiency and the energy transfer. Likewise, ferrous material in the magnetic pathway will also alter significantly the energy transfer. What makes this experiment unique is that it is undertaken at a frequency which is conducive to self resonance in easily formed coils at room distances of 1 to two meters. I believe the distance used in the experiment allows for two variables to be satisfied. One is that the two coils have minimal interaction at self resonance while simultaneously achieving a reasonable power transfer due to proximity of the two coils. I believe Maxwell's equations should give a reasonable prediciton of the power transfer in this experiment. As to the device being non-radiating, I don't see how it would be possible to not have significant radiation and therefore produce both interference and possible exposure problems. If we make the rash assumption that the oscillator is at least 50% efficient, then it is easy to assume the remaing lost power is radiated. If all these assumptions are correct, I make that radiated power at about 50 watts.