WiTricity is the new technology that you might have read about last year that could power a 60w lightbulb wirelessly over 7 feet. It’s supposedly an improved form of induction. One article described it as ‘an obvious concept’ that for whatever reason wasn’t investigated before. Does anyone know more details, both about the reality and the theory?
I found a couple demonstration videos at http://www.witricitynet.com/ (videos #2 and #3). For all the talk about about ‘7 feet’, 2W LEDs can’t light up further than a couple inches, and the 60W lightbulbs less than 1/4".
Ok, I’ll answer my own question some more: The improvement over plain old induction is to use some sort of ‘resonance’ between the two inducting coils. However, another source implied that the principle bordered on radiation coupling (eg, beaming microwaves or lasers) but the receiver had to be less than 1/4 wavelength from transmitter (a few meters given MHz frequency). This way only a small fraction of energy was lost to space and transfer efficiency was high. The ‘resonant coupling’ is just words for saying the two antenna have to be tuned to the same frequency.
Does the above give any physicists here a clue for what’s really going on?
Well the basic theory is that of magnetic induction. Induction is the property whereby a current flowing through a conductor generates a magnetic field around said conductor. If the generated field can cut through another nearby conductor, it will induce an identical current flow in that second conductor (losses ignored for sake of simplicity).
This is the principle upon which power transformers operate. The only connection between the primary and secondary coils is a magnetic one, and thus the two coils are said to be electrically isolated from each other.
This isn’t a new idea in consumer battery recharging - Sonicare uses this design to charge the batteries in its toothbrush. The hand held part rests in a base which has primary coild wound around the “cup” where the toothbrush sits. There is a secondary coil in the toothbrush. Electromagnetic energy from the primary coil induces a current flow in the secondary coil without the need for an electrical “plug-in” contact between the two.
The problem that WiTricity is likely trying to overcome is that the coefficient of coupling is critical in order to ensure maximum power transfer between the two coils. As soon as the two coils fall out of an ideal point of orientation with respect to each other, power transfer efficiency between the two coils drops off pretty rapidly. That, I think, is going to be the biggest challenge with trying to use magnetically coupled power to charge a car in motion.
OK well I see you’re all studied-up on the concept of induction so all of that probably wasn’t necessary.
If they’re using tuned circuits to create a high level of inductive coupling at a particular frequency, I don’t see how you’re going to get a lot of power from source to load. You need a heavy wire to support a high current load, but a small gauge wire to get your tuned circuit to resonate at a high frequency.
I guess I should go spend some time on their web site.
I was going to point you to this thread which discusses the subject with regard to electric cars, but then I noticed that you started that thread, so never mind.
It was rumored that about 80 years ago a lone mad scientist was working on a method to send power over the air 
What Ludovic is alluding to is the work of Nikola Tesla, who developed the first electromagnetic induction systems capable of powering electric devices. The concept of this induction has been known to science for at least a century. The only reason it isn’t in widespread use today is that transmission of electricity via wires was easier to develop and implement. As Patty O’Furniture says, getting a high rate of power transfer is tricky at best.
Come on… there’s got to be physicists here!
What are the physics of an antenna and receiver separated by less than 1/4 wavelength?
What kind of answer are you looking for? The transmitter sets up electric and magnetic fields in its vicinity. If the receiver is resonant at the transmitted frequency, it will set up relatively larger currents, allowing it to couple more energy from the fields. Those larger currents also mean there will be larger losses due to the finite conductivity of the wires in the receiver.
In addition, both the receiver and transmitter will radiate energy as well. Presumably, they are designed to not be good antennas, since that energy is lost. They’d want the energy to be coupled via the near-field only. The far field is the portion of the field which “normal” transmitting antennas are designed to radiate. But there are other components of the fields whose amplitudes fall off as higher powers of distance than the far field. As you farther from the antenna, the near-fields make up a smaller and smaller fraction of the energy.
I guess I was looking for formulas and/or an intuitive, but detail explanation.
Ok, to be clearer, let me pose some concrete questions:
What are the differences between witricity and induction? In how it works? In efficiency? In devices involved? (I.e., in induction you deal, to generalize, with an electromagnet and a coil of wire. You don’t have ‘antenna’.)
Are there strict orientation requirements? (E.g., in induction the receiving loops should be perpendicular to the magnetic field, which itself takes a certain path through space.)
Does witricity use the electric field or electromagnetic waves? Because I’m pretty sure induction uses the magnetic field only.
EDIT: In post #3 I wrote up what witricity was seeming to say about its technology, and was asking if any physicsts here knew what they were on about.
Actually, this could be kind of cool for low power devices.
Induction, like in a transformer, relies on the field from the transmitter being highly concentrated in the vicinity of the receiver. The primary and secondary windings of a transformer are right together, and that’s how good coupling is achieved. It’s not resonant, so you can run a wide range of frequencies through a transformer, and it will still work.
As the receiver is moved away from the transmitter, the energy density of the fields falls off rapidly, 1/R^3 for near fields, so the energy coupling will drop off rapidly with distance. To get around this somewhat, they make the receiver resonant, to improve the coupling. This resonance, I think, is the difference between Witricity and induction. But if you wanted to say Witricity is induction, I wouldn’t argue.
It sounds like they’re using primarily the magnetic field, to minimize coupling into other things. In principle, you could use electric near-field also. Keep in mind, though, time varying magnetic fields generate electric fields, and vice-versa, so you won’t have strictly just one or the other.
I’m a little dubious, because they talk about transmitting power over room-sized distances, but for all the examples they show, they’re really pretty close. If you just need to set your cell phone on your bedroom chest and it charges overnight from some transmitter in the corner of your room, that’d be pretty cool. If you have to set it on a special tray, well, maybe, but its not that much harder to set it in a normal charging unit.
Their website has that snake-oil look to it, but at least there is real physics behind what they’re selling.