My own question in this thread got me thinking about this.
Is such a concept even POSSIBLE? Would it ever happen? Like, plugging a transmitter into my wall outlet, then a receiver into my electronic device?
You mean powering devices wirelessly, using RF or microwaves to deliver the power? It’s possible, in theory, but in practice, you’ll have huge engineering headaches to deal with. Remember that electomagnetic radiation drops off roughly as the square of the distance (it would be exactly, if the transmitting antenna was a point source), so you have to ensure sufficient power can be delivered at the maximum expected range. There have been discussed similar schemes for delivering power from solar satellites, by beamign the energy to Earth in the form of microwaves, but this involves a tightly-focused beam, aimed at a stationary receiving antenna. Broadcasting power to portable devices, like you’re asking about, is orders of magnitude more difficult. It’s definitely not a trivial problem, and likely impractical, to boot.
Nicola Tesla seemed to think that one day we would be beaming power through the air all over the place, but so far it hasn’t happened much, due to the engineering headaches QED mentioned. However, it is done in many small applications. RF tags, which are used on shipping containers, security badges, EZ-pass toll booths, and even in your pets (ask your local vet), are quite often powered by the incoming radio beam. The incoming radio beam is first used to charge a capacitor, and once it has enough of a charge on it, the semiconductor on the tag activates and uses the stored power to beam back a reply. As a practical matter, you don’t get much power this way, and beam powered RF tags always have a fairly short working distance. Longer distance tags have a built in battery instead of relying on beam power.
It’s also done on occasion to “beam” power through a plastic cover, to charge batteries inside something that is sealed and waterproofed.
Broadcasting power from one large antenna to another one, as QED talked about, has also been done in test systems, but not for anything large scale and practical, and as far as I know no one has actually tried to beam power from a satellite to earth or vice-versa, although it has been talked about for years.
I suspect that with nano devices it will be entirely feasible. For example, for someone with a physical condition needing intermittant attention, nano-implements could be implanted along with a telemetering device to send pictures. Then when the need arose, doctors miles away at a hospital could perform maintenance by radio.
Covert listening devices, too.
Hmm, what if you flood the area with a low-power EM radiation? Choose a frequency range that doesn’t cause harm to humans - say, visible light. It might be enough to power small electronic devices, like calculators and wristwatches.
The major problems with beamed power are (1) the human body is a very effective absorber of EM radiation, and (2) any power that doesn’t hit the device is wasted. The only way around is to use low enough power so it doesn’t cause harm (e.g. the aforementioned RF tags) and negligible waste, or use tightly focused beams (e.g. the proposed solar power satellites).
#1 is not insurmountable since we now regularly receive radio inside enclosures that reflect virtually all of the energy that impinges on them, such as aircraft fuselages.
#2 is a bit sticky. However, nano motors and the like require very little power and the total is so small that it might pay off in critical situations even if 95% is wasted.
It’s definitely possible, but it’s so horribly inefficient that it’s not common. Let me point out some places that it is used, though.
Have you seen key cards – for access to doors, parking lots, and so forth – that you just have to wave at the receptor from some distance, and it opens something up? Or have you seen a warehouse where the contents are tagged, and a computer system lets you know where the different packages are, and what they are? Or do you use a speed tag at your gas station, or through a tollway? They use a form of beamed energy.
The card or tag has an antenna, a capacitor that can be charged up, a small transmitter, and a minimal amount of logic circuitry. The “reader” transmits a signal (constantly or on demand, depending on the application). The card picks this up, and uses it to store energy in the capacitor, which runs the circuit on the card. It then transmits its ID code back to the reader.
These are very low power applications, and tend to work at a short range. Transmitting energy runs into several problems:
[ul][li]Attenuation: The power drops off exponentially with distance even in a perfect vacuum. Unless you very carefully design your transmitting antenna, it’s sending power in all directions, most of which is wasted. Even with an efficient directional antenna, you’re not getting all of the power to the receiver.[/li][li]Absorption: Air, and other materials between the transmitter and receiver, further absorb or deflect some of the power.[/li][/ul]
As you can see, it’s difficult to beat having conductors carry your power. But it is possible, and we even have some everyday applications that use power transmission.
A magazine article about some of these here:
They don’t even need to be powered at all for that: http://www.spybusters.com/Great_Seal_Bug.html
RR
You mean Arf-ID tags?
Another aspect that hasn’t been brought up yet is the side effect of transmitting power. Transmitting power at 60 Hz would be hard and inefficient, but transmitting at 2.4 GHz is pretty easy. Trouble is in the power. Your cordless phone may operate on 2.4 GHz, but at rather low power - perhaps three watts. What can you run on three watts? One christmas tree bulb is about it. Want more power? Prepare to get warm. Warm as in what’s happening inside your microwave oven - this is pretty much what will happen to anyone near a power transmitter.
FWIW, there certainly is enough “wild” RF in the air to be useful. Years ago, I had a random-length long wire antenna connected to a rudimentary crystal radio - pretty much just the wire and a germanium diode. This snagged enough power from the air to run a tiny audio amplifier and speaker. A “real” crystal radio supplied audio. It wasn’t loud enough to interrupt conversation, but it was enough to be heard without having to pass around an earplug.
You’re almost certainly referring, not to RF power, but to the Earth’s natural electric field, which averages about 100 V/m (it can get to several kV/m in a thunderstorm). You’ve got about 200 V between the top of your head and the ground. The source impedance is very high, however, so your available current (and therefore power) is very low as well, which is why you don’t get zapped by that 200 V. But it’s enough to run small electronic devices and trickle charge small batteries with a long enough wire. The higher up you get your wire into the air, the more power you can snag.
Ah, they’re 1920s style…oh, never mind.
Don’t we already use this method with Solar power? Sun = EM radiation…
And we already have unmanned Drones, which we recharge with Microwave power. (read it in PopSci…)
It was my understanding that the Great Seal device indeed worked via wireless electricity. The link seems to support that. Good read, by the way.
Quoth NE Texan:
Two problems with this. First, in a vacuum, power isn’t attenuated at all with distance, only flux, or the amount of power you can intercept with a receiver of a given size. Completely surround your transmitter, and you’ll receive all of the power.
Secondly, the power does not drop off exponentially, it drops off as an inverse square (this is a very common way for things to fall off with distance, in a 3-dimensional universe). An exponential falloff would be much faster, and therefore in general shorter range.
You’re completely right, that was sloppy writing on my part. I did intend inverse square rather than “exponentially”. My error.