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Is RCA's Airnergy Snake Oil?
Ben Newman writes "Of all the tech that's come out of CES this week, nothing has gotten the blogosphere more excited then the RCA Airnergy. A lot of people love the thought of an ever-recharging cell phone, and the Airnergy promises to constantly charge its internal battery through 2.4GHz wireless signals. Neat idea, but as some commenters have pointed out the energy just isn't there to make this work — BOTECs for a full charge range from 100 days to 32 years. Plus, don't let the RCA brand fool you into thinking this must be from a legitimate company: RCA hasn't existed as anything more then a licensed brand name for a couple of decades. So what do Slashdotters think — real deal or 21st century hokum?"
Is totally gonna charge up your battery and run your cell phone for days.
The inverse square law and dBm being a logarithmic unit can all go to hell.
Yes, when I am confronted with an RCA TV, the first thing I think is, "a legitimate company produced this."
YES
There's no -1 for "I don't get it."
Not necessarily, frequency is just as important as voltage and current.
I think it is the _change_ in magnetic flux that generates a current in a conductor, not just the presence of magnetism.
They whose government reduces their essential liberties for temporary security, receive neither liberty nor security.
Yes. Solar power from visual light (EM radiation) works very well. We know that.
http://michaelsmith.id.au
I'm sure we can look forward to a vigorous debate, where both sides bring up excellent points. I certainly cannot say where the slashdot community will land on this question, and the article certainly doesn't give any hints! Thanks, Ben, for your valuable question, and I hope you find the answers both challenging and enlightening.
Carry a cell tower... I think I'd rather carry a microwave oven - much more compact and convenient, since it can reheat my lunch too.
Excuse me, but please get off my Pennisetum Clandestinum, eh!
Assume a wireless router broadcasts at 1W, uniformly outward. Suppose the charger has an effective surface area of 4" * 2", or about 50cm^2. Assume the charger is 10m away fro the router; then the charger can receive no more than (1W) * (50cm^2) / (4 * pi * 1000cm * 1000cm) = 4 * 10^-6 W. A Blackberry battery on Bestbuy.com claims to store 1100 mAh @ 3.7V of energy, so the device could charge a powered-off Blackberry in (1100 mAh * 3.7V) / (4 * 10^-6 W) = 116 years... I'm wrong, or the device doesn't work as claimed.
Nasa HAS tried this.
http://en.wikipedia.org/wiki/Electrodynamic_tether
You can generate electricity as you move around the earth. Being in orbit, you are going fast enough to make worthwhile magnetic flux, and you are free of air resistance that would keep you from deploying the tether if you were lower in the atmosphere.
...unless you want to cook the user holding the phone at the same time.
Your ideas are intriguing to me and I wish to subscribe to your newsletter.
Literalism isn't a form of humor, it's you being irritating.
A *changing* magnetic field generates a current. If you just take a coil with some wires attached, and hook up a voltmeter, nothing will happen. Only when you start moving your coil through a magnetic field will you start to see volts. (Earth's field is extremely weak, but with a big coil and a sensitive meter you could see a small current.)
The reason this can't be used for infinite power generation is that the coil will resist movement. Any flow of current generates a magnetic field of its own, and if you do the math, it turns out that the induced current in your coil creates a field in opposition to the field it's moving through. It works against you like a kind of friction, or like air resistance. If you just give the coil an initial kick, it will quickly run down to a stop. In order to generate power you have to keep putting energy into the system.
In other words, you're not draining energy from the magnetic field, you're just converting the kinetic energy you put in.
This is in fact how generators work. A big conductive coil is spun around inside the field of some permanent magnets. If your generator is connected to a water turbine, you're converting the kinetic energy in falling water into the kinetic energy of a spinning coil and thence to electrical energy in a wire.
Tesla seemed to think this idea was workable. Can't say about RCA's product but I'll trust Tesla.
At what price learning? At what cost wisdom? The price is a man's peace of mind, and the cost is his life.
I'll get more energy with a hand-crank generator...
Frequency effects your ability to absorb the energy. I can only pick up NSA broadcasts on my dentalwork, for example.
If you were blocking sigs, you wouldn't have to read this.
The greek nu is the standard notation for frequency in physics, or at least it generally was when I did that kind of thing. f is sometimes used, though less commonly. However, photons are not relevant for RF - the photon energy is so small that the quantum nature of the radiation is not apparent, and it behaves for all practical purposes as a classical wave.
Frequency plays a massive difference in EM radiation and magnetic induction.
For EM, you really need an antenna that's close to the wavelength, and for 50Hz that's 6000km. For 2.4GHz it's about 12cm.
For induction, frequency affects the overall number of turns required. A 50Hz transformer that copes with 300W is the size of a shoe box, but for a switchmode power supply at 100KHz it's the size of a match box.
Meh, according to the German broadcasting agency I'm already stealing radio waves by owning a computer.
I'm not kidding.
You use the "nu" when dealing with particles. For waves it is f. In electronics it is practically always f.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
Actually you only need about 1/4 of the wavelength to get a good antenna. A full wave antenna doesn't provide a good match for real work situations.
It's easier to fight for one's principles than to live up to them.
I can feel the energy from the sun on my hand. I can't feel the energy from a router. Not even with my EM allergies :-)