Slashdot Mirror
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?"
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.
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Yes. Solar power from visual light (EM radiation) works very well. We know that.
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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.
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.
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.
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.
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