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Wireless Electricity

  1. Dec 22, 2012 #1

    In this video, I have two questions:

    1) Why have a capacitor as part of the two circuits? Is it so no actual current flows around the coils, that's all I could come up with.

    2) In the picture he shows of the two conductors joining at resonance, why does the magnetic field of the bottom one point in the same direction as the top (source) coil? Wouldn't we expect them to oppose each other like two bar magnets anti-aligned since Lenz's Law would say the induced electric field points clockwise viewed from above in the bottom coil?

  2. jcsd
  3. Dec 22, 2012 #2
    The Capacitor in the two resonant circuits makes each into a self-resonant device, capable of storing energy. The energy is stored in the magnetic field when the capacitor is discharging electric field, and in the capacitor when the coil's magnetic field is collapsing. This is different from a transformer because energy is not stored in an ideal transformer.

    This design is almost exactly a tesla coil. Tesla coils transfer energy from a tank circuit to a secondary circuit that share a common resonant frequency. The tank has a big capacitance and a small inductance, and the secondary has a tiny capacitance and a big inductance. The sparks are due to the voltage multiplication due to the ratio of turns from the tank inductance to the secondary. In tesla's design, a spark-gap is used on the tank side to initiate ringing by suddenly joining the tank L and C together in parallel through the low resistance plasma of the ark.

    In the design on TED, the idea is the same. The turns ratio is different from a tesla coil since the purpose is to power devices that run on 120Vac or 5Vdc, and the power must be harvested in some way. I imagine that they insert a full wave rectifier and regulator for the phones, and an additional transformer coupled to the one for the TV that is rectified and powers an inverter of some kind which makes 120V@60Hz. Modern electronics can make these conversions pretty efficient.

    a guess: The fields are in the same direction because the two coils are magnetically coupled, combining to form a single resonant system. A system like that would have harmonic resonances which would represent the two systems oscillating at different phase shifts with respect to eachother. I imagine that the phase shift would be dependent on the amount of power transferred, maybe its 0° at zero power transfer and shifts to 180° at maximal power transfer. I can only assume that when the primary coil is energized for the first time, lenz's law will be obeyed and the reverse field will be initiated in the secondary, but as the energy of the secondary grows it becomes energetically favorable for the two fields to be in alignment. This could be where the energy to power the device comes from, the difference in magnetic dipole potential between two aligned fields vs. two misaligned fields. But I don't know, coupled oscillators can be crazy.

    Its possible that the artists rendering is just for illustrative purposes.
    Last edited: Dec 22, 2012
  4. Dec 23, 2012 #3
    I noticed in the video that the transmitter and receiver coils are basically parallel and share an axis. In principal, is this alignment required to make this work?
  5. Dec 23, 2012 #4
    Intuitively I assume yes.
  6. Dec 24, 2012 #5


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    I think the resonance in question is parallel (high impedance) rather than series (low impedance), which makes the E field across the coil much higher that it would be without the parallel C.

    The LC circuit does store ('reactive') energy when it resonates but that energy is not getting from A to B because PD and Currents are largely in phase quadrature. Probably a better way to explain the function of the resonance would be to say that it drastically changes the ratio of the E and H fields in the vicinity of the coils (as suggested above) - that is to say, the Impedance is high. For power to be transferred, there need to be significant in-phase components of E and H. This will be when those two impedances are matched, improving the energy transfer so that more energy goes from coil to coil than is being lost by radiation.
  7. Dec 24, 2012 #6
    Agreed. In the situation I was describing the L and C would be in parallel, just like in a Tesla Coil.
  8. Dec 24, 2012 #7
    I watched the video, and came out with a much simpler idea of how it works. I am an electrician by trade for 25 years now and have an electronics background, so all the resonance inductance and so forth I am familiar with.
    I see the ring in the transmitter acting as a resonating rotor in a generator, which causes the receiving Ring to resonate and act as it's own Rotor and vibrates inside it's own coil and thus generate (induce) power. My guess is that ring is magnetic.

    Much like a vibrating tuning fork placed near a non vibrating tuning fork will cause the non vibrating fork to vibrate at the same frequency, now make the tuning forks magnetic and place a coil of wire around them.

    But it's all just my guess, I really don't know
  9. Dec 25, 2012 #8
    Yeh, The more I thought about my first answer, the less I like it. Nevermind
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