The key: Magnetically coupled resonance
In contrast, WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. A child on a swing is a good example of this. A swing is a type of mechanical resonance, so only when the child pumps her legs at the natural frequency of the swing is she able to impart substantial energy.
Another example involves acoustic resonances: Imagine a room with 100 identical wine glasses, each filled with wine up to a different level, so they all have different resonant frequencies. If an opera singer sings a sufficiently loud single note inside the room, a glass of the corresponding frequency might accumulate sufficient energy to even explode, while not influencing the other glasses. In any system of coupled resonators there often exists a so-called "strongly coupled" regime of operation. If one ensures to operate in that regime in a given system, the energy transfer can be very efficient.
While these considerations are universal, applying to all kinds of resonances (e.g., acoustic, mechanical, electromagnetic, etc.), the MIT team focused on one particular type: magnetically coupled resonators. The team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields; they were able to identify the strongly coupled regime in this system, even when the distance between them was several times larger than the sizes of the resonant objects. This way, efficient power transfer was enabled.
Magnetic coupling is particularly suitable for everyday applications because most common materials interact only very weakly with magnetic fields, so interactions with extraneous environmental objects are suppressed even further. "The fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations," Kurs, a graduate student in physics, points out.
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