WOULDN'T it be great if your cellphone or laptop started to charge as soon as you walked into your house of office? A phenomenon called "evanescent coupling" could make charging your electronic gadgets this easy.
Researchers have been looking for a wireless charger for some time. One idea is to use electromagnetic induction - passing an electric current through a coil to create a magnetic field that induces a current in a neighbouring coil. This is the way devices like electric toothbrushes are charged, and has been proposed as the basis of a universal re-charger pad before (New Scientist, 22 January, p 21). The snag as far as mobile devices are concerned is that the charger and device must be in close contact with each other for it to work. Alternatives such as transmitting electromagnetic waves in all directions to reach any device would be hugely wasteful.
"Placing one wireless source in each room could provide universal coverage
Instead, Marin Soljacic at the Massachusetts Institute of Technology wants to use evanescent coupling, which allows electromagnetic energy trapped at a source to be tapped by a drain device if the two have the same resonant frequency. "The energy is trapped at source, until I bring a device that has the same resonant frequency close to it. Only then can the energy tunnel through," says Soljacic.
Soljacic and colleagues Aristeidis Karalis and John Joannopoulos have carried out numerous computer simulations to see if the idea will work. They discovered that a small copper ring could be constructed with a resonant frequency that would allow it to trap electromagnetic energy at a frequency of 3 to 4 megahertz without emitting radio waves to its surroundings.
Alternating current from the mains is converted to this frequency and injected into the ring, which consists of an inductor loop and a capacitor. The current travels round the ring, generating a magnetic field as it passes through the inductor loop and an electric field as it passes through the capacitor. The magnetic field extends up to 5 metres around the ring, and the switching between electrical and magnetic states makes the circuit resonate at the required frequency.
If a device fitted with a ring with the same resonant frequency enters the room, this magnetic field induces an electric current in its inductor loop. This travels round the second ring, constantly switching between electrical and magnetic states, just like the first ring. The energy can then be drawn off as current by a wire in the device, to begin charging. Placing one source in each room could provide coverage throughout a home or office.
Soljacic presented the results at the American Institute of Physics Industrial Physics Forum in San Francisco on 14 November. The team is now trying to develop a prototype device.
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