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## Cutting the cord - Wireless electricity

First came cordless phones, then wireless internet. The next step to making our lives truly mobile is getting rid of the tangles of power cables lurking in all our homes.

Instead of channelling current down wires, wireless electricity makes use of magnetic fields. When electricity is flowing down a coil of wire, it creates a magnetic field. The reverse is true as well – a magnetic field can produce an electric current in a coil. This form of power transmission is called magnetic induction and is used widely in gadgets like electric toothbrushes.

In a sense, wireless electricity has therefore been around for years, but a team from MIT lead by Dr Marin SoljaÄiÄ‡ A had bigger ideas for it.

Magnetic induction is efficient only over a very small distance as the strength of the field tails off rapidly. ‘You have to put the toothbrush in a particular spot,’ comments Dr Aristeidis Karalis, a member of the research team. ‘There’s almost no difference between that and attaching it to a wire. It’s contactless but I wouldn’t necessarily call it wireless: there’s a mobility concept attached to wireless and that’s what we’re trying to address.’

The MIT team’s solution was to add sharp resonance to the equation, dramatically improving the efficiency of the energy transfer. Resonance is the phenomenon at play when an opera singer causes a glass to shatter at distance. This is possible only if the note sung has the same frequency as the resonant frequency of the glass, causing it to absorb the acoustic energy and to break.

In the case of wireless electricity, electricity from a normal power socket runs through a coil,  producing a  magnetic field. Elsewhere in the room, a second coil transforms the magnetic field back into electrical current to power your TV or charge your phone. The pair of coils are fine-tuned to have the same resonant frequency, ensuring that only the receiver coil is targeted by the magnetic field.  This means that no energy leaks elsewhere and that there is no danger for humans, pets or anything else in your home.

### Goodbye wires

Whilst bidding farewell to cables is a definite perk, it’s only one of the benefits that wireless electricity could bring. ‘A second, more interesting class of applications is where you actually improve performance,’ comments Karalis. ‘Inside a factory, efficiency is often limited by the fact that there are wires running along all the machines and robotic arms. If we can get rid of those wires, then we can definitely improve the efficiency of the whole production line.’

Liberated from the constraints of wires, inventions that had previously been impossible will soon emerge. Only time will tell what these might be, but Karalis has a few ideas, including wirelessly charged artificial organs and nanobots. SoljaÄiÄ‡, Karalis and their colleagues have gone on to found a company, WiTricity, aiming to commercialise their invention.

‘It’s incredible how much interest we have from all kinds of companies, from electronics to industrial companies,’ says Karalis. Car manufacturers for example were quick to understand how wireless electricity could be applied to hybrid cars. ‘It would be very nice if you could just drive into your parking spot and your car would start charging automatically without you having to think about it,’ explains Karalis.

### Efficiency and the environment

Whilst the imminent arrival of wireless electricity has generated much excitement, some critics are questioning its green credentials. Inevitably, transmitting power wirelessly is fundamentally less efficient, but things aren’t quite so simple. ‘Efficiency is not a number, it’s a function of the application. So depending on what the application is you can tolerate a higher or lower efficiency,’ explains Karalis. ‘We always make sure the efficiency is reasonable.’

‘If you are trying to recharge a car and you are transmitting kilowatts, you’ve got to be above 90% efficient,’ he elaborates. ‘Whereas if you are powering your wireless mouse, you are talking about milliwatts, so you can compromise lower efficiency for greater distance.’

Likewise, if operating without the constraints of power cables greatly improves the efficiency of a factory production line, again the transmission efficiency itself may be less relevant.

Reducing our reliance on the 40 billion disposable batteries produced worldwide each year is another way in which wireless electricity might bring a positive impact. ‘The environmental cost of batteries is extreme,‘ adds Karalis.

Whilst wireless electricity seems like an integral part of our vision of the future, the humble power cable is unlikely to disappear from our homes in the short term. ‘Of course there will always be applications for which wireless electricity isn’t beneficial,’ says Karalis. ‘At this stage we’re not trying to power every device on the planet wirelessly!’

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Find more sites about resonance with physics.org

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