Soon it may be possible to recharge your laptop and mobile phone wirelessly.

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SYDNEY: You can surf the net using a wireless connection, but soon it may also be possible to recharge your laptop and mobile phone the same way, according to U.S. scientists.

Research presented this week at an American Institute of Physics meeting in San Francisco, California, showed how wireless energy could power gadgets in the future using electromagnetic fields.

Inspired by the irritating sound his mobile phone makes every time he forgets to plug in and recharge, Marin Soljacic of the Massachusetts Institute of Technology (MIT) in Boston, tried to formulate new ways of transmitting energy to solve this common problem.

"Needless to say, this always happens in the middle of the night," said Soljacic. "So, one night, at 3 am, it occurred to me: wouldn't it be great if this thing charged itself?"

Soljacic and fellow MIT colleagues are now looking forward to a future without the hassle of wires attached to recharging laptops and mobile phones. According to Soljacic, wireless also has the potential to power other common household gadgets.

"At home, I have one of those robotic vacuum cleaners that cleans your floors automatically. It does a fantastic job, but after it cleans one or two rooms, the battery dies," he said.

For nearly two centuries, scientists have known that electrical power can be transferred without wires. Electric motors and power transformers contain coils that transmit energy by electromagnetic induction - a current running in an emitting coil induces another current in a receiving coil; the two coils are in close proximity, but do not touch.

Scientists later discovered electromagnetic radiation in the form of radio waves, and they showed that another form of it - light - is how we get energy from the Sun. But turning light into electrical power is notoriously difficult, and requires a direct line-of-sight between transmitter and receiver.

Radio waves, and in particular microwaves, can be used to transfer energy, which can be picked up by antenna. But transferring energy from one point to another through ordinary electromagnetic radiation is typically very inefficient, and can even be dangerous: the waves tend to spread in all direction, so most of the energy is lost to the environment.

However, Soljacic realized that the close-range induction taking place inside a transformer - or something similar to it - could potentially transfer energy over longer distances, say, from one end of a room to the other.

Instead of irradiating the environment with electromagnetic waves, a power transmitter would fill the space around it with a 'non-radiative' electromagnetic field. Energy would only be picked up by gadgets specially designed to 'resonate' with the field, and most of the energy not picked up by a receiver would be reabsorbed by the emitter.

While rooted in well-known laws of physics, non-radiative energy transfer is a novel application that no one seems to have pursued before. And figuring out the details was not easy, Soljacic said - something he and his colleagues did through theoretical calculations and computer simulations.

"It certainly was not clear or obvious to us in the beginning how well it could actually work, given the constraints of available materials, extraneous environmental objects, and so on. It was even less clear to us which designs would work best," said Soljacic.

With the proposed designs, non-radiative wireless power would have limited range, but Soljacic and colleagues calculate that an object the size of a laptop could be recharged within a few metres of the power source. Placing one source in each room would be sufficient for coverage throughout your home.

Wireless energy could also be adapted to industrial applications, for example powering freely-roaming robots within a factory.