How wireless electricity could make power cables history
Own an electric toothbrush? Then you already have wireless electricity at home. Toothbrush chargers use inductive coupling to provide power without electrical contacts. When current from the mains runs through a coil of wire in the charger unit, it produces a fluctuating magnetic field which induces a current in a second coil embedded inside the toothbrush. This principle also underlies charging mats which power up phones and cameras at close range.
The catch, however, is that inductive coupling is only effective over a very short range – stray by just a few millimetres and the magnetic field tails off rapidly.
One solution is to throw resonance into the mix. Resonance is the phenomenon which allows an opera singer to shatter a wineglass. For this to happen, the frequency of the singer’s voice has to match the glass’s resonant frequency – the frequency at which the glass naturally vibrates.
To apply this idea to wireless electricity, scientists fine-tune two coils to resonate to the same frequency of magnetic field. This makes power transmission across a few metres possible as the second coil amplifies the energy of the first. The low-frequency magnetic fields used don’t interact with people or pets, making it safe to use in a domestic environment.
If you want to beam power over much greater distances, converting it into electromagnetic radiation (eg light or microwaves) is the way to go.
Laser-transmitted power has already been used to power unmanned aircraft. First, electricity is converted into a high-powered infrared laser beam; a photovoltaic cell at the other end turns this back into electrical current.
Microwave-transmitted power follows the same idea, converting energy into microwaves, then back into current with the aid of a rectifying antenna, or rectenna. This is more efficient than laser beams but requires much bulkier equipment.
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