Resonance and Wireless Power

Household devices produce relatively small magnetic fields.
For this reason, chargers hold devices at the distance
necessary to induce a current, which can only happen if the
coils are close together. A larger, stronger field could
induce current from farther away, but the process would be
extremely inefficient. Since a magnetic field spreads in all
directions, making a larger one would waste a lot of energy.

In November 2006, however, researchers at MIT reported that
they had discovered an efficient way to transfer power
between coils separated by a few meters. The team, led by
Marin Soljacic, theorized that they could extend the distance
between the coils by adding resonance to the equation.

A good way to understand resonance is to think of it in
terms of sound. An object's physical structure -- like the
size and shape of a trumpet -- determines the frequency at
which it naturally vibrates. This is its resonant frequency.
It's easy to get objects to vibrate at their resonant
frequency and difficult to get them to vibrate at other
frequencies. This is why playing a trumpet can cause a nearby
trumpet to begin to vibrate. Both trumpets have the same
resonant frequency.

Research at MIT indicates that induction can take place
a little differently if the electromagnetic fields around
the coils resonate at the same frequency. The theory uses
a curved coil of wire as an inductor. A capacitance plate,
which can hold a charge, attaches to each end of the coil. As
electricity travels through this coil, the coil begins to
resonate. Its resonant frequency is a product of the
inductance of the coil and the capacitance of the plates.

As with an electric toothbrush, this system relies on two
coils. Electricity, traveling along an electromagnetic wave,
can tunnel from one coil to the other as long as they both
have the same resonant frequency. The effect is similar to
the way one vibrating trumpet can cause another to vibrate.

As long as both coils are out of range of one another,
nothing will happen, since the fields around the coils aren't
strong enough to affect much around them. Similarly, if the
two coils resonate at different frequencies, nothing will
happen. But if two resonating coils with the same frequency
get within a few meters of each other, streams of energy move
from the transmitting coil to the receiving coil. According
to the theory, one coil can even send electricity to several
receiving coils, as long as they all resonate at the same
frequency. The researchers have named this non-radiative
energy transfer since it involves stationary fields around
the coils rather than fields that spread in all directions.

The MIT team's preliminary work suggests that this kind of
setup could power or recharge all the devices in one room.
Some modifications would be necessary to send power over
long distances, like the length of a building or a city.
The team is making progress -- in June 2007, the MIT team
published a paper detailing a successful demonstration of
their prototype. They used resonating coils to power a light
bulb over a distance of about seven feet (two meters).

Other wireless power theories involve enormous distances --
like from space to the Earth. We'll talk about them in the
next post.