Natural Science Forum / Physics / Relativity / November 2006

That is precisely what light/radio/microwave/television/cellphone
transmission does: pass electric energy wirelessly from one place to
another.

But it depends a little on what you're calling "electricity". If you
mean by electricity moving *electric charges*, that's one thing. But in
fact, that's not a very good definition, because a lot of what's
involved in an electric circuit is the creation of a potential drop (a
voltage) between two places, viz. an electric field. Then charges that
happen to be laying around locally (in any conducting material) will
respond to that voltage and create a current. The setting up of the
voltage is what's easy to create wirelessly.

PD

Dear Norman Bates:

Do you define "electricity" as the flow of electrons?

Electron beams (used in a CRT).  No conductor.  Very lossy.  Tends to
disperse due to particle charge.

There are emitter - receiver pairs that you can buy that will accept DC
power at the emitter, and the receiver will generate ~24vdc about 1
inch away.  But it uses changing magnetic fields (so "bound photons")
to convey the "electricity".  Good for powering hermetically sealed (or
rotating) devices.

Probably need to define what you mean by "electricity" a little better.

David A. Smith

Yes. It isn't "like" light or radio waves, it *is* light or radio
waves.
Specifically, you transmit the energy in the form of electromagnetic
radiation.

Tesla famously did some demonstrations in this area over 100 years
ago.
http://www.tfcbooks.com/articles/tws8c.htm

Here is a more modern experiment:
http://www.tsgc.utexas.edu/tadp/1996/general/wpt.html

And MIT put out a press release on this subject (local power
transmission to power consumer electronics) a couple of
weeks ago, that was picked up by a lot of news sources.
http://web.mit.edu/newsoffice/2006/wireless.html

The press release fails to mention Tesla, but a paper by these
authors does mention him in their introduction:
http://arxiv.org/ftp/physics/papers/0611/0611063.pdf

Note that all of the above involves small amounts of power,
not the output of a commercial power plant. Concepts for
transmission of large amounts of power have been around for
decades (going back to at least my childhood 40 years ago).
Probably the idea of actually pointing a multi-gigawatt beam
anywhere near a residential area gives investors enough pause
that it isn't considered practical.

                     - Randy

Transformers do it with great efficiency, but it's hard to retain the
efficiency when you split them into two separate sections.  That said,
you can probably transmit efficiently by magnetism over short distances
using a suitable apparatus.

Beyond that, only radiation of various kinds such as light and sound
seems to fit the usual meaning of wireless.  You can of course transmit
electricity wirelessly from an electricity socket to a home device in
numerous other ways such as rechargeable batteries.

Cf. "Sneakernet", the equivalent concept for information transfer.

- Gerry Quinn

Why would you want to do such a thing? It is not impossible but it is
not practical.
Hint: air is a very poor conductor.

 No, you cannot (unless you consider an electric arc a means of
transmitting electricity). What you are calling "electricity,"
is the power produced  by quasi-static electric fields, i.e.,
the fields associated with and tied to moving charges. By contrast,
radiation fields are the fields which can escape to infinity and
therefore cannot be tied to the charges that radiate them (and still
be causal). Loosely speaking, radiation fields are the transverse
components of the 4-current density, j^u, while what you are calling
electricity is the longitudinal and time component. Current
conservation
requires the four divergence to be zero. That is automatically
satisfied
by the transverse components, but requires the condition, dq/dt + div J
= 0
for the other two. dq/dt is a moving charge.

Over shortish distances, yes.
By short I mean up to a few metres.
Bill Beatty has a good site that delves into this:
www.amasci.com/tesla/tesceive.html

Randy Poe wrote:

..

..

If transmission is limited in distance to 15mm, you can transmit power
this way...
http://www.designnews.com/article/CA85861.html?industryid=43651

.. not as "exciting" as Tesla... more along the lines of RFID
excitation.

David A. Smith

In principle, the efficiency of transmission can approach 100%.
In this sense, there is no more efficient way.

In practice the radiation goes into directions you don't want, and the
recievers and transmitters are lossy.
If we could solve this, we would help human civilisation, since we could
then harvest solar energy with greater effciency.

Gerard

As in this recent article?

  MIT's Soljacic proposes wireless energy systems
  http://news.bbc.co.uk/2/hi/technology/6129460.stm

If by "electricity" you mean electrical energy, then be aware that
"electricity"
is the same as radio waves and light.   It's just of lower frequency.
It
appears at 60Hz on your radio dial!

Whenever EM energy flows between the primary and secondary coils of
a transformer, or between the plates of a capacitor, that's "wireless
electricity."  The article above is just a description of nearfield
magnetic
EM energy transfer.  If we add a resonant circuit to the primary and
secondary coils, we get amazingly higher coupling between distant
coils,
and the coils can be quite small.   See:

  Energy transfer in resonant antennas
  http://amasci.com/tesla/nearfld1.html

As long as the spacing between coils is << than a wavelength at the
operating frequency, the device is a transformer rather than a radio
transmitter and receiver.   If the coils are 10M apart, we must keep
our operating frequency below 1MHz or so.   To transfer significant
energy, either crank the value of current up very high, or add a
capacitor to make the coil resonate (which accomplishes the same
thing: high current and intense b-field.)

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William J. Beaty              Research Engineer
beaty@chem.washington.edu     UW Chem Dept,  Bagley Hall RM74
billb@eskimo.com              Box 351700, Seattle, WA 98195-1700
ph425-222-5066                http://staff.washington.edu/wbeaty/

Another way to do that is through microwave beaming. If you're in the near
field it may be efficient. It see more on the subject look up rectantennas.
But the problem transmitting the power via wireless is the efficiency. If
you eventually need DC or low frequency AC then a wire with transformers to
a high voltage is the most efficient way. Only do the wireless transmission
of energy if it is small, or one of the objects can be fixed with as much
power as you need and the other object is mobile and must be light and
cannot be tethered to the fixed object.