physics question #1934



Karan Rustagi, a 19 year old male from Ontario asks on February 25, 2004,

Q:

I heard that electricity can be transmitted in other forms than electron flow; like electromagnetic beams. This way we could power appliances or large robots and satellites in space without power lines or batteries. It could also help rovers on the dark side of the moon to operate without worrying about charging their batteries all the time. Could someone give me more specific information on this and how this could be made possible?

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the answer

Donald J. Barry answered on February 26, 2004, A:

Transmitting energy from one location to another is an entire subject of study. One major division of Electrical Engineering is power transmission, and of course most of that concerns just the technology in our electrical grid. So obviously I must be very brief.

There are many ways to move energy from one location to another. One very general set of methods involves conversion of energy in some form into a specific frequency of electromagnetic radiation and then transmitting it. As a matter of fact, our own alternating current power lines are just that: they serve as "waveguides" for the transmission of high-intensity 60 Hz electromagnetic radiation in one sense. At different frequencies, very different problems arise in how does one generate the electromagnetic signal to begin with, how does one steer it (in a wire, cable, hollow waveguide, propagated free in air or a vacuum), how does one receive and convert it into a useful form.

Because of this, some frequencies have seen much more use than others, because the problems seem easy in them even if the nature of the solutions are very different. So in addition to the very very low frequency of power lines, where many issues of generation, reception, steering all seem very easy, we now know we can propagate energy also at almost any other wavelength we choose. But not with, in most cases, anywhere near the efficiency of our very low cost and very low loss power lines.

Some examples: Light can be generated many different ways from an energy source. To be effectively steered, though, it's better for that light to be emitted generally in one direction to begin with. The best collimated beams are produced by lasers, but even the best lasers -- solid state infrared lasers, are only a few percent efficient. At least beam losses do not have to be large with these generators! Solar cells generally reach peak efficiencies of about 20%, though most affordable ones are somewhat less. So you see there are some appreciable drawbacks to this technology for sending power where other means will suffice!

Efficiencies are somewhat higher in the microwave area of the spectrum but it is less possible to steer a tight beam of microwaves without it spreading over long distances.

One final example: The gifted inventor but also half-crackpot Nikola Tesla once proposed setting up networks of antennas to beam large intensities of radio waves at a frequency that would be reflected by the Earth's ionosphere (such as happens in shortwave radio). His idea was that with enough stations broadcasting such high energy waves at a particular frequency, that the spherical shell between the earth's surface and the bottom of its ionosphere would be filled, as a waveguide, with this radiation, and then anyone on the surface of the earth could "tap" the energy for local use by merely erecting an appropriate antenna.

Of course -- the energy necessary to power this system would have been absolutely enormous, the losses incredibly high, and thus the cost beyond prohibitive. But what an idea!

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