|MadSci Network: Physics|
Hi Brian, Probably the first thing to examine is what makes a useful current. In order to do work using electricity, we require power, which is the product of current and voltage. Believe it or not, most tesla coils are actually relatively low power devices. The primary idea in the design of a tesla coil is to make the circuitry resonate so as to transform a high-current but low voltage input to a high voltage but low current output. It is essentially performing the same task as a standard transformer, though it is taking advantage of a resonant circuit to make the conversion much more dramatic. Nevertheless, the total amount of energy is conserved, and except for some energy which is lost to heat and such, the incredibly high voltages of a tesla coil imply that they have currents typically on the order of microamps. Incidentally, this is less current than you get when you test an AA cell using a wet finger and your tongue. As a necessary disclaimer, I am not saying that a Tesla coil doesn't have enough juice to hurt you, but just because it has a half million volts does not mean that it has a half million watts. It is necessary to consider the amperage too. Now, a tesla coil can certainly be influenced by magnetic fields. The question is how big does a magnetic field have to be, and how fast does it have to change in order to produce a usable current. On the surface of the Earth, we have a fairly constant magnetic field with a strength of roughly 1/2 gauss. This is actually not a very strong magnetic field. By comparison, a bar magnet is often several thousand gauss. Really, this makes sense when you consider that it is necessary to place a compass needle on a very fine balance in order to detect the field at all. If you've ever used a sticky compass needle, I'm sure you'll appreciate how weak the field actually is. Next, in order to generate electricity from a magnetic field, you either have to change the strength of the magnetic field, or you have to move through the magnetic field. The former, unfortunately, won't be particularly useful, for while the Earth's magnetic field does change, it changes by very small amounts. I have seen the Earth's field stay constant to one part in 10 for days on end, and for a field the size of 1/2 Gauss, that means that there is effectively no significant change in magnetic flux. You'll get no usable current that way. Therefore, the last option is to move your coil into space and fly it around the Earth. This is in essence the Space Tether project that NASA was working on. Rather than use a tesla coil, they were using a simple wire to convert the motion of the space shuttle into current in the tether. Unfortunately the tether broke before the experiment was complete, but it definitely demonstrated the production of electrical current using the space shuttle's motion through the Earth's magnetic field. Of course, this comes at a price: as the current is generated, it creates it's own magnetic field which interacts with the Earth's and causes the space shuttle to slow down. This is really just the conservation of energy principle at work, but it does mean that in order to generate power for Earth using this method, you are really transforming rocket fuel into electricity. As you can guess the economics of powering houses with rocket fuel is not exactly favorable, but the technology does have a useful application in converting electricity into rocket fuel! That is, a satellite in the future may be able to use a tether to change its orbit rather than maintaining reservoirs of rocket fuel. Since it costs thousands of dollars per pound to put something in space, saving a few pounds of rocket fuel is a very big deal! Satellites, of course, have "lots" of electricity because they have solar panels. (But don't forget that solar panels and tethers have weight too!) You may be interested in NASA's quick description of the space tether experiment: http://www-istp.gsfc.nasa.gov/Education/wtether.html So, keep working on solving Mankind's energy problem. We need every hand we can get! Zack
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