MadSci Network: Physics

Re: can corotation field magnatism be harnesed with a tesla coil 'reciver'?

Date: Sun Nov 20 23:37:20 2005
Posted By: Zack Gainsforth, Undergraduate, Physics, U.C. Berkeley
Area of science: Physics
ID: 1131598823.Ph

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

So, keep working on solving Mankind's energy problem.  We need every hand
we can get!


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