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Q & A: Charged-sheath vortex?

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Most recent answer: 10/22/2007
Q:
What exactly is a charged sheath vortex (CSV)? Could you put something in the middle of one? What’s the difference between a CSV and a fictionary force-field?
- Courtland (age 15)
GA, USA
A:
Hi Courtland,

I did a little scouting around the web to see what this could possibly be. One site explained that you can get these by arranging charges (like electrons, or maybe ions of some sort) in otherwise empty space and arrange it so they travel around in circular paths on the surface of a cylinder. The electrostatic forces tend to make the charged particles fly apart, and the magnetic forces tend to make them want to stay together.

I don't think such things exist, largely because the electrostatic forces will be stronger, and without some other structure holding the charges together, they will fly apart. The electrostatic forces are quite strong, although the magnetic forces get stronger as the charges move faster. But when the charges move faster, they require more force to keep them moving in a circle. It's a no-win situation I'm afraid.

You can charge up a cylindrical object (insulating) and set it spinning, however. The object then supplies all the forces needed to make the charges follow their circular paths. In this case, you just get the ordinary electric and magnetic fields from the moving charges. It is easier to make charges flow around a cylinder by wrapping a wire around the cylinder and flowing a current through it. Such a device is called a "solenoid" and is very useful in making magnetic fields and inductive electronic components (quite useful in making oscillators and frequency filters). Here, the atoms of metal in the wires steer the electrons around in the correct path. A very strong solenoidal magnet will want to crush inwards on its core however, and strong magnets often need strong mechanical support to hold them in place.

A fictionary force is one that is introduced in a non-inertial frame for convenience purposes. Newton's laws hold in inertial frames -- i.e., those frames of reference which do not accelerate or rotate (something "fixed" in a rotating coordinate system, such as a person riding a merry-go-round, is constantly accelerating. Acceleration is the change in velocity with time, and even a change in the direction of velocity counts as a change in velocity and thus and acceleration). Someone riding a merry-go-round may refer to a "centrifugal force" pulling him outwards, and may even solve physics problems conveniently in his frame of reference using the idea of centrifugal force.

In the inertial frame of a bystander observing the merry-go-round, the rider is moving in a circle, and is constantly accelerating towards the center of the merry-go-round. The inward force needed to sustain this motion is supplied by the wooden horse or the brass pole the rider is holding on to. This is a real force, and causes a real acceleration. The centrifugal force only exists in the non-inertial, rotating frame of the rider and balances the inward force of the pole so there is no "acceleration" in the rider's frame.

As far as I can tell, there is little connection between these two ideas. One probably doesn't happen, and the other is an invented convenience for talking about the motion of objects in accelerating frames.

Tom

(published on 10/22/2007)

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