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Just out of curiousity

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LAN_403

luds

Fri Apr 23 2010, 12:12AM

Registered Member #2820 Joined: Fri Apr 23 2010, 12:05AM
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Posts: 1

Assuming the application was for a coiled electromagnetic projectile accelerator.

We'll say it is a single stage with a single coil.

Would there be any theoretical benefit in having the coil spinning at a high velocity when the coil is energized to release the projectile?

The two ends of the coil could be electrically connected by carbon brushes that are stationary.

I was just curious, if you were to accelerate the coil to a high RPM in the same rotation of current flow if that would have any benefit or if it has ever been experimented with.

GhostNull

Fri Apr 23 2010, 12:49AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Are you talking about an inductance or reluctance coilgun here?
What kind of coil are you talking about?
If you're not sure this should help:http://www.coilgun.eclipse.co.uk/ coilgun_basics_1.html

these two pages have information on different coilgun designs.
And how you you think this would have a benefit?
A diagram of your idea might help

Solenoids are axis-symmetrical and so are their magnetic field, so unless you are thinking of a asymmetrical coil it would theoretically have no effect all.

If you want to impart spin onto the projectile normally several asymmetrical offset coil stages are used to case an asymmetrical projectile to spin.

-see more here
A high RPM spinning asymmetrical coil would have little time to impart any spin onto the projectile, at least thats what I think.

On practical terms the brushes would create extra resistance, any resistance even a few milli-ohms will effect the pulse and an asymmetrical coil spinning at high RPM would likely cause a lot of vibration and the energy spent spinning the coil.

Well that's my thoughts

-Cheers

Turkey9

Fri Apr 23 2010, 01:45AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

As said before, the magnetic field would have an even effect on the projectile axially. Even if the coil was asymmetrical, the projectile isn't so there would be VERY little effect on it. Now, if the projectile was magnetized with the poles running in strips down the length of it instead of at each end, then an asymmetrical spinning coil might have some effect.

If you were thinking that it would increase the speed of the electric current flowing in the coil and hence increase the magnetic field, that's not the case. The speed of light (which is the speed of electricity) will always stay constant to the observer. So even if the coil was spinning at close the speed of light, the current would still flow at the same speed according to you (the observer). If you were observing from the coil, time would slow so that the current is still flowing at the speed of light. Yay relativity!

klugesmith

Fri Apr 23 2010, 06:40AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Turkey9 wrote ...
...If you were thinking that it would increase the speed of the electric current flowing in the coil and hence increase the magnetic field, that's not the case. The speed of light (which is the speed of electricity) will always stay constant to the observer.

Not so fast! The electron drift velocity is very small, because metals have such a huge density of mobile charge compared to familiar electric current densities.
Consider a stationary coil wound with AWG12 wire, carrying 1000 amperes for 3 milliseconds. For that current, the electrons drift a bit less than 1 inch per second. They move less than the thickness of a human hair before the current pulse is over.

"Speed of electricity" in the usual sense has to do with how fast a change in current at one end of the wire propagates to the other end.

In a physically rotating coil, the motion of its electrons would produce a huge magnetic field - perfectly cancelled by the magnetic field from equal motion of the positive atomic nuclei, since there is no net charge on the wire. A rotating charged ball -does- generate a magnetic field.

As for special relativity... that's all you need to explain the circumferential "magnetic field" near a current-carrying wire. (Magnetic field does not affect stationary charges, only moving ones).

A test charge moving parallel to the wire is attracted or repelled according to its charge X its velocity X the "magnetic field strength" at that distance from wire. In the laboratory reference frame, the wire has equal density of fixed + and moving - charges. But as observed from the moving test charge, the wire is relativistically contracted by a tiny amount. The contraction ratio (apparent density increase) is different for the + and - charges, even at ordinary drift velocities, which gives the wire a net charge in that reference frame. If you work out the resulting electrostatic attraction or repulsion of the test charge -- voila, it is identical in magnitude and direction to what we call the "magnetic" Q x V x B force.

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