Electrostatics question

[speakerguy]

I have a solid steel cylinder, an aluminum ring surrounding the cylinder, and a steel ring surrounding that such that all are concentric. I want to put a large static charge on each (in the kV range) such that the alu ring will have a tendency to stay centered concentrically. How do I figure out the repelling force between the alu ring and the inside/outside cylinders? all the force equations I find are for point charges in a field. Also equations to predict when I will run into arcing. Thanks!

 

-speakerguy

[Externet]

Hi. Why arcing ?

All bodies will have the same charge polarity in order to be repelled. The rings would have to be suspended with strings, the cylinder vertical - Is that the way you envision?

Miguel

[speakerguy]

i don't want to say too much about my app but let's say the cylinders are standing up (circular shape if you are looking down from above) in free air not connected to anything, just hovering vertically in free air. I can keep everything aligned vertically but not radially which is why i am looking into electrostatics for the radial centering. I need to generate a lot of centering force on the alu ring and the clearance between the metal bits is quite small. i figure I will need to have hundreds if not several kV on the rings to get the force i need but i am not sure since I don't have the equations. all parts would be same polarity yes. alternatively the center steel cylinder and the alu ring may be +kV while the outer ring would be grounded, centering relying only on repulsion from alu ring and inner steel cylinder. not sure if that would work though. oh also consider the inner steel cylinder and outside steel ring fixed in position, only the alu ring can move around away from a concentric position.

 

Edited to add: by reading this you are hereby NDA'd

[chilehed]

It won't work, it's an inherently unstable arrangement.

 

With charged shells, the electrons go to the outside surface, so you'll have a negatively charged OD on the inner shell and a positively charged ID on the outer one. If you have shells of atomically perfect form and exactly on center with each other you can theoretically have a resultant attraction of zero between them, but the slightest imperfection in form or location will cause a nonuniform concentration of electrons on the shells. This will result in a non-zero resultant force and the shells will move toward each other.

[speakerguy]

nuts

 

any ideas about how to center them without any physical attachments? EM and electrostatics are the only things I can think of. and I would have to add something to the alu for em to work

[YT2095]

it should be possible to do it Magneticly, your idea of a "fritionless speaker/transducer" isn`t bad but even with magnetic suspension, it`ll be tricky to align them

[speakerguy]

Shhhhhhhhhhhhhhhhhhhhhhhhh!!!!

 

 

Motor structures on subs can be quite loud mechanically, and dipole speakers are en vogue right now, which both require mega excursion and low mechanical noise. Electromag suspension ideas are not new wrt restoring force (Cms) but I have not seen any good implementations for frictionless VC centering.

 

Vertical magnetic suspension should be easy given a properly designed B field in/around the gap and proper coil implementation, and by setting bias current I can set things like Cms, Vas, Fs and such. Dial-An-Fs if you will It's just voice coil centering that gets me. VC must be light and not interact with magnetic ckt, but it looks like electromag is the only way I'm going to center it. The more centering force I can generate, the smaller the amount of voice coil rocking at high excursion, the tighter I can make the gap, the higher the B field becomes, the more efficiency I get out of the unit since force =BIL.

[YT2095]

IIRC, I seem to recall an idea using aluminised Mylar sheeting set us as the "cone" if you like (it was actualy flat and pulled tight. and then some sort of electrostatic method of making it vibrate to music using a step up transformer, so the output of the hi-fi (about 50 volts) became something lke a few Kv.

I`m not sure of the exact details though

[speakerguy]

Do you mean planar magnetics like Magnapan or electrostatics like Martin Logan? Those are nice for >100Hz but are not so good in the bass range due to limited diaphragm displacement/

 

OOH! I just had an idea. I can't post it, I gotta evaluate it to see if it might work first. It's very simple.

[YT2095]

why not have a moving Magnet and keep the VC stationary?

much much easier to align that way

[speakerguy]

Alas, that has been tried before - I think back in the 50's and they were called field coils. They were given up on not too long after. I think the problem is too high moving mass and it kills efficiency. I could look into it though, maybe with modern magnets (Neo 50 and the like) it may be more feasible. How would it help with voice coil centering though? I am not understanding how that would work.

 

Thank you for the help!

[YT2095]

yeah, I was thinking NIB magnets, small and very powerful.

the VC wouldn`t be the standard type around the varnished paper tube, I was thinking more like 2 typical electro-magnets you`de make at school with wire around a soft iron nail, one ether side of the magnet but in opposition North South with the windings, a similar effect is seen in these "perpetual motion" machines seen in gadget shops that rock a cradle and little silver balls spin around etc...

(there`s a 9 volt battery in the bottom).