Coin-shrinking theory / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Coin-shrinking theory

1 2 next

Move Thread

LAN_403

kc1980

Fri May 07 2010, 12:06AM

Registered Member #2849 Joined: Thu May 06 2010, 11:34PM
Location: Los Angeles, CA
Posts: 2

I apologize in advance if this was cover in another thread. I did my due diligence by searching the forums and google but came up with nothing good. I was not satisfied with the explanation on capturedlightning.com. I asked the following question in the physics forums, but received no response.

Can someone explain to me EXACTLY how the coin-shrinking process works?

Here is my current understanding:
A large transient current is passed through the coil which sets up a very large, rapidly-increasing magnetic field through the coin. The changing magnetic field then induces eddy currents in the coin; the direction of the eddy current is such that it creates a magnetic field in the opposite direction as that of the coil field. The current in the coin is rotating in an opposite direction than that of the coil, thus the resulting Lorentz force imparts a radially-inwards force on the charge carriers within the coin. I'm assuming that the coin material does not need to be magnetizable -- just electrically-conductive.

Question #1: Assuming that the current is forced towards the center of the coin, why does the coin shrink? Unlike a deflecting wire or crushing can, there is no metal boundary -- i.e., the coin is not hollow in the middle and so the current should be able to go inwards without deforming the metal coin.

Question #2: Does the skin effect cause the current to concentrate along the circumference of the coin? I can readily conceptualize the skin effect in a wire, but I just can't envision how it would apply to a coin (presumably of negligible thickness).

THANKS IN ADVANCE!!
Ken

radiotech

Fri May 07 2010, 02:43AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"The changing magnetic field then induces eddy currents in the coin; the direction of the eddy current is such that it creates a magnetic field in the opposite direction as that of the coil field. The current in the coin is rotating in an opposite direction than that of the coil,"

Is this a reexpression of Lenz's Law with eddy currents?

kc1980

Fri May 07 2010, 04:26AM

Registered Member #2849 Joined: Thu May 06 2010, 11:34PM
Location: Los Angeles, CA
Posts: 2

yes, that would be a much more concise way of putting it. My understanding of electromagnetics is a bit rusty. I could probably just barely understand a proper, math-based theoretical explanation of the phenomenon. What I seek instead is a conceptual explanation (i.e. right hand rule, lenz's law, lorentz force, etc.)

any help would be appreciated! thanks. :)

Proud Mary

Fri May 07 2010, 08:47AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It's yer actual theta-pinch. The mass and volume - and so density - of the coin is unaffected. The coin is distorted, not shrunken, in the implosion phase of the Bennett pinch.

There's a lot of experimental literature about theta and z-pinches, because people have hoped that putting the squeeze on a plasma would lead to fusion of a useful kind.

There's a good crib sheet about pinches on the Plasma Universe site here:

radiotech

Fri May 07 2010, 03:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Magnetostriction is another area to look at for shape changes. Its also why pylons and substation transformers hum.

Steve Conner

Fri May 07 2010, 04:08PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

kc1980 wrote ...

Question #1: Assuming that the current is forced towards the center of the coin, why does the coin shrink? Unlike a deflecting wire or crushing can, there is no metal boundary -- i.e., the coin is not hollow in the middle and so the current should be able to go inwards without deforming the metal coin.

This is an interesting question. You might as well ask why, in an induction motor, the electrons in the rotor drag the rotor round with them instead of just whirling round uselessly inside it.

(An induction motor works equally well with a solid metal cylinder for a rotor, no "metal boundaries" are needed.)

Edit: When the path of a current in a conductor is influenced by a magnetic field, that's the Hall effect.

I guess the Hall effect represents a deviation away from how the current would normally like to flow. The electrons want to go back to their normal path, the strength of this tendency can be expressed by the Hall voltage and the resistivity of the material.

So to get back to the original topic, when you try to smash all the electrons into the middle of the coin, the surplus of electrons in the middle creates a potential gradient that makes them all want to flow back out again. And the force required to smash them in against this gradient is probably just the Lorentz force.

WaveRider

Sat May 08 2010, 01:09PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

The coin-shrinking process is dominated by 2 effects: the extremely high currents induced in the coin heat the metal, softening it. Secondly, the interaction of these induced currents with the coil magnetic field produces a crushing-force that deforms the softened metal. These forces are enormous and are evident on the coin-crusher coil...reaction forces often make the coil self-destruct!

Yes..the current tends to penetrate slightly initially, but as temperature of the metal rises, its conductivity decreases, enhancing current penetration. Studying the actual progression of the coin-shrinking process would require some serious multiphysics modeling (electromagnetism, thermal, viscoelasticity)...

radiotech

Sat May 08 2010, 06:20PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Induction devices, tranformers, motors or even coins depend on the transfer of charges by producing, and then retarding magnetic fields, in the object part of circuit. The tranfer of charges
by electromagnetic means is why the heat/force that mutually coupled systems experience cannot be simultaneous.

Electrons are not charges in the model because they have mass.

Lenz and Ampere had no concept of electrons. The closet science
was the electrochemnistry of Faraday.

The fifth force wont be found in easily in our sequental time limited mechanics.

WaveRider

Sun May 09 2010, 09:35AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Huh?!! Could you explain further? There is no transfer of charge to the coin during shrinking...

Electrons are indeed charges...and they have mass. Your post has more than a whiff of pseudoscience in it, but I will give you the chance to explain yourself better in the context of what this has to do with coin shrinking...

radiotech

Mon May 10 2010, 06:47AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The conduction of an electric current is by charge transfer, not by movement of electrons. (There is a proof which shows one ampere through a conductor 1 mm^2 causes electrons to move and it takes minutes to travel 1 meter.)

Electron movement does not constitute an elecric current. Even in a CRT the electron beam shooting mass particles at the screen does not complete the circuit, the charge flow does, moving at <c limited by the velocity factor of the beam in terms of inductance and capacitance. The electrons move much slower. (if you ramp up the anode voltage to approach c there is a relativistic correction needed to predict beam curvature (learned when colot TV's were in early development)

Across a transformer magnetic circuit from primary to secondary, from the flow of charges in the primary from the start of the winding to the end ,energy is transfered (joules) to the magnetic field which transfers the same energy from the magnetic field back to the secondary winding (joules) through the load R doing work.

If the winding is held stationary energy is transfered to load R . if not it is pushed away from the field at a rate of speed times the force equal to the energy transferred from the magnetic field.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob