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How about measuring losses directly? Put the tube vertically, let neodimium magnet fall through it and mesure the speed at the end. See how much energy it lacks comparing to the free fall. Copper tubes slow the thing down to the snail speed.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Yandersen wrote ...
How about measuring losses directly? Put the tube vertically, let neodimium magnet fall through it and mesure the speed at the end. See how much energy it lacks comparing to the free fall. Copper tubes slow the thing down to the snail speed.
What an elegant idea. Hats off to Yandersen! Now let's see if some reader has both CF tube and NIB magnet that fits. Don't overlook the magnets in those silly old shake flashlights.
That's also be a way to demonstrate the effect of slitting a metal barrel.
I bet the eddy current retardation of a falling magnet in a CF tube will be hard to detect. An intermediate case would be nonconductive tube wrapped with a single layer of thin Al foil. Overlap the edges just a little, and bind with something stretchy (garden tape?) for good contact pressure.
But I guess that losses in CF tube will be that small, that friction will simply overcome them. Anyway, the longer the pipe, the more precise result will be. In generall, I'm against any slotting of the barrel. Just because of forces: axial force is dozens of kg, so the squizeeng pressure could be even higher and slotted barrel will not be able to sustain that force - it will change it's diameter under the coil.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I've been studying magnetic field acting on conductive surfaces for maglev stuff I'm doing.
FWIW the effect of the tube varies with the frequency/linear speed; at high enough frequency the tube wall will be opaque- the magnetic field will not penetrate any more.
There's a critical speed/frequency which depends proportional to the 'surface resistivity'.
So for example, 3mm thickness of copper might have a surface speed of 5m/s. 1.5mm thickness would be 10m/s, or the equivalent magnetic longitudinal wave frequency.
It may be that wound carbon fiber, because it's pretty resistive and not particularly thick, won't matter much below several hundred metres per second or more, although it will have a small effect.
I guess, basically, you should calculate the skin depth and use that.
You could simulate it pretty well with FEMM; just ignore the winding direction and plug a material in with the same average conductivity; dilute the conductivity of the carbon over the thickness and run an AC analysis.
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Barrel material
