Magnets, Solenoids, I forget physics...

[PVC Arsenal 17]

Everyone knows you can generate electricity by moving a magnet back and forth through a solenoid, for example:



My question is, how would I have to arrange a bunch of magnets to invert this system and still have it work?

Old fashioned MS Paint diagram for explanation:




I want to have a long tube (PVC or whatever) with small neodymium magnets embedded in the wall throughout its length. Inside the tube will be a solenoid attached to some other circuitry such that when the solenoid slides through the pipe, it provides power to the circuitry.

Can I get away with one row of magnets all aligned with their poles in the same direction, for example North always facing the inside of the pipe?

If I have to make two opposing rows across 180 degrees apart from each other like in the diagram, how do I orient their poles with respect to the other row? Always the same pole facing inwards? One inwards and one outwards?

I'm only concerned with the solenoid moving in one direction, let's say "forwards" and I want to produce DC so no rectification would be necessary.

Any E&M buffs around here?

[Technician1002]

Take apart an old loudspeaker. It is exactly what you are looking for. One magnet gap and a long solonoid coil will do the job. DC is produced while the coil is moving in one direction. The Voltage drops to zero when the coil stops and the DC polarity reverses when the coil moves back through the magnet gap. Only the turns of the selonoid in the gap cutting magnetic lines of force produce electric current.

http://en.wikipedia.org/wiki/Faraday's_law_of_induction
http://en.wikipedia.org/wiki/Maxwell's_equations
http://hyperphysics.phy-astr.gsu.edu/hb ... axeq2.html

[PVC Arsenal 17]

Looks like those are circular magnets axially polarized with north/south facing down the length of the tube.

Maybe I can arrange my small magnets in a ring shape?

[Technician1002]

Imagine a coin with a conductive edge going through the center of this ring magnet. As it approaches, it cuts lines of force and a curent is generated. As it passes the center of the ring, it's travel is parallel to the lines of force so NONE are crossed and the current drops to zero in the center. As it exits, the line crossing direction is reversed and so is the current. This will produce an S curve output like my magnetic chronograph. A long solonoid with turns entering and exiting at the same time will have a net output of zero. The larger the selonoid diameter the more lines of force it will cross. In a loudspeaker, the use of a pole piece in the center changes the shape of the flux lines to be concentrated in the narrow gap the voice coil travels in.

[jackssmirkingrevenge]

"Is that a fleshlight?"

"No, I'm recharging my flashlight!"

"Yeah..."



Interesting concept, I had never seen these. A sort of reverse coilgun.

Won't the connections to the coil interfere with motion the way you drew it? Better if you make it run up and down conducting rails rather than with a loose wire connection.

[PVC Arsenal 17]

A long solonoid with turns entering and exiting at the same time will have a net output of zero.

Is that so? You couldn't get any power from it? I thought if that was the case then the flashlights couldn't possibly work. Maybe I'm misunderstanding something.

Edit: You said "net output" and mentioned "s curve," do you mean that in this configuration, the device would produce AC despite only moving in one direction?

Edit 2: I think I get it now. Essentially my idea is worthless. Rails it is.

Better if you make it run up and down conducting rails rather than with a loose wire connection.

That was my initial thought, but induction just seemed so cool.

If you haven't guessed it by now I'm charging something as I fire it from an airgun.

[jackssmirkingrevenge]

If you haven't guessed it by now I'm charging something as I fire it from an airgun.

Ahhh...

For a tracer type light or something more sinister?

The Oerlikon-Contraves AHEAD/Skyshield (which, in case I hadn't mentioned it before, is fuçking awesome!) uses coils at the muzzle to set the time fuse as the projectile passes through, after measuring the velocity of the projectile. For a supersonic projectile that is just incredible. Science!

[PVC Arsenal 17]

If you haven't guessed it by now I'm charging something as I fire it from an airgun.

Ahhh...

For a tracer type light or something more sinister?

You're good...

And that weapon reminds me of something I saw when I visited Picatinny Arsenal (the XM25, maybe?) which had airburst rounds programmed in-gun. Before firing, though.

[Technician1002]

The flashlight works because the coil is short enough for the magent to enter and exit it. This produces current as it enters the selonoid, and then exits it. If the coil was stretched out the entire length of the flashlight so the maget was inside the coil 100% of the time and could not reach the ends of it, the the flashlight would not work as turns the maget enters is countered by tuns the maget is leaving.

In summary, it works because the magent can exit the ends of the coil causing flux line crossings all in one direction at a time.

Yes, an AC S curve is generated in the coil as the magnet passes through. Voltage ramps up to a peak as the magnet enters, then drops to zero as the maget crosses the center and then peaks in the opposite polarity as the magnet exits the other side.


Multiple S curves as a magnet passes through a series of coils of wire. The magent is being accellerated through the coils as it it fired from a spudgun.

The line from left to right makes several fast polarity changes from negative to positive. This is where the magent passes the center of the coils. The slow drop back negative is between coils as it leaves one coil and approaches the next 1 foot away.

[jackssmirkingrevenge]

Other than mechanical interest, I don't see the benefit of this over an LED and button cell. You're going to need some sort of cell to charge anyway...

[Technician1002]

I wonder if the use is not to charge a cell but to do something prohibited here..

The voltage generated could be used to switch on an SCR and turn on a light as it is fired or start a timer.

Do not believe the voltages in the above scope trace. They were amplified with a mic mixer.

[PVC Arsenal 17]

Plan was just to charge some caps with a coil wrapped around them and wired to LEDs... sort of like reusable tracers I can shoot up in the air at night. No real purpose, just for fun.

But a quick search revealed that I can get 100 button cells and 100 LEDs for under $5 shipped from China. Case closed.