With this Instructable I'll show you a simple rig that can be used to experiment with electromechanical resonance.
Step 1: Forewords
Repulsive force for some reason is more entertaining than the attractive one. In this field, I mean.
We know that when a current flows through a metallic wire a magnetic field is generated; that's what electric motor are based upon.
The wire can be wound in the shape of a bobbing to make the field stronger and a metallic core can increase the strength of the field. The equations linking number of turns, coil diameter and current can be found here (http://en.wikipedia.org/wiki/Electromagnet) but it is not strictly necessary to understand the underlying principles.
A direct current, the one from batteries, generates a constant magnetic field. The direction of the magnetic field with respect to current is established conventionally by the so-called right hand rule : with reference to the picture, grabbing the coil with our right hand with the thumb pointing the same direction of the current, the magnetic field will have the direction of the other four fingers.
The direction of current is, again, conventionally fixed going from the + pole of the battery through the wire to the - pole.
Alternating current (a.c.), the one that comes out a regular wall socket, will have the same effect except that the intensity of the current flowing through the coil wire cyclically rises to a maximum then decreases to a minimum in the opposite direction passing through a zero current level. As a consequence, a.c. powering a coil generates an alternating magnetic field that cyclically rises to a maximum value then decreases to a minimum passing through a zero field value.
It is important to note that the field does not rotate in any direction. Simply rises and falls in intensity then reverse its verse and rises again.
Step 2: Two Is Better Than One (and Three Would Be a Crowd)
With these two coils a region of reasonably uniform magnetic field (B field) is obtained between the coils.
The two coils are connected in series so as that the two windings have the same winding verse. I couldn't visually check the verse so I had to experiment. Later on on this point.
The two coils are powered by a wall a.c. transformer. Extremely important: do not try to power the rig directly from the mains: it doesn't work, it would destroy the coils and possibly cause burning and electric hazard. An a.c. low voltage output transformer is mandatory.
Also, make sure that the output of the wall transformer is not direct current (d.c.) as it won't work for us.
In case you can't find a wall a.c. output transformer you can build one yourself. In case, it shouldn't be difficult for a friend to help you.
Step 3: Construction
1. The coils I took from a small stepper motor. I demolished the one I used here a while ago so I haven't any picture. Anyways, a small/medium size stepper motor can be taken off a printer. Larger ones have a different internal structure so no useful coil can be taken from them. In case, a bobbin could be made out of thin cardboard or plastic covered with 200 turns of magnet insulated copper wire (Amazon p.n. B000IJW7YU should do)
2. The wall adapter I used was part of a dead DECT wireless telephone. For some reason it supplies d.c. and a.c. (on different pins of its plug). The output is 6Vac, different values are just fine.
3. Rare earth magnets: essential part of any modern household with kids they can be bought easily. May I suggest the original Plastwood (r) Supermag (r) ? they are proudly made in Sardegna ( http://www.plastwood.com/en/ ). Do not settle for substitutes ! They can be bought everywhere where good things are sold like Amazon ( e.g. p.n. B000TGU7FS ) for as low as 13 USD. The more you have in your home the more can be ingested by small kids...be careful.
4. Rubber bands.
5. Insulated wire, both for holding the coils in place and for electric wiring.
6. Small chunk of scrap wood 8"x8". A hard cover book can be used instead. You could buy it from Amazon...there's a plenty in there to choose from ! I was 'bout to use a Harry Potter I never read.
In retrospect I should have done it : the magic of magnetism and the magic of Potter really match.
7. Safety goggles (Amazon.com p.n. B000F712UA, just to point to one of the many you can find there, should do).
First you need to mark the wood board and drill the holes to keep the coils in place as in the picture.
You'll then need to hold the coils in place with some insulated wire and connect the two coils in series. Try to connect the two coils so as the two windings have the same verse; should it be hard to find the verse connect then disregarding the verse. Connect the power the two coils and keep the magnet between the two coils you should feel a strong vibration. If not, reverse one of the two windings.
Tie the magnet to a rubber band. The North and South poles of the magnet should be along the direction of the two coils.
Step 4: Showtime !
Anyways, you can see that after attaching the power the rubber band with the magnet is placed between the coils. Pulling or releasing the rubber band rises or lowers the natural frequency of oscillations of the system rubber band+magnet. When the natural frequency equals the one of the mains, 50 Hz or 60 Hz depending on where you live, the oscillations become very smooth. When the natural frequency is off the mains' the oscillation is very irregular.
When the oscillation is smooth the systems is said to be in natural forced oscillation and the maximum energy is transferred to the system. In the video oscillations don't look as smooth as they are in reality.
You may have watched the footage regarding the Tacoma bridge falling a while ago. In that case the wind made the air vibrate closer and closer to the natural frequency of oscillation of the bridge till a great deal of energy was transferred to the bridge making it collapse.
Less catastrophically, sure you have pushed a swing: it is easier to push a swing at its natural frequency (the one we experiment when in free oscillation) than to force it to oscillate at any other user-defined frequency.
Please wear your safety goggles when operating the thing.
One note: after connecting the power to the circuit check the coils for heat: if they feel too hot, disconnect the power and go for a lower voltage transformer or coils with more turns. Sorry, otherwise the fun wouldn't last long.
Again : do not operate this rig directly from the mains, it won't work and very likely you would burn the coils, ignite something and cause hazard.
Step 5: Final Considerations and Hints for Further Investigations
A regular electric razor is one example of device whose efficiency is based on a resonance.
I opened an old Braun razor and cleaned it. Amazing how dirty can be the INSIDE of a razor, almost as a TV set.
From the pictures you can see the structure of a razor: two coils generate an alternate magnetic field. Between its polar expansions (the metallic U-shaped core inside the two coils) a magnet with spring is placed. The a.c. generates an alternating magnetic field which in turn forces the magnet to oscillate and swing the razor blades.
Now the interesting part: the red paint dot keeps a tuning screw locked. The screw is necessary to tune at the factory the natural resonant frequency of the system magnet+spring and match it to the mains' so as to transfer the maximum energy to the spring+magnet (and blades) and give the razor a more natural handling feel and regular blade travel.
I dismantled the razor and tried to use the bobbins and polar expansions for my circuit but the position of the expansions is not very suitable.
Be carefully if playing with and open razors. Electric shock is always awaiting for careless curious people and metallic part could spring out of the oscillating bridge. Wear your safety goggles. Don't operate and open razor unless you know exactly what you are doing. Always have someone around you whom know what to do in case.
Also, do not be tempted into touching the razor's coils when the razor is powered.
A further interesting experiment with my circuit could be to place a low value resistor (1 Ohm or less) in series with the coils and check the current flowing through it with an oscilloscope and see what happens to the current in and out of resonance.
Ciao e buon divertimento (have fun !)