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Coils and Magnets

  1. Nov 22, 2003 #1
    How do I figure out how much electricity would be generated by dropping a magnet down a coil of wire? What are the specific variables that would determine this? Is the number of coils more importants than the thickness of the wire and If it does generate electricity whats the best way to store it and make use of it? Would a capacitor be the best route or is there a better way? Would a capacitor even work to power a simple motor or would it only work on a small light bulb or something like that?

    Any thoughts on the topic are much appreciated
  2. jcsd
  3. Nov 22, 2003 #2
    You would have to know how fast the magnetic field is changing so the speed at how fast the magnet is goign would obviously count. As for everythign else I'm not too sure.
  4. Nov 22, 2003 #3
    'ere we go

    Faradays formula:

    Induced voltage = change in magnetic flux / change in time taken
  5. Nov 22, 2003 #4


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    The current generated would depend on the number of coils per unit length, the strength of the magnet as well as how fast it moves. I thing you would have difficulty generating usable currents with this arrangement. You should be able to measure an emf while the magnet is moving.

    The trouble with using a capacitor in place of a battery is that the voltage on the Cap depends on the amount of charge stored, so as you use the stored charge the voltage will drop. A battery on the other hand provides current at a voltage which is determined by the chemical reaction generating the excess electrons.
  6. Nov 22, 2003 #5
    What Jakrabb describes is a linear generator, the underlying physics of which would be the same as a rotary generator or a “ding-dong” doorbell in reverse. Never the less, a gravity-powered generator seems interesting. Suppose buoyancy was utilized to recover the magnet after dropping through a 1 mile long sealed vertical coil in the ocean. So it doesn’t crash through the bottom, a braking system would be needed as well as a watertight ejection system.
  7. Nov 22, 2003 #6
    so, let us add more specific information to the questions. We can estamate the acceleration of the magnet at 10m/s^2 (Gravity). we can estamate the number of coils to be around 12 coils per cm. Let us use a strong rare earth magnet like a neodymium (NdFeB) magnet. Lets make a goal of enough electricity to spin a small motor which requires 1.5 v at .16amps for a few seconds. So, how far would we have the magnet fall to achieve this. Also, what happens if we place another coil outside of this original one, but not have them connected in any way. Would the outer coil generate any electricity as the magnet fell?

    As always, any thoughts on the topic are much appreciated.
    Last edited: Nov 23, 2003
  8. Nov 23, 2003 #7
    I don’t believe it’s as simple as that. The energy would by acquired by extracting the kinetic energy of the magnet as it’s falling. An opposing magnetic field from the coil would retard, possibly almost stop the magnet momentarily if the connected load was large (low impedance). I believe the spacing of the wires comprising the coil or the load or both would be critical to keep the opposing force from exceeding gravity. A free fall acceleration distance would be necessary before the magnet entered the coil. The field strength of the magnet would be reduced at each pass through the coil. It would have to be re-magnetized for constant output. I suppose the efficiency would be very low, but with a free energy source it’s not a concern. All that would be necessary would be a net gain, however small.

    I don’t think it would be practical to drive a motor directly. It would be easier to store the energy in a capacitor, which could be discharged into a voltage regulating circuit, and than used to drive a motor. If the motor can bring the magnet back to a little above the original drop height, it’s a winner.

    I find this intriguing and hope some of the more knowledgeable posters will comment.
  9. Nov 23, 2003 #8
    I have a doubt on the approach of GINIERE.. how can we continuously generate power from a magnet falling down a tunnel? After the inital moments, almost all of the magnetic field lines of the magnet is enclosed within the coils. Hence no more change ( or very little change) in magnetic flux takes place. So no more current. This is unless one takes a magnet of dimensions in the order of the tunnel itself... which I think is overkill. Also.. magnetic field strength of the magnet is not weakened through every pass. Only the kinetic energy from the fall is utilized. But the problem is... once the magnet is near the bottom... its difficult to get it back up. You can use the bouyancy trick only once. After that, u have to spend an equal amount of power that is generated from the 2way trip to pump the water out.... or mebbe more... because of magnetic loss, hysterisis loss in the coils, eddy currents, frinctions etc etc... so its not feasible.... unless u wait for water from a 1 mile long tunnel to evaporate :):)

  10. Nov 23, 2003 #9
    Water would never enter the coils interior and it would be evacuated of air. Designing the extraction system and containment tube would be difficult because of the immense pressures, but is an engineering vs. cost problem. It need not be a single coil, but many coils separated by an acceleration distance thereby avoiding the static flux issue. I do believe the magnet would weaken it passed through the coils and also be subject to great heating. Of course there would be many losses, culminating in heating the structure. The heat could be beneficial if its energy could also be extracted, say by circulating water around the coils.

    I envision a rotating insertion/extraction system similar to a “six-shooter” pistol with ceramic seals which inject into the coil tube and extract into a side-by-side water filled tube.
  11. Nov 23, 2003 #10
    see, what I am picturing is not one magnet but lots of magnets on a conveyor/pully system driven by the motor. so all you have to do is generate enough energy to raise one magnet a few inches each time. When it gets to the top it falls down the coil and generates enough for the next magnet to be dropped. so you need to only generate enough electricity to spin the motor and gears enough to rotate the pully 'one notch'. I think using this technique I can keep the scale down. Of course, there wouldnt be any excess electricity which makes it more of a novelty than an ample source of energy, at least at first.
  12. Nov 24, 2003 #11
    I think it would not even be a novelty... it would come under those perpetual motion machines that violate the second law of thermodynamics... I think.

    Dont people always want something for nuttin :smile:

  13. Nov 24, 2003 #12
    Unfortunatly we dont get to violate the second law because it requires a lot of mechanical energy to stage it. Putting the magnets into all the little slots on the pully and all. Plus the slow loss of energy due to heat and noise, etc. I think it might be a nice novelty because it would have the illusion of being in perpetual motion but would in fact die out after a time. I just wish I could figure out what math I can do to know the scale that would be required. Are Maxwells Equations the ones to focus on for that?

    As always, any thoughts on the topic are much appreciated
  14. Nov 24, 2003 #13
    I was about to ask the same thing...

    I learnt bout maxwell's four equations but I dunno how to relate the current produced or change in flux and its rate to the force exerted on the coil. If I knew the field strength of a magnet and parameters of a coils, can I calculate the velocity of the magnet as it falls under the influence of gravity at any instance of time? Also, if I was given a damping parameter for the model given by jakrabb, is it possible to calculate how long the thing would go round?

  15. Nov 24, 2003 #14


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    I believe the equations you need can be found in a Calculus level introductory Physics text such as Haliday & Resnick. I am looking through my old copy but time is limited.

    Basically you will need to find the strength of your magnets as you need the change in flux though your loop. First thoughts would say your changing flux will depend on the speed of the magnet wrt to a coil and the distance from the coil.
  16. Nov 24, 2003 #15
    as for the strength of the magnets, the information that I have to reference gives us this: they are Nickle plated, neodymium-35 magnets with dimensions of .375" dia. x .20" thick. It also makes reference to it being between 32MGOe to 45MGOe in stength, but that seems to be any NdFeB (neodymium) magnet. Are there any simple empiracle ways to demonstrate the strength of a magnet?

    since the magnet is .425" on its longest axis, diameter of .75" for the coil is what I would expect. Does it matter if the magnet bumps the edge of the coil while in free fall? or, if I were to wrap the coil around a polyeurithane tube with an interior diameter of .5" and an exterior diameter of .75", how would the tube impact the experiment?

    Does spin or rotation on the magnet during freefall effect anything?
  17. Nov 24, 2003 #16
    To measure the force empirically, hot glue one magnet to workbench. Hot glue another magnet to a spring. Spring strength will have to be chosen to provide best result. Lower the magnet slowly to the fixed magnet until there’s a ¼ inch separation. Measure spring stretch vs. un-stretched spring. Disconnect spring from magnet. Hang weights from spring to achieve same stretch. I think it’s possible to relate gauss (Newton-second?) to gravity but I’m having trouble doing so. I think its best to use two magnets rather than a piece of iron of unknown properties compared to similar specified magnets. I think the ¼ inch separation can be extrapolated to 0 inch.

    Best choice would be to have inside coil diameter barely wider than magnet diameter, but to practical, coil should be wound around a thin wall PVC tube. The further the magnet is from the coil, the lower the field strength. Bouncing is bad but unavoidable in a simple device. Don’t impede air exhaust.

    I’m also going to construct a device and try to charge a capacitor(s) through a blocking diode. After multiple drops, I’ll discharge it through a resistor to determine current and voltage. I don’t have an oscilloscope, so I’ll have to get at least 2 readings for dv/dt. Big problem, is weekends are almost full up until after the new year.

    PS - Rotation of the magnet would have no effect.
    Last edited by a moderator: Nov 24, 2003
  18. Dec 24, 2003 #17
    sorry its been a while since my last post but I've been away on business for the last 3 weeks..... I've attached a mock up of what I was thinking for this concept. I was wondering, with the coil, would it be more effective to have multiple coils or one big coil. The research I've done so far seems to point to multiple coils being more effective. As always any thoughts on the subject are much appreciated.

    Attached Files:

  19. Dec 29, 2003 #18
    Yes Maxwell's equations describe exactly what your asking about.
    Let's tho look at your example (in RE maxwell), You could pull an infinitely long wire thru a circular magnet and measure a induced charge ( with a measuring device placed strategically along the infinite length). Conversely You could drag a magnet down the infinite length of a wire and see the same induced current.The eureka moment is to recognize that a torus is an infinite line with no beginning and no end and a coil is just a wrapped up long lenght of wire , that is why we don't have 2 mile linear reciprocating generators bulging from automobile hoods.
  20. Dec 29, 2003 #19


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    Consider this application of a coil and magnet. I have one of these, it takes about forty passes of the magnet thorugh a coil to power a LED for ~2min.
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