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Homework Help: How much voltage is created when a magnet passes over a coil?

  1. Jan 11, 2009 #1
    I need to calculate how much voltage is created as a magnet passes over a coil. However I don't know a formula to calculate this.

    Could anyone help me, I just need a formula

    PS: sorry, spell check does not work
  2. jcsd
  3. Jan 11, 2009 #2
    You need Faradays law of electromagnetic induction
  4. Jan 11, 2009 #3
    you mean E = -(N*(change in flux)) / t

    How would I find the change in flux, would I use, flux = B*A*cos(theta), to culculate the max and min flux?

    How would I calculate the strenght of the magnetic field at the coil? If I knew the strenght of the magnet and the distance between the coil.

    PS: I don't know the symbol for flux, I think its phi

    PPS: sorry again, no spell check
  5. Jan 12, 2009 #4
    You have the right equation but written in non calculus notation.Without knowing the details of the question I can only hazard a guess which is that two of the three quantities,B, A,and theta are constant.Suppose that B and theta are constant then you will need to calculate the change in area per second.
  6. Jan 12, 2009 #5
    OK, let me say this a better way, how do I calculate the force between two magnets if I know the strenght of both of them.
  7. Jan 12, 2009 #6
    More information is needed.Is it a project you are doing or do you want the answer to a specific question? If it is a question send the full details.In either case it is the magnetic flux density(B) which is relevant,along with the other factors such as coil area etc.
  8. Jan 13, 2009 #7
    OK, I am trying to build a magnet propelled generator.
    I am trying to use a magnet lined stator, as well as a magnet lined armature.
    To figure out how much energy I can pull off of this I am trying to write an equation to encompass all of the variables.

    However, I need to know how to calculate the force acting between two magnets.

    Thanks for your help
  9. Jan 14, 2009 #8
    Your armature will need a coil of wire or something similar for the current to flow through.Perhaps you are considering joining the rotor magnets together so that they serve as the coil and if so this seems,at first sight, to be quite a novel idea(well I have never heard of it and I am still trying to picture this in my head)The picture I see at the moment is that both stator and armature are ringed with magnets such that alternate poles face inwards across the gap and if this is the case I cannot see you achieving any more than you would get by using a more conventional armature which itself becomes magnetised when the current flows.

    The variables are B, A, N, and t.

    To generate a large emf;

    1.Make B as large as possible ie use as strong a magnetic field as you can make.

    2.Use a large cross sectional area(A)for for your armature.

    3.Use a large number of turns(N)

    4.Move the armature(or stator) as quickly as possible so as to reduce the time (t)

    Increasing one thing may comprimise something else so it will boil down to design.As for the force between magnets(I am assuming you mean permanent magnets ,well that is a very tricky one because it depends on the geometry of the set up as well as the strengths of the magnets themselves and I think the relevant factor here is B)The answer you get using Faradays law depends on the geometry of the design also but no matter what the design is to generate a large voltage BAN must be large and t must be small.I hope some of this helps.
    Last edited: Jan 15, 2009
  10. Jan 14, 2009 #9
    Having thought some more about your intention to use magnets on the rotor I see nothing but disadvantages in doing this.The design will be more complex,the bearings will need to cope with heavier loading other design features will be comprimised and the performance, when compared to more traditional designs, will be reduced.The output you get from a generator depends on the work done against the induced back emf and any currents and magnetic polarities resulting from loading your generator,these being dependant on the conductive properties of the rotor. I am not an expert on generator design so perhaps you should seek advice from somebody who is.In the meantime may I suggest that you start your project more modestly for example by investigating the effects of moving a magnet towards a coil at varying speeds and then investigating the other variables.Good luck with it.
  11. Jan 16, 2009 #10
    Thanks for your help

    I think I am actually going to cannibalize an automotive alternator, pull off the regulator, and bridge rectifier. Then build the generator off of that.
  12. Jan 16, 2009 #11
    Here is a news report of some British guy building something like this on a much larger scale

    Last edited by a moderator: Sep 25, 2014
  13. Jan 16, 2009 #12
    Good luck! Looks like a con job in the making to me!
  14. Jan 17, 2009 #13
    I agree with Dr. D and I think it could be a hoax perhaps first broadcast on April the first.History is littered with peoples failed attempts to build perpetual motion machines but the bloke in the film is going one step further and claiming to get more energy out than goes in this making him some sort of creator.Always remain sceptical in science but I would advise you not to get involved in this particular venture because you could end up investing too much of your time and money for nothing.
  15. Jan 18, 2009 #14
    I don't know about the report (I think that sounds a little bit farfetched too), but my design is not going to be perpetual (that can't be achieved). It is going to draw power from the potential energy stored in the magnets. As this is used the magnets will depolarize, and have to be repolarized (which will take more energy that was pulled off the magnet).
    Last edited: Jan 18, 2009
  16. Jan 18, 2009 #15
    OK new direction

    Can someone help me by explaining this equation?
    E = (mu * q-sub-m1 * q-sub-m2) / (4 * (pie) *r)

    E = emf (voltage)
    mu = ?
    the q's = ?
    r = the distance between the centers of the magnets?
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