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Homework Help: Can anyone explain this anomaly?

  1. Dec 21, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi, so during an experiment, I spun a magnet inside a coil of wire in order to induce an emf, and measure the input power and output power. One of the things I found was that the slower the magnet spun (ie the smaller the input voltage to the motor) the larger the induced voltage and the greater the output power from the coil. Here is some of my data;

    9.98v and 0.25A -> 0.06V and 0.04µA
    4.95v and 0.15A -> 0.08V and 0.01µA
    6.01v and 0.15A -> 0.11V and 0.15 µA

    This was a general trend I found when changing variables such as the number of turns in the coil and the strength of the magnet. All my tests showed this trend. According to the Faraday Law and the equation for induced emf (E=Blv where B is the magnetic flux density, v the velocity of the coil/magnet and l the length of wire) the faster the magnet is spinning the higher the expected emf and therefore voltage. Can anyone think of any reasons why this might be happening?

    2. Relevant equations

    3. The attempt at a solution
    I thought maybe the skin effect, where resistance increases with higher AC frequencies, might have something to do with it but it requires frequencies much much higher than the ones I am using to have any effect.
  2. jcsd
  3. Dec 21, 2015 #2


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    Staff: Mentor

    Most likely the setup was not what you thought you had. Perhaps the voltmeter you were using may not have been suitable for the frequency range involved?
  4. Dec 21, 2015 #3


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    Science Advisor
    Homework Helper
    Gold Member

    0.01µA is just 10nA and the output power <1nW.

    Would it take much noise to cause a problem? You have a 1W brushed motor in close proximity?
  5. Dec 21, 2015 #4
    I may be reading this wrong. So the magnet is spinning with 4.95 velocity and producing .08V and 0.01 micro-amperes? Then when you increase the velocity to 6.01 the produced voltage and current is also increased? Isnt that what is expected?
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