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Moving magnet through coil

  1. Jun 5, 2011 #1

    I'm a new face here and have an easy question.

    I know how to calculate induced current when magnet moves through coil (it's E=BLv) but how do you calculate how much a magnet will decelerate when it goes through the coil? Will it decelarate at all?

    I have one of those mechanically-powered flashlights and the magnet won't slow a bit probably because the strength of magnetic field isn't so great.

  2. jcsd
  3. Jun 5, 2011 #2
    Yes it will decelerate. The easiest way to calculate is the Energy produced. If you know the power consumed by the light, you know how much kinetic energy the magnet had to lose.
  4. Jun 6, 2011 #3
    This is Lenz's Law and it is the electromagnetic formulation of Newton's Third Law, in other words conservation of energy and momentum.
  5. Jun 6, 2011 #4


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    Once you've calculated the electric field through the coil, that electric field will make charges go round the coil, creating a current.
    The current going round the coil creates its own magnetic field (just like in a solenoid) that opposes the magnetic field of the magnet.
    This opposing magnetic field causes a force on the magnet, decelerating it.
    The actual equation for this deceleration is not immediately obvious from the laws of electromagnetism... I guess it depends on the form of the magnetic field created by the magnet...
  6. Jun 6, 2011 #5
    Correct OP, his is the reason why positive and negative voltage gets created from the EMF of the coil passing in and out through the magnetic field.
  7. Jun 6, 2011 #6
    If you were really anal you could treat the magnet (or the coil, that relative stuff) like a simple harmonic oscillator. It would be a perversion of whats really going on with the muscles in your hand but it would be easier. The last statement really sounds a bit strange, sorry.

    Anyways, length of the path of the magnet, get a good rhythm going, time it... weird stuff.

    And that E is EMf?
  8. Jun 6, 2011 #7
    I was also assuming you had the commercial model with the cylindrical magnet that moves back and forth. It bumps a spring on each side to change direction and go back through the coil... I did not realize what you were looking for, WHEN IT GOES THROUGH THE COIL. My bad, as we say.
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