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Eddy Currents and Lenz's Law

  1. May 17, 2014 #1
    A metallic disc attached to a rod swings between two opposite magnetic poles, and its oscillation dies away very quickly. I thought this happens because the eddy currents formed inside the disc are established in a direction that would cause their magnetic field to oppose the magnetic field which induced them, causing a resistive force on the disc, just like air resistance but much stronger. And since the amplitude is a measure of the total mechanical energy of the disc, and work is done against the "resistive forces", the amplitude decreases. However, my A level book says otherwise. According to my book, eddy currents cause a heating effect which dissipates energy in the disc causing the oscillations to die away. Thoughts on this please?
  2. jcsd
  3. May 17, 2014 #2
    Both explanations are correct.
  4. May 17, 2014 #3
    Could you please elaborate further?
  5. May 17, 2014 #4
    Both things you said are correct. The induced currents produce a magnetic field that opposes the preexisting field forcing the pendulum to slowdown. The energy of the pendulum is dissipated which means it is transformed into heat. That heat is deposited in the metallic disk because the eddy currents have a heating effect. Both things are true. One explanation does not preclude the other explanation. They are both correct.
  6. May 17, 2014 #5
    But a heating effect would be caused by the current anyway, regardless of its direction, right?
  7. May 17, 2014 #6
    Yes, but only one direction works. Lenz law gives you the correct direction.
  8. May 17, 2014 #7
    May be an analogy will help. If you shoot a bullet into a water tank the water will slow down the bullet. You can say that the drag produces a force that slows down the bullet but you can also say that turbulent motion converts the energy of the bullet into heat. Both the force and the energy points of view are correct.
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