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How much will the speed of a magnet be reduced when it is passed trough a coil?

  1. Jul 28, 2010 #1
    I need some help to calculate the retardation of a magnet cylinder passing trough a coil with an open end which lies horizontal. If you have a 0.5 m long circular coil which is 50 mm in diameter and a circular magnet cylinder fitting in to the rod. The magnet cylinder has an initial speed of 10 m/s when it enters the coil. What speed will the magnet have when it has passed trough inside the coil? And how much will the voltage be between the two open ends of the coil? The number of turns in the coil is 2000 and the weight of the magnet is 0.5 kg and magnetic force is 2 T. I have seen an example where Newton’s second law is set to equal to Lenz’s law in order to calculate the retardation of a metal bar in a magnetic field. But how should I do to take the length and the number of turns of the coil into consideration? Any help would be appreciated.
     
  2. jcsd
  3. Jul 28, 2010 #2
    The magnet passing through the coil induces a voltage in the coil. If the two ends of the coil are open, there is no current in the coil, and no retarding magnetic field. There will be a retarding magnetic field if the two ends of the coil are connected together through a resistance.

    Bob S
     
  4. Jul 28, 2010 #3

    sophiecentaur

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    The retarding force will be proportional to the current induced which, in turn, will be dependent on the resistance of the coil so you would need to know that if you wanted to have a quantitative idea of what's going on. For zero resistance, the magnet would possibly stop dead in its tracks!?
     
  5. Jul 28, 2010 #4
    Even under ideal conditions, if the magnet did stop, guess what? There would no longer be an induced field and the magnet would start moving again, then eventually stop again, then start moving again and so on.
     
  6. Jul 29, 2010 #5

    sophiecentaur

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    :biggrin:Oh yes, of course.

    But the resistance of the coil will affect the current and, hence, the terminal velocity. After all, with an open circuit, there is no braking effect.
     
  7. Jul 31, 2010 #6
    Thanks for all replies!
    If a resistor is conecter between the ends of the coil which makes the total resistance including the coil to 10 ohms, how much will the speed be reduced then?
     
  8. Aug 24, 2010 #7
    Sometimes it's much cheaper to build a machine than to model it with math. In this case I would suggest a small scale setup. Have 3 coils and three magnets of differing strengths. Drop a magnet through an open coil and time how long it takes to fall through the coil. In fact drop the magnet from 10 different heights. Start at 10cm and increase by 10cm for each additional reading. Now short the coil and repeat. Add resistance in the coil and repeat. Repeat all of the above with the other magnets. Switch the coil and repeat all of the above. With all this data you should be able to come up with an equation to solve your problem. At the very least you'll have some nice graphs to show for your efforts.
     
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