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Induced voltage in a solenoid

  1. Aug 9, 2003 #1
    Hello all,

    We've been studying electromagetism and motors in physics. I was just wondering, if you placed a rotating magnet near the end of a current-carrying solenoid, what would the relationship be between the magnet's speed of rotation and the induced voltage in the solenoid?

  2. jcsd
  3. Aug 10, 2003 #2
    Hi ajax,
    I don't understand why you say 'current-carrying'. What you are interested in is voltage, so what for do you need a current?
  4. Aug 11, 2003 #3
    Welcome to Physicsforums Ajax!

    I can't remember the name for this effect, but I think rotation of the magnet would change the direction of the current in the solenoid in about every half a turn. This effect stated when the magnet is approaching a current carrying solenoid, the current carrying solenoid would create a magnetic pole repell the magnet.

    e.g. If the north pole of a magnet is approaching a current carrying solenoid, the solenoid would adapt a current in which it produces a similar pole to repell the magnet.
  5. Aug 11, 2003 #4
    Thanks for your replies. As I understand it (and I could well be wrong), the effect is indeed something to do with half-cycle current reversals (ie. AC), but other than that I'm not sure.

    As for a current-carrying solenoid, I don't think there would be any effect of a plain magnet on the coil, but when the solenoid has a current, it then has an electromagnetic field that can be "fiddled" with. I suppose rather than voltage, I'm interested in any effects on the solenoid.

    That's also why placing the magnet near the solenoid (rather than inside) is important. As I'm sure everyone knows, the field within a solenoid is linear.

    Perhaps the answer has something to do with how the magnet interferes with flux????
  6. Aug 11, 2003 #5
    I still don't quite understand your experimental setup. Please tell me what causes the magnet's rotation? And what causes the current in the coil?
  7. Aug 12, 2003 #6
    Sorry for not having been quite clear with a description.

    As I picture it, you would have a solenoid connected to a DC source voltage (perhaps also in series with a rheostat so as not to overheat the solenoid).

    As a separated piece of equipment would be a bar magnet attached to a small motor, so that the motor would cause the magnet to rotate (I was thinking the magnet would be attached so as to rotate about its longitudinal axis).

    Finally, an oscilloscope or digital multimeter could be connected across the solenoid to measure any effects of altering the rate of the magnet's rotation.

    I hope this helps. If I'm still not making sense, I can draw up a circuit diagram for you if you like.
  8. Aug 12, 2003 #7
    Yes that helps.

    But I still need more information concerning the magnet's rotation:
    - What is the relative orientation of the rotation axis relative to the solenoid axis?
    - What is the orientation of the magnet's north-south axis relative to the rotation axis?

    Maybe a drawing of the whole setup would indeed do good.
  9. Aug 15, 2003 #8
    think of it this way,

    if the magnet is spinning/rotating faster, then there would also be a greater changing magnetic field [ greater as in the difference between the change of the magnetic field for a particular spot, if u know what i mean].

    thus by referring to faradays law, the induced current is proportional to the changing magnetic field, u can deduce that induced current would be greater.

    let me know what u think, i am in a similar situation[high school].
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