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Homework Help: EMF with coil

  1. Apr 23, 2004 #1
    A small coil is laid on a flat table and a magnet is held vertically over the magnet. Both are stationary. I move the magnet so that it passes the coil and then held it stationary again. I move the magnet over the coil and hold it stationary there again. What am I supposed to conclude about the generation of a current as a function of the magnet motion?
  2. jcsd
  3. Apr 23, 2004 #2
    Not sure how specific the answer's supposed to be, but I think this is a question about induction. There's a formula that says:

    [tex]EMF = -\frac{d\Phi_B}{dt}[/tex]

    Where [tex]\Phi_B = \oint \vec{B} \cdot \vec{dA}[/tex]

    Which means that EMF is only induced when there is a change in the magnetic flux. Magnetic flux can be changed by changing the field (which doesn't happen in your case), or changing the area (which does happen) or changing the angle between the coil and the magnet. So, basically, current is only generated when EMF is generated, which only happens when you're moving the coil or moving the magnet such that the area changes.
  4. Apr 23, 2004 #3
    What do you mean when you say "area change" in your last sentence? Moving the magnet/coil doesn't change its physicsl area does it? Could you explain more about this? I thought that you meant magnetic flux can be changed by changing the physics area of the coil. I'm not sure.
  5. Apr 23, 2004 #4
    My interpretation has always been that magnetic flux is changed by the area that the magnetic field passes through, not the area of the coil. When you're moving the coil into the magnetic field, the area through which the magnetic field passes through increases, so the flux changes. If you have area of the coil = A, and the magnetic field only passes through half the coil, you'd use A/2 in your calculations.
  6. Apr 23, 2004 #5
    so the more the magnet moves, the more area it covers, the more the magnetic flux?
  7. Apr 23, 2004 #6
    The faster the magnet moves, the faster the area changes. Remember, since induction relies on the rate of change of flux (not the flux itself). It doesn't matter what the area is, but how fast it is changing. Think of it in terms of derivatives. If you had a small slope, no matter how large the value of y is, dydx will still be incredibly small.
  8. Apr 23, 2004 #7

    Doc Al

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    Staff: Mentor

    The field changes, not the area

    Since the coil is unmoving and unchanging, it's quite a stretch to claim that its area is changing! Of course the magnetic field through that coil is changing. As you move the magnet around, the flux changes due to the changing magnetic field.

    UrbanXrisis, didn't you bring up this same experiment in a recent post?
  9. Apr 23, 2004 #8
    Oh my gosh! You're right! :tongue: That's pretty stupid of me. Basically, a current is present when there is a magnetic flux, and the flux is created by the magnet moving. If the magnet isn't moving, then there isn't a magnetic flux, therefore no current. Correct? The possitive and negative of the current is determined by Lenz's Law.

    Am I getting this right?
  10. Apr 24, 2004 #9


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    Science Advisor

    Not quite. A magnet has flux whether it is moving or not. A current is generated only when the magnet is moving. The current is generated by the changing amount of flux through the coil.
  11. Apr 24, 2004 #10
    There is always a flux, just not a changing flux while the magnet is not moving? So there is always a magnetic flux, but current is generated when there is a changing magnetic flux...aka moving the magnet?
  12. Apr 25, 2004 #11

    Doc Al

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    Staff: Mentor

    Right! Just as krab explained.
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