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Induced Emf: Magnet Passes through a Coil

  1. Jul 23, 2011 #1
    I'm doing a lab write-up for physics 2. The experiment is about the title, a bar magnet being dropped through a solenoid.

    I have to explain four graphs that plot the change in emf (y axis) vs. time (x) axis. There are incoming and outgoing peaks on these graphs. I have to tie these results with the equation emf = -N(ΔΦ/Δt).

    For example, if a bar magnet is dropped with north facing downward through the center of the coil, why is the incoming peak positive? How do the magnetic field, induced magnetic field, and emf all tie together to produce a positive peak?

  2. jcsd
  3. Jul 23, 2011 #2
    How the coil is wound and attached to the voltmeter would make a difference. You should know the right hand rule. Pick a direction, either up or down through the coil. What was the sign of the rate of change of flux through a surface inside the coil that is normal to this direction, as the bar magnet approached the coil?

    Now that you know the sign of rate of change of flux for this surface, using the normal vector, the right hand rule, and the emf formula, what was the direction of the emf (if you viewed the coil from the top, would it be clockwise or clounter clockwise)? Now, based on the emf direction and the way the coil is wound, would the wire at one end of the coil have higher or lower voltage than the other end, if a voltmeter was placed across the coil?
  4. Jul 23, 2011 #3
    The way in which the coil was wound and attached to the voltmeter is irrelevant (for this experiment).

    My confusion is this: the lab manual states that when the magnet is entering the coil, the magnetic flux through the coil increases with time. Wouldn't that mean the magnetic flux always increases if it is entering a coil? (But that can't be true).

    The negative sign in emf = -N(ΔΦ/Δt) also confuses me.
  5. Jul 23, 2011 #4
    It's not irrelevant if you care about whether the peaks are positive or negative.

    Why do you think that?

    Stick your right thumb out and curl your fingers in a loose fist. If the rate of change of flux is positive in the direction the tip of your thumb is pointing, then the emf circles around in the opposite way that your fingers are curling. That is, it would go around like going from the tips of your fingers to your knuckles. The minus sign is due to it being the opposite way (fingertips to knuckles as opposed to knuckles to fingertips).
  6. Jul 23, 2011 #5
    I don't mean to ignore what you said earlier, but before I can understand that, I need to address this problem:

    My lab manual says "When the magnet is entering the coil the magnetic flux through the coil increases with time." How is this possibly true? If north is downward when the magnet enters, the emf makes a positive peak. On the other hand, if south is downward, the peak is is a trough (instead of a crest).

    Magnetic flux does not necessarily increase just because the magnet enters. Doesn't it depend on the orientation of the magnet as well?
  7. Jul 23, 2011 #6
  8. Jul 24, 2011 #7
    First of all, you need to visualize the magnetic field of the magnet.

    Like you said, a magnet has a north pole and a south pole. Magnetic fields are visualize via closed magnetic lines, these magnetic lines come out of the north pole and go back in through the south pole...then, they travel through the inside of the magnet to come, again, through the north pole...

    ...you need to draw this, with arrows...

    and you will see that when to insert the north pole into the solenoid, the arrows in the magnetic lines will be in opposite directions as compared to when you insert the south pole first...

    ...according to the right hand rule, the best induction is produced when a wire is crossing magnetic lines that are perpendicular to it...and so, when you introduce a bar magnet into a solenoid, most of the induction is pretty much happening in the turns around the ends of the bar magnet and not much in the ones next to the body of the magnets....again, pay attention to the direction of the magnetic lines...
  9. Jul 24, 2011 #8
    so the polarity of the peak is relative to the polarity of the magnet (north down vs. south down) and the way the coil wraps around the solenoid?

    i.e., a crest could be a trough and a trough could be a crest, in that it all depends on the experiment?
  10. Jul 24, 2011 #9
    it also depends in which direction the magnet is moving...whether it is going down or up
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