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Induced emf when dropping a magnet into a solenoid

  1. Sep 23, 2012 #1
    I am having some trouble understanding why and how the emf changes when a bar magnet is dropped into a solenoid.

    As the magnet moves down (have not entered the solenoid yet) it experences an increase in magnetic field. As it enters the magnet it experiences an increase in magnetic still until it has entered all the way. At which, now at the back there is a decrease in magnetic field while at the front there is an increase, however because in the front there are more field lines overall it is still a increase. Once, it reaches the middle there is no everall change in magnetic field. As it drops down further, i am clueless on what happens. I think that since there are now more coils at the back, so overall there is a decrease in magnetic field.

    Here is a diagram of what i think: http://i.imgur.com/feMhv.png

    Thanks for the help! :smile:
  2. jcsd
  3. Sep 24, 2012 #2

    Philip Wood

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    Easiest way to understand what's going on is in terms of flux linkage. This is a topological idea. Consider two closed loops, A and B. A represents an electric circuit, B represents magnetic field lines (which are closed loops). If A and B are linked, like adjacent links in a chain, we have flux linkage. If the loops are not linked, we don't. If the loops are unlinked and we link them, an emf is induced in the circuit loop while the linkage is changing. If we unlink them an emf is induced in the opposite direction while the linkage is changing.

    On the thumbnail I use this idea to try and explain what's happening in the case of the magnet dropping through a coil.

    Attached Files:

    Last edited: Sep 24, 2012
  4. Sep 26, 2012 #3
    Hi Mr Wood :smile: Thanks for the explanation and diagrams it help me a lot. However, i am still confused about this.

    when the magnet reaches this position: http://postimage.org/image/he6l7v6qd/full/, shouldn't the emf induced become zero?
    This is because the magnet field lines do not cause any net magnetic flux linkage. And only when the magnet reaches this position:http://postimage.org/image/he6l7v6qd/full/ then a new emf should be induced.

    Thanks for the help!
  5. Sep 26, 2012 #4

    Philip Wood

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    I'm a bit confused about your diagrams. Both your links take me to the same pair of diagrams! But in each diagram (the one with the magnet near the top of the solenoid and the one with the magnet near the bottom) there's lots of flux linkage. Are you visualising the turns of the coil and the lines of flux like the links of a chain? [Incidentally it's a good idea in this context to continue the magnetic field lines round inside the magnet, as I did in my diagrams, emphasising that the field lines are themselves closed loops.]

    Don't know what you mean by 'not causing any net magnetic flux linkage'. There's plenty of flux linkage in each case. The flux is linked with different turns of the solenoid, but that doesn't matter at all, as all the turns are all in series.

    As the magnet goes from the top position to the bottom, the flux linkage doesn't change, so that's why there's no emf while the magnet is inside the coil. Not that there's no flux linkage.
  6. Sep 26, 2012 #5
    hi, sorry my first diagram is the first position i was mentioning while the box is the solenoid. So since there is no change in the flux linkage once it reaches postion 1 from the link to position 2, there will not be any emf produced? Thanks for the help!
  7. Sep 26, 2012 #6

    Philip Wood

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    Some physicists think about the magnet and solenoid situation in terms of cutting of lines of flux. This is absolutely fine, but I chose to discuss in terms of flux linkage, and changes in flux linkage. The two concepts are closely related, but shouldn't be muddled up together. Flux linkage is more versatile, because there are plenty of cases (such as inductors and transformers) where flux linkage changes without obvious cutting of flux, because there's no macroscopic movement of conductors.
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