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Homework Help: Having trouble explaining this phenomena

  1. Sep 12, 2012 #1
    1. The problem statement, all variables and given/known data

    I was doing an experiment to verify faraday's law

    Where we were basically dropping a magnet through a coil and measuring the induced emf

    Now here is the thing I cant explain;

    we connected 2 coils in series and found the induced emf was the same as that of one coil

    then we connected 2 coils in parallel and found that the induced emf was half that when using one coil.

    How do you explain this... I thought it had something to do with the resistance but when I checked it using a bit of math for resistance in series and parallel I did not get a result like that from the experiment.

    What is happening in terms of the physics here. Thanks

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 12, 2012 #2


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    Does it help to think that a single coil is a whole lot of loops joined in series, so to add a whole lot more loops in series should mean the same result - but given the physical size of the set-up, that emf may exist for a longer period of time.

    Not sure of the parallel connection.
  4. Sep 12, 2012 #3
    Yeah I kinda gathered that but im more unsure of the parallel result... I just cant seem to figure it out.
  5. Sep 12, 2012 #4


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    I wonder if the parallel situation is giving you an "average" result? The magnet can't be in both coils at once [unless one fits inside the other] so at each phase you are getting "emf+0"/2.

    One might argue that a magnet falling through a coil effectively produces a power supply, like a battery.

    What happens when you connect two similar batteries in Series?
    What happens when you connect two similar batteries in parallel?
  6. Sep 12, 2012 #5
    Well in series the voltages would add and the current capacity would stay the same

    in parallel it would be the opposite

    so your saying that the ε ≈ dBA/dt

    so for ε/2... BA stays the same but the time the battery spends falling throught the loop approx. doubles since it is two loops.so N=2n/2l

    Is this what your saying?

    for the first case (series) it acts like this

    2ε = dBA/2dt

    thats why we had the same result.

    second case (parallel)

    ε = 2BA/2dt

  7. Sep 12, 2012 #6


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    I have never actually considered this situation before, so don't have a definitive answer - I merely put forward some thoughts to prod you along.

    This last consideration where you refer to rate of change of flux looks promising.
  8. Sep 12, 2012 #7
    Ok thanks
  9. Sep 13, 2012 #8


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    Staff: Mentor

    Can you explain how you went about this? How you dropped a magnet through two parallel coils simultaneously?
  10. Sep 13, 2012 #9
    the coils were placed on top of each other so the length of the coil (x) is now 2x.

    then we dropped the magnet through them so at some point I guess the magnet could have been in both coils at the same time since the length of the magnet was ≈x/2.

    does this make sense.

  11. Sep 13, 2012 #10


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    That's how you placed them for parallel coils. How did you place them for series connection?
  12. Sep 13, 2012 #11

    the same way
  13. Sep 14, 2012 #12


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    http://imageshack.us/a/img717/4080/3110y.gif [Broken] ...... there lies your problem.

    To succeed in demonstrating what you expect, then the EMFs in each coil will have to be identical, i.e., generated simultaneously. If the magnet has to pass through one coil before it enters the second, then (without careful analysis) it's anyone's guess how the result will pan out.

    Were I to do it, I would try concentric windings.

    What device were you using to measure the voltage?
    Last edited by a moderator: May 6, 2017
  14. Sep 14, 2012 #13
    just a voltmeter
    Last edited by a moderator: May 6, 2017
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