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Forces acting on bar magnet inside a charged solenoid

  1. Oct 24, 2016 #1
    This is not a specific homework problem. It is a lack of understanding in theory relating to a lab report I have to write.

    We (my group and I) wrapped a 40 cm PVC pipe in copper wire (very densely and for almost the entire length), applied a current to it, and dropped a permanent magnet through the tube. The current was applied such that the magnetic field of the solenoid (as we modelled it) would point upwards, opposite gravity.

    By connecting two extra coils of wire at each end to a special data capture instrument, we could record the induced current in these coils as a function of time (giving us an accurate way of measuring the descent time of the magnet).

    When we applied increasing levels of current, the data (and observation) showed very clearly that the magnet was slowing - at one point it even stopped at the top of the tube (until we pushed it).

    That's the context - the problem is, then, what is the force opposing the magnets motion? (Not numerically but conceptually)

    I originally thought that the eddy currents induced in the solenoid (by the passing magnet) would affect the drop time (thus explaining the force opposing the magnets motion). However, when no current was applied, the drop time of the magnet was very similar to what we would expect from classical kinematics (little change), and I don't believe eddy currents explain why a greater applied current affects drop time.

    Any assistance is much appreciated.


    1. The problem statement, all variables and given/known data


    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Oct 25, 2016 #2

    NascentOxygen

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  4. Oct 25, 2016 #3
    Yes, I am wondering why and how the permanent magnet passing through the magnetic field of the solenoid is experiencing a repulsion (thus slowing it's descent time).
     
  5. Oct 25, 2016 #4

    Merlin3189

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    How about drawing a bar magnet in the middle of the solenoid and sketching in some field lines like Nascent0 did?

    Would you have the same question about two permanent magnets repelling each other?
     
  6. Oct 25, 2016 #5

    NascentOxygen

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    Did you find that your magnet experienced a repulsion if you did one of the following:
    (i) reversed the orientation of the permanent magnet, or
    (ii) swapped the wires connecting to the DC voltage supply?
     
  7. Oct 25, 2016 #6
    I did not think of it that way - so it is being decelerated as a result of like poles repelling each other - which would explain why the deceleration increased with current, since the magnetic field inside the solenoid increased?

    I believe we did observe a difference when swapping the orientation of the magnet, but did not test thoroughly - we didn't test the effect of swapping the wires... That might have helped a lot.
     
  8. Oct 25, 2016 #7

    Merlin3189

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    Of course, no sooner had i pressed send than I realised that the repulsion I'd thought about does not apply "in the middle of the solenoid", only at the ends!
    But you would have noticed the repulsion from the bar entering the solenoid and detected it reaching the end before it was greatly accelerated out of the solenoid.

    The permanent magnet in the solid copper pipe experiences drag all the way down, because it is continuously generating current just in front and just behind it all the way through - being repelled in front and attracted behind.
     
  9. Oct 25, 2016 #8
    I see, I am wondering if you have an resources I could read up on this? I would need to formalise this theory for my lab report.
     
  10. Oct 26, 2016 #9

    Merlin3189

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    I'm afraid I don't know of any useful explanation I could direct you to.
    I am coming to it simply in a qualitative way, based on the notion that a bar magnet is like a solenoid (externally.) So the force between a solenoid and an external bar magnet is like the force between two bar magnets (which I think you may find explanations of.) Inside a solenoid there is a uniform field, so I'd expect there to be no net force on the bar magnet when it is parallel to the field. When it is not parallel, there would be a torque trying to make it become oriented with its field in the same direction and sense as the solenoid field. When it is parallel to the solenoid field, but its field is in the opposite sense, then it could be in unstable equilibrium and would turn around at the first opportunity. (Maybe some of the slowing you observe is due to the magnet being pushed out of line and rubbing against the plastic tube?)

    All of this can be seen qualitatively by using Faraday's idea of field lines and I don't suppose anyone is expecting any quantitative calculations.from you.

    Your observation
    is consistent with the idea that the repulsion happens only at the end of the solenoid. Once you push it past this point, I presume it traveled through the remainder of the solenoid without stopping, then accelerated away from the other end..
     
  11. Oct 26, 2016 #10

    NascentOxygen

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    It probably would have been more informative had the PVC pipe been sat horizontally on the lab bench, allowing students to guide the bar magnet into, then through, and out of either end of the horizontal solenoid free of the confounding effect of gravity. That way investigators would be able to feel the force the bar magnet experiences at each point throughout its travel over the full length of the solenoid and beyond.
     
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