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Homework Help: Electromagnetic induction on iron core question

  1. Aug 19, 2012 #1
    1. The problem statement, all variables and given/known data
    An iron core with a solenoid coiled around it is in a circuit with a switch and a dc current supply. In front of it, there is a iron ring tied to the ceiling such that it faces the solenoid without touching the circuit. When the switch is turned on what is observed and explain your answer.

    2. Relevant equations


    3. The attempt at a solution
    I sort of understand the first part:
    The iron ring will be repelled away as when the switch is turned on, the magnetic field strength of the solenoid becomes stronger. This results in a change in magnetic flux linkage and by faraday's law there is an induced emf across the ring. Since the ring is a closed circuit, by lenz's law there is an induced current flowing to form an induced pole to repel the iron core.

    However, when we consider what happens after the magnetic field of the solenoid becomes constant I'm unsure what happens. Once it reaches it's maximum strength, the ring would no longer experience a change in the magnetic flux linkage. So it should fall back to its original position right? But as it falls back it would experience a change in magnetic flux linkage as it would go into a stronger region (closer to the solenoid means stronger magnetic field) so would it be repelled again such that there will be a jerking motion?

    I think that that would happen as it makes sense? Thanks for the help :-)
  2. jcsd
  3. Aug 19, 2012 #2
    You got the first part right. Here is a video for you:

    For the second part, hint: when the current in the solenoid becomes constant, the solenoid is essentially a magnet.
    Last edited by a moderator: Sep 25, 2014
  4. Aug 19, 2012 #3
    Since it becomes a magnet, if it starts to move back it experiences a change in magnetic field? As when the iron core is further the magnetic field is weaker, so it causes it to get repelled again due to lenz's law? I'm pretty confused, that's the only thing that came to mind..

    Thanks for the help! :-)
    Last edited by a moderator: Sep 25, 2014
  5. Aug 19, 2012 #4
    Think about a magnet and a piece of iron. How do they react to each other? I am sure you have seen that many times.
  6. Aug 31, 2012 #5
    They attract? I don't think that happens though I mean the iron ring is sort of a solenoid so shouldn't it have the solenoid effect?
  7. Aug 31, 2012 #6
    When the current is constant, the solenoid is just a magnet. How would an iron ring interact with a magnet? What forces would act on it?
  8. Sep 1, 2012 #7
    Isn't the current in the solenoid always constant while it's the magnetic field that increase or decreases? So once the magnetic field becomes constant then it would get attracted? But even as it becomes constant and the iron ring falls back then the iron ring would experience a change in magnetic field again and get repelled? Sorry but I don't quite understand this case. Thanks again :)
  9. Sep 1, 2012 #8
    You got this almost right. The iron will feel a constant attractive force. But any motion toward the attractor will create a resistive force. This is similar to the motion of an object in the air under the force of gravity. Gravity is constant, drag depends on the motion. What happens with the object?
  10. Sep 2, 2012 #9
    Oh okay! I think I will start explaining all the way behind: when the switch is turned on, the magnetic field of the solenoid increases, this causes a change in magnetic flux linkage on the iron ring. So by faraday's law, there is an induced emf across the iron ring and since the iron ring is a closed circuit, by Lenz's law there is a induced current flowing in the manner such that it opposes the magnetic field of the solenoid. As the solenoid's magnetic field stablizes, the iron ring only experiences the attractive force of the solenoid and the gravitation force pulling it back. This causes the iron ring to fall back into position, however as it moves closer to the solenoid it experiences a change in magnetic field (weaker to stronger) causing it to get repelled away until it achieves an equilibrium due to the iron ring's inertia.

    Is this correct? Thanks for the help!
  11. Sep 2, 2012 #10
    It is important to observe that in equilibrium the ring will have only three forces acting on it: constant magnetic attraction from the solenoid; gravity; tension in the string. No repulsive force!
  12. Sep 2, 2012 #11
    But I thought it will constantly be repelled then falls back and repelled? So much that the ring just stays in one position without moving eventually?

    By the way, can we classify an increase in magnetic field as repulsion and decrease in magnetic field as attraction? Because in all the scenarios I've seen it's like that. Thanks!
  13. Sep 2, 2012 #12
    In a constant magnetic field, there is nothing that would cause back and forth kind of motion. It just slows any moving conductive loop, no matter where it is going - think about it. It is just like a viscous fluid. That will damp oscillations, not induce them. If the ring were non-ferromagnetic, that would be the only action of the magnetic field. But iron is ferromagnetic so it is also attracted, no matter whether it is moving in any particular direction.
  14. Sep 2, 2012 #13
    In the picture given, there is a solenoid facing an iron ring with is connected to the ceiling by a wire. So once it reaches stability, the iron ring will move back but as it moves back de to gravity and magnetic force, isn't there also a change in magnetic field? I mean when it is far away the magnetic field is weaker than when it is closer. So again there is that repulsion. Something like moving a coil closer to a bar magnet - when it is far it experiences less magnetic flux linkage than when it is closer right? So it experiences a change (weak to strong) causing it to be repelled. So the same thing here right? It goes closer to the source of the magnetic field so again by faraday and Lenz law won't there be a repulsive force?
  15. Sep 2, 2012 #14
    It does not matter whether the loop goes from weak to strong or from strong to weak. Either way it will be subject to a force in the OPPOSITE direction to its motion. This force is neither attractive nor repellent; it's braking.

    Last edited by a moderator: Sep 25, 2014
  16. Sep 2, 2012 #15
    So as it moves back it experiences a braking force until it stops again and the cycle keeps repeating?
  17. Sep 2, 2012 #16
    I'd like to see what actually happens... Iron isn't a very good conductor so the induced current may not be that great. Iron is readily magnetised. The former will want to make the ring jump away and the latter attract it to the solenoid.

    The MIT video looks like it used an aluminium ring. Much better conductor than iron (3.5x) and no ferro-magnetism.
  18. Sep 2, 2012 #17
    What would make the cycle repeat?
  19. Sep 2, 2012 #18
    Again as it repels the iron ring away, it stops again causing the effects of gravity and magnetic attraction to pull it back. So at this point of time, the iron ring will have the same repulsive force. So the cycle repeats until it finds an equilibrium position?
  20. Sep 3, 2012 #19
    It will NOT have the same repulsive force. It will feel a sort of a magnetic drag. Which does not cause oscillations, it suppresses them. You have seen a pendulum in magnetic field.
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