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Homework Help: A steel rod and an iron rod is placed inside an ac current soleoid

  1. Feb 10, 2012 #1
    What happens? I think nothing should happen since the rods will not get magnetised at all due to the ac current..

    Thanks for the help!
  2. jcsd
  3. Feb 11, 2012 #2
    If you have low frequency AC then the bars will become magnetised with changing magnetic polarity. The bars could repel each other although the fact that one bar is steel and the other is iron will produce different strengths so .....? Not sure !!
    At higher ( mains) frequencies I think you are maybe correct!.......I know that mains AC solenoids can be used to demagnetize steel bars by withdrawing them slowly.
    I imagine there is not one simple answer.
  4. Feb 11, 2012 #3
    Hi thanks for the help. I'm Singaporean so ky syllabus is a lot different, we only touch the basics of magnetism so I'm unsure about this. May I ask what is the relationship between magnetic domain and magnetic field? And why when magnetic shielding occurs, the shielding object does not get magnetised and have its own field?

    This question came out in a test. I'll let you know the answer my teacher gives. Thanks!
  5. Feb 11, 2012 #4
    That's probably not true, right? I mean nothing does not happen...I am sure something happens, it is a matter of whether what happens is of interest to you or not or whether it affects you or not...so, maybe some context would have been nice.

    Maybe the rods do not get magnetized because of the changing magnetic field, but something happens...because the relative motion between the magnetic field and the rods, eddy currents will be induced into the rods and they will heat up due to their internal resistance...certainly the iron rod, I don't know about the steel one, if by making it out of steel, they meant non-magnetic steel?
  6. Feb 11, 2012 #5
    The question just said, if the dc current was changed to a low frequency ac current, describe what will happen to the rods and explain why.

    I know that steel is a hard magnetic material so its harder to get magnetised, but I thought the field will still pass through it so they will still repel no matter what. Thanks for the help
  7. Feb 11, 2012 #6
    Nope, the original posting said absolutely nothing about dc.
  8. Feb 11, 2012 #7
  9. Feb 13, 2012 #8
    Can I get more help in this
  10. Mar 2, 2012 #9
    This is what my school teacher said: there are 2 scenarios that can happen
    1) the iron will keep changing polarity with the changing current. While steel will retain its polarity from the first current direction. This is because steel is a hard magnetic material so it retains it magnetism longer. Thus, it will continuously attract and repel.(this is when the frequency is very very low)
    2) the steel and iron will keep repelling each other. This is because the iron is a soft magnetic material so it is able to get magnetised easily. Even though steel is a hard magnetic material, it is able to switch polarity so it will repel from the iron rod.

    But won't the steel be unmagnetised?
  11. Mar 2, 2012 #10


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    Staff: Mentor

    To demagnetise a steel object you slowly withdraw it from within a strong AC field. Emphasis on the "slowly".
  12. Mar 2, 2012 #11
    I don't quite get it, slowly is meant to demagnetise it, but I'm still unsure about what if the observations in this case.
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