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How do you determine the polarity rods placed in a solenoid?

  1. May 31, 2015 #1
    Now well the question is pretty straight forward , and please do not pass this by , I am in high school but I am a still a very accomplished physics student. I am currently doing Magnetism and I came by a question , where people are saying a solution that seems very
    counter-intuitive to me.
    Observe the image below :
    https://scontent-atl1-1.xx.fbcdn.net/hphotos-xpt1/v/t34.0-12/11303617_1076701942357842_1492731568_n.jpg?oh=104c35896d5f63fc35c944b032598b97&oe=556DD2E5

    Others are saying that D is the answer but I say that the answer is B. Now understand I am very open to suggestion, I mean what is a physicist without an open mind but my reasoning has kept leading me to B being the answer , now look at this , if we place these two materials in the Solenoid , in the way the current flows , we have a north on the right side based on the Right Hand Grip Rule . But here is where the division occurs , they believe that the north of the soft iron material is on the same side but I am saying that the iron material has to have the opposite polarity of the solenoid because that is how it normally works when something is magnetised. So I say the north of the Solenoid creates a south in the material on the right and vice versa. What do you think ? Is my answer a misconception ?
     

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  3. May 31, 2015 #2

    Simon Bridge

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    When applying the right hand rule, the dotted lines indicate wires going behind the solenoid.
    Letting your fingers follow the current (look at "I") up and behind will put your palm facing you and your thumb pointing to the right.
    The iron material inside the solenoid will have the same polarity as the solenoid - otherwise iron-core electromagnets will not work.
     
  4. May 31, 2015 #3
    But the flux does not initially travel through the iron cores
    I am sorry but your answer is not elaborate enough for me
     
  5. May 31, 2015 #4
    Please I am familiar with magnetic circuits , feel free to explain making references to the one created here
     
  6. May 31, 2015 #5

    Simon Bridge

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    The question is about the steady-state condition, not the initial transient condition.
    Transients can be arbitrarily complicated.

    This is not a magnetic circuit question - soft iron is ferromagnetic so it's magnetic domains line up with the applied magnetic field.
    see: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html
     
  7. May 31, 2015 #6
    So what you are saying is at the end of the day , the flux travels through the iron just as how it would if air were there ?
     
  8. May 31, 2015 #7

    Simon Bridge

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    Sort of - at the end of the day, the flux goes through the iron in the same direction as it would if it were just air or a vacuum - but it flux density is much higher.
     
  9. May 31, 2015 #8
    Ah yes thank you , s I know because iron has much greater permeability than air , the flux density part, Ok so no matter how I take it , it only provides a path for the flux that already existed assuming of course that it itself was not already magnetised.
     
  10. May 31, 2015 #9
    Thank you, I made a rookie mistake in not examining holistically.
     
  11. May 31, 2015 #10

    Simon Bridge

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