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Mutual induction coeff question

  1. Aug 28, 2009 #1
    here is the question and solution:
    http://i29.tinypic.com/30cy4cn.jpg


    i cant understand certain things in here:
    i cant understand the 3d picture.
    why the magnetic field in the z direction??

    i cant understand how did they find the mutual inductance here.
    because by this formula

    [tex]M_{12}=\frac{N_2\phi _{12}}{I_1}[/tex]

    in our situation we need to find the current in the celonoid
    but they for some weird reason find the emf
    and right after without any calculation
    they show the mutual induction coeff

    why??
     
  2. jcsd
  3. Aug 28, 2009 #2

    gabbagabbahey

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    Homework Helper
    Gold Member

    They set up their co-ordinate system so that the z-axis runs parallel to the solenoids axis, that's why the field points in the z-direction.

    As for [itex]M[/itex], it comes straight from its definition, together with Faraday's law:

    [tex]\Phi_{\text{loop}}=MI_{\text{solenoid}}\implies \varepsilon_{\text{loop}}=-\frac{d\Phi_{\text{loop}}}{dt}=-M\frac{d I_{\text{solenoid}}}{dt}[/tex]
     
  4. Aug 29, 2009 #3
    thank :)
     
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