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Magnetism of an electromagnet

  1. Apr 14, 2014 #1
    Hey I'm new here so I don't know if this is the right forum, but from what I could see this was the most relevant.

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

    A long solenoid with 50 turns per cm carries a current of 0.2 A. The solenoid is filled with an iron
    alloy. The magnetic field B inside the iron alloy is measured to be 1.58 T acting parallel to the axis of
    the solenoid.

    Calculate the:
    i) size of the applied field Bapp;
    ii) size of the magnetization M in the iron alloy;

    2. Relevant equations

    This is my problem really as I don't know so far I've been using the magnetic field of a solenoid = (N/L)
    *I*μ0, however I don't know if this correct or if it needs the permeability of the core to produce an accurate value.
    I've also been using Bt= Bapp0*M rearranged to M= (Bt-Bapp)/μ0 to answer the second question.

    3. The attempt at a solution

    Well this is just plugging numbers into the equations so I'm confident that I haven't done anything wrong here.
    Assuming the solenoid field is Bapp, Bapp=5000*0.2*1.3×10−6 = 0.0013.
    Putting that into the rearranged equation. M = (1.58 - 0.0013)/1.3×10−6 = 1.2106.

    My problem here is those values seem very small and ridiculously large respectively, have I done something wrong or am looking for errors where there are none?
     
    Last edited: Apr 14, 2014
  2. jcsd
  3. Apr 14, 2014 #2

    BruceW

    User Avatar
    Homework Helper

    hey, welcome to physicsforums! uh, your answers look good to me. Although when you hand in your work, you should probably give units, and maybe be a bit more careful on the rounding-off. But it's essentially good. And yes, one is very large and the other is very small. But this is just because SI units are like that. Looking at your equation M= (Bt-Bapp)/μ0 And since μ0 is very small in SI units, the ratio between Bt-Bapp and M will always be an 'extreme' ratio, in SI units.
     
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