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Homework Help: Energy Stored in Magnetic Field of Solenoid

  1. Apr 16, 2009 #1


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    1. The problem statement, all variables and given/known data
    The magnetic field inside an air-filled solenoid 39.9cm long and 2.00cm in diameter is 0.800T. Approximately how much energy is stored in this field?

    2. Relevant equations

    => U = (1/2)(B^2)(area*length)/(u)

    3. The attempt at a solution
    I discovered how to derive the bottom equation from the top equation from http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/indeng.html#c2

    The derivation on the site seems to make sense, but when I try to use the bottom equation I get:
    U = (1/2)(0.8^2)(Pi*0.01^2)*(0.399)/(8.85*10^-7) = 45.324 J

    What am I missing here?

  2. jcsd
  3. Apr 17, 2009 #2


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    Staff Emeritus
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    Gold Member

    Check the value you use for permeability.
  4. Apr 17, 2009 #3


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    Oh wow do I feel stupid now..... Thanks for the help!
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