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Magneto motive force question

  1. Aug 26, 2009 #1
    1. The problem statement, all variables and given/known data
    A cast steel magnet has an air gap of length 1mm and an iron path of 30cm. Find the magneto motive force (MMF) necessary to produce a flux density of 1Wb/m^2. The relative permeability of cast steel is 900. Neglect any leakage or fringing effects

    2. Relevant equations
    I think these are relevant

    Flux density B = mu H
    Magnetic field strength H = (NI)/L
    Permeance Mu = Mu0 MuR where Mu0 = 4pi *10^-7 and MuR is the relative permeability
    MMF = NI

    3. The attempt at a solution

    i first equated flux density and magnetic field strength together to make

    B = 1 = Mu * (NI)/L

    i worked Mu out to be approx 1.131*10^-3

    Then substituted L as 0.3metres.

    this then worked out to be

    NI = 0.3 * (1.131*10^-3)

    NI is then 3.39*10^-4. Which is the MMF.

    Is this correct? Do i have to use the air gap of 1mm for anything?

    Your help is appreciated.
  2. jcsd
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