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Magnetic Flux through a solenoid

  1. Apr 1, 2009 #1
    1. The problem statement, all variables and given/known data

    The magnetic field inside a cylindrical solenoid of area 4 cm^2 is 0.15 T along the axis of the solenoid. What is the magnetic flux through a disk of radius 3 cm placed perpendicular to the solenoid axis?

    2. Relevant equations
    [tex]\phi = \zeta B * dA [/tex]


    3. The attempt at a solution

    Well the area is 4 cm^2 and B is 0.15, so 6E-5 Wb is the answer, but I'm a little confused here. I thought flux for a closed surface was always 0 (gauss' law for magnetism)? What's going on here?
     
  2. jcsd
  3. Apr 1, 2009 #2

    alphysicist

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    Homework Helper

    A closed surface means that it encloses a volume. A sphere, cube, etc. would be a closed surface, but a disk would not. If you constructed a cylinder with the disk in the problem at one end, and calculated the flux through the cylinder then the flux would be zero.
     
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