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Homework Help: Magnetic flux and emf help?

  1. Mar 8, 2010 #1
    The Earth’s magnetic field at a certain location in the UK has a magnitude of 48 μT and is directed at 66° below the horizontal.

    a)Determine the magnitude of the flux of the Earth’s magnetic field through a
    wedding ring of diameter 2.2 cm when the ring is held in a horizontal plane.

    b)What is the magnitude of the flux change when the ring is flipped through 180° about a horizontal axis?

    c)Use Faraday’s law to calculate the EMF induced in the ring when it is flipped
    through 180° in 0.2 s, assuming the flux changes uniformly during this time.

    can any one help me please i am completly stuck and dont know where to start :(
  2. jcsd
  3. Mar 8, 2010 #2
    You would actually probably be better off posting this in introductory physics, but that's okay, let me give you a hand anyway.

    a) Remember that the equation for flux is [tex]\int \int \mathbf{B}\cdot d\mathbf{a}=\int \int B cos(\phi) da[/tex]

    b) You should have an intuitive guess about this, but go ahead and work it out.

    c) Remember that EMF is [tex]\frac{d \Phi}{dt}[/tex]
  4. Mar 15, 2010 #3
    Hi ghostbuster remember the B dot A cross product rule that helped me and the angle is below thw horizontal so work out the angle you need
  5. Apr 1, 2010 #4
    Not all that astonishingly, I have the same homework as ghostbuster. it's getting pretty late now though, but here are my tenuous attempts at the three questions:


    Magnetic flux = B dot A; this is the dot product of the B vector and the area vector, A

    B = 48 x10^-6 T at 34 degrees from the normal vector to the the area

    The A vector has magnitude = the area, and is directed perpendicular to the surface.
    Mag A = (2.2 x 10^-2 m)^2

    flux = (48 x 10^-6 T) (Pi) [(2.2 x 10^-2 m)^2] (cos 34)
    = 2.75 x 10^-6 T m ^2


    The A vector changes direction

    Δ flux = B A cos (156) – BA cos (34)
    = ( -0.9135 - 0.8290) x 10^-8
    = 1.7425 x 10^-8 T m2


    Using Faraday's law

    EMF = -N

    = -1 -1.7425 x 10-8 T m2
    0.2 s

    = 8.7125 x 10^-8 V

    I had some help from a guy, who's reasoning I can't quite follow now he's gone. Part (a) makes most sense to me, but am rather lost with b and c. any help, or hints would be very gratefully recieved
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