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Homework Help: Induced EMF and wire coil

  1. Feb 11, 2004 #1
    Ok, so here's the problem:

    Induced EMF
    A 5.80 cm diameter wire coil is initially oriented so that its plane is perpendicular to a magnetic field of 0.770 T pointing up. During the course of 0.140 s, the field is changed to one of 0.240 T pointing down. What is the average induced emf in the coil?

    and here's how i tackled it:

    E=-N(change in magnetic flux/change in time)

    change in magnetic flux = B2A - B1A = A(B2-B1) = 0.058^2pi(.770 - 0.140)
    change in time = 0.140 s

    N = 1

    So I get..

    E= 0.0100 V

    Apparently that's wrong. Any suggestions ?
  2. jcsd
  3. Feb 11, 2004 #2


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    Homework Helper

    The flux is a vector quantity. Check the B-fields in your equation again.

    Draw a picture.
  4. Feb 11, 2004 #3
    i thought cuz they were both perpendicular, jus pointing different directions, it wouldnt matter.. guess i was wrong.. i'm really not sure exactly how to proceed now that i know the flux is a vector quantity since angles aren't given..
  5. Feb 11, 2004 #4


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    You can pretty much just assume that the initial and final vectors are 180 degrees apart. Other than that, don't worry about angles. I sorry for saying "vector," as it probably made you start thinking about oblique directions, sines and cosines and whatnot. That isn't what I wanted to point out. I was trying to draw your attention to the negative sign that you're missing. The flux is a magnitude and sign (which is a 1-D vector). So, pick a sign for up, assign the opposite sign to down, and put the values into the formula accordingly. Your procedure is basically correct; it's the details that are killing you.

    Oh what the hell, here's what I get:

    0.0191 V ccw

    I just noticed something else in your first post. You squared the diameter, but you should square the radius of the loop.
    Last edited: Feb 11, 2004
  6. Feb 11, 2004 #5
    I was gonna rush in here to say Nevermind I figured it out but lol I guess I'm too late, and yeh I finally ended up with 0.0191 as well after figuring out I had to add the fields..Thanks anyways mann
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