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Rate of change of the magnetic field

  1. Jul 26, 2009 #1
    1. The problem statement, all variables and given/known data
    The resistance of the loop is 0.55 Ω. At what rate the magnetic field strength increasing or decreasing? (a=8.4 cm, I=148 mA. Give answer in T/s.)
    The loop is a square, so all the distances around are a=8.4 cm

    2. Relevant equations
    dI=rate of change of the flux
    B=magnetic field

    3. The attempt at a solution

    I found V by using Ohm's Law V=IR, which is the same as emf

    but now I am lost, I feel like I need to find B, obvious reasons. Do I just use the equation


    N=1 because only 1 loop
    Uo= constant
    R=radius, so half of a?
  2. jcsd
  3. Jul 26, 2009 #2
    I am still lost, no matter how I go about solving this I seem to be missing 1 or 2 bits of information..... usually B is one of those.
  4. Jul 26, 2009 #3


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    Okay, let me restate this:
    Does that accurately describe what the problem is asking?

    If so, you do have enough information. Figuring out the EMF was a good first step, but now you need an equation that relates that EMF to magnetic flux. Do you know what it is?

    (your B=Uo*N*I/2R is not the right equation for this problem)
  5. Jul 27, 2009 #4

    dI is delta flux?
  6. Jul 27, 2009 #5


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    Yep, that's it. But note that nobody really uses I for flux. Normally the formula would be written
    [tex]\varepsilon = N \frac{\mathrm{d}\Phi}{\mathrm{d}t}[/tex]
  7. Jul 27, 2009 #6
    right, I just didnt know how to right Phi on the computer
  8. Jul 27, 2009 #7


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    Some people have signatures with the Greek letters that you can copy and paste from. There are other ways too, depending on what operating system you're using.
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