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Homework Help: Faraday's Law help, not sure if I'm using the equations correctly.

  1. Nov 15, 2011 #1
    1. The problem statement, all variables and given/known data

    A coil 3.55 cm radius, containing 480 turns, is placed in a uniform magnetic field that varies with time according to B = (1.20e-2 T/S)t + (3.05e-5 T/s[itex]^{4}[/itex] )t[itex]^{4}[/itex]. The coil is connected to a 620 Ohm resistor, and its plane is perpendicular to the magnetic field. You can ignore the resistance of the coil.

    2. Relevant equations

    [itex]\Phi[/itex][itex]_{B}[/itex]=BA
    ε = - [itex]\frac{\Phi_{B}}{t}[/itex]

    3. The attempt at a solution

    First I divided by t and removed a t from each term in "B"

    r = .0355
    N = 480

    A = [itex]\pi[/itex](.0355)[itex]^{2}[/itex] = 3.959192142e-3
    AN = (3.959192142e-3)(480) = 1.900412228

    So all we're left with is:
    BAN = 1.900412228 [(1.20e-2 T/S) + (3.05e-5 T/s[itex]^{3}[/itex] )t[itex]^{4}[/itex]]

    ε = (2.28e-2 V) + (5.80e-5 V/s[itex]^{3}[/itex])t[itex]^{3}[/itex] should be my final answer but mastering physics says I'm wrong. Please help thanks!

    EDIT:

    I just found a posting where they multiplied the second term in B (3.05e-5 T/s[itex]^{3}[/itex] )t[itex]^{4}[/itex] by 4. When I did that I got the correct answer! But I don't know why I would multiply the term by 4. Any thoughts?
     
    Last edited: Nov 15, 2011
  2. jcsd
  3. Nov 16, 2011 #2
    I have not checked all of your maths but did you remember to x4 when you differentiated t^4 ??
     
  4. Nov 16, 2011 #3

    Redbelly98

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    [tex]ε = -\frac{d \Phi_{B}}{dt}, \ \text{ not } \ - \frac{\Phi_{B}}{t}[/tex]
     
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