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At what rate must the current be to changed to produce 40V emf

  1. Dec 8, 2005 #1
    A 20 H inductor carries a steady current of 4.0 A. At what rate must the current be changed to produce a 40 V emf in the inductor?

    I odn't get this one because it gives you all the information!!
    The formula is:
    EMF = -L di/dt
    they want u to find di/dt, but they already give you it has a steady current of 4.0A, so isn't di/dt just i, which is 4.0A?
    Then the EMF would be 40, and they say L = 20H, so i'm stuck on what intermediate step i'm missing! thanks
    Last edited: Dec 8, 2005
  2. jcsd
  3. Dec 8, 2005 #2


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    Gold Member

    For a steady current, at any value,

    [tex] \frac {dI} {dt} = 0 [/tex]
  4. Dec 8, 2005 #3
    So basically, I can disregard that they told me it had a steady current of 4.0A right? and then just use
    di/dt = EMF/-L to find the di/dt?
  5. Dec 8, 2005 #4
    thanks that was the trick!! i did that the last time, but forgot the minus sign!
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