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Homework Help: Current, Power and Energy in an Inductance

  1. Sep 18, 2011 #1
    The problem statement, all variables and given/known data
    The graph below shows the voltage across a 820 mH inductor vs. time.
    Find the current (Ix), the power delivered (Px) and the energy stored (Wx) at time tx = 9 μs.

    The attempt at a solution

    I tried adding up the areas of the rectangles up to tx and multiplying it by 820m but this gives the wrong answer for Ix.

    Attached Files:

  2. jcsd
  3. Sep 18, 2011 #2
    You are very close! Keep plodding along.

    The voltage across an inductor is given by:

    [itex]V = L\dfrac{dI}{dt}[/itex]

    Now, after rearranging to isolate [itex]\dfrac{dI}{dt}[/itex], and then integrating, what do you get? Hint: you shouldn't be multiplying the sum of the areas of the rectangles by 820mH.

    Also, are you ensuring that you subtract contributions from areas beneath the [itex]t[/itex] axis?
  4. Sep 18, 2011 #3
    cool....not sure on how to get voltage in order to get power
    Last edited by a moderator: Sep 19, 2011
  5. Sep 18, 2011 #4
    ...and what if the area was 0 altogether?
    Last edited by a moderator: Sep 19, 2011
  6. Sep 18, 2011 #5
    The question asks for the power at tx = 9 μs. I.e. it's asking for an instantaneous power, and so you only need the voltage at that point in time. This you can easily read from the graph to be 700 V.

    If the voltage V = 0, then the power (at that point in time) P = VI = 0.
  7. Sep 18, 2011 #6
    cool got the answer thanks for the help :smile:
  8. Sep 18, 2011 #7
    No worries!
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