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Series-Parallel Capacitive Circuit

  1. Dec 9, 2012 #1
    Hi there, can someone help me check if the answer for part (iv) is correct? I got 7.26uC.


    safari.jpg


    Thanks.
     
    Last edited: Dec 9, 2012
  2. jcsd
  3. Dec 9, 2012 #2

    gneill

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    Staff: Mentor

    Yes, your result looks correct. Also, the "ANSWER" for part (iii) looks slightly off, too. Should be more like 4.24V rather than 4.26V.

    So the Answer key guy gets 6 marks out of 11 from me :smile:
     
  4. Dec 9, 2012 #3
    lawl.. phew.. i thought i was wrong, i have been doing this question for the whole day..
     
  5. Dec 9, 2012 #4

    gneill

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    Staff: Mentor

    Glad I could help!
     
  6. Dec 9, 2012 #5
    gneill thanks for your help so far, can I ask you some questions on the rate of change of influx? I have been reading through my lecture notes but I can't seem to understand how should I go about it.

    I am confused. My book is showing alot formula, whereas the formula sheet is showing a different one. Can you explain to me which formula is for finding which value?


    Thank you.

    http://i303.photobucket.com/albums/nn129/icefrogftw/Picture10.jpg
    http://i303.photobucket.com/albums/nn129/icefrogftw/Picture11.jpg
    http://i303.photobucket.com/albums/nn129/icefrogftw/Picture12.jpg
    http://i303.photobucket.com/albums/nn129/icefrogftw/Picture13-1.jpg
    http://i303.photobucket.com/albums/nn129/icefrogftw/for.jpg

    I know some part of the book is showing how the formula is being derived. So should i ignore formula (4),(5),(6) and just go with (1),(2) from the formula sheet?
     
    Last edited: Dec 9, 2012
  7. Dec 9, 2012 #6

    gneill

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    Staff: Mentor

    The two formulas on your Formula Sheet summarize the important results of the derivations in your book; they are the two formulas that you will be likely to use most often.

    The first one, ##V_L = L\frac{di}{dt}## relates the voltage induced across an inductance of value L to a change of current through it. This will be handy when writing the differential equations for circuits where the variables of interest are voltage and current (think Kichhoff's laws).

    The second formula, ##V_L = N\frac{d\Phi}{dt}##, will be handy when you're looking at voltages induced in an inductor by external magnetic fields. This comes up when looking at solenoids, transformers, loops of wire in changing magnetic fields, and so on.

    The book's derivations show how these items are interrelated, so that you aren't left thinking that they are mutually exclusive concepts. You may come across situations where understanding this interrelationship will be important, but mostly you'll use these two formulas in their given form.
     
  8. Dec 9, 2012 #7
    For this question, I would use VL=N∅/dt.

    VL=200
    N=500

    How do I find the phase angle? Do i need to differentiate the phase angle? ∅/dt

    ap.jpg
     
    Last edited: Dec 9, 2012
  9. Dec 9, 2012 #8

    gneill

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    Make that a capitol ##\Phi##, the standard symbol representing flux, and it should be ##\frac{d\Phi}{dt}## in that formula, the rate of change of flux with respect to time.
    There's no phase angle. ##\Phi## represents the flux. ##\frac{d\Phi}{dt}## is the rate of change of flux, and happens to be what the question is looking for (the value that ##\frac{d\Phi}{dt}## takes on with the given number of turns and the resulting induced voltage).
     
  10. Dec 9, 2012 #9
    How do i make the dt go away?
     
  11. Dec 9, 2012 #10

    gneill

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    You don't have to. ##\frac{d\Phi}{dt}## as a whole is what you're looking for; it's the rate of change of the flux with respect to time. Replace it with ##\dot{\Phi}## (##\Phi## with a dot over it) if you wish to give it a variable name...
     
  12. Dec 9, 2012 #11
    ah.. i see. thanks for the explanation :)
     
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