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Confusion RLC circuit

  1. May 21, 2013 #1
    1. The problem statement, all variables and given/known data

    Hi i got this question ?

    [Broken]

    have to find Vr and VL

    so can i use this formula :

    VR = IR
    VL = I * XL???


    confuse ?? need help ??

    am i on right track?
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. May 21, 2013 #2

    ehild

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    Impedance is used for AC circuits, when the current and voltage have sinusoidal time dependence. Here the time dependence of the current is different from a sine function. Go back to the definition of the voltage across a capacitor (in terms of charge) and inductor (in terms of derivative of current).

    ehild
     
  4. May 22, 2013 #3

    so answer is 0.002 * (20/1) = 0.04 for Vl

    right?
     
  5. May 22, 2013 #4

    ehild

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    ??? You have to show the time dependence of voltages. The answer is not a number.


    ehild
     
  6. May 22, 2013 #5

    whaaattt???

    are you kidding mee !!

    need help please !!!
     
  7. May 22, 2013 #6

    ehild

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    I try to help and I am not kidding. Read the problem text, please.


    ehild
     
  8. May 22, 2013 #7
    yeah i know its time dependency .. this is the graph for voltage in inductor vs time in ms

    [Broken]
     
    Last edited by a moderator: May 6, 2017
  9. May 22, 2013 #8

    ehild

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    You sent the plot of voltage across the resistor. The shape is correct, but watch out the magnitude. The current is given in A (ampers) The resistance is 2Ω. What is the maximum voltage across the resistor?
    What about the inductor?

    ehild
     
  10. May 22, 2013 #9

    [Broken]
    Got it right?
     
    Last edited by a moderator: May 6, 2017
  11. May 22, 2013 #10

    gneill

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    No, your diagrams are not correct.

    Can you write the general expressions (definitions) relating voltage and current for a resistor and an inductor?
     
  12. May 22, 2013 #11

    ehild

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    No. The maximum voltage for the resistor is about right now, but the shape is not. You remember Ohm's Law: The voltage across the resistor is U=IR, proportional to the current. The shape of the U(t) function follows the shape of I(t).

    As for the inductor, remember Faraday's law about induced emf in a coil and inductance. How was it defined?

    ehild
     
    Last edited by a moderator: May 6, 2017
  13. May 22, 2013 #12
    I know deal with resistor but i have no clue about inductor
     
  14. May 22, 2013 #13

    ehild

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    The voltage across the inductor is U=L(dI/dt) (it is proportional to the time derivative of the current). Determine the derivatives at the different sections and multiply by L.

    ehild
     
  15. May 22, 2013 #14
    Along with U=L(dI/dt) , We should point out that the voltage across the capacitor v = (1/C) integral (i(t)) and of course voltage across the resistor is v = IR. The plot of current is simple enough that taking the integral or the derivative is simple.
     
  16. May 22, 2013 #15
    I agree with you. If the graph is current against time then this is the voltage across the inductor for the first part of the graph AND the last part of the graph. This voltage will be constant over these time intervals.
    For the middle part it will be -0.04V.
    For the flat bits of the graph there is no change of current with time....what will the voltage across the inductor be ??
     
    Last edited: May 22, 2013
  17. May 22, 2013 #16

    ehild

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    The current changes 2 A in 1 ms,( Edit:20 A in 1 ms) so the voltage on the inductor is not 0.04 V.

    ehild
     
    Last edited: May 22, 2013
  18. May 22, 2013 #17
    It looks to me like 20A in 1ms .....
     
  19. May 22, 2013 #18

    ehild

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    I lost a zero, thanks. I edit the previous post. But still, UL is not right.

    ehild
     
  20. May 23, 2013 #19
    so it will be this graph right because di/dt is derivitve !!!

    [Broken]
     
    Last edited by a moderator: May 6, 2017
  21. May 23, 2013 #20

    ehild

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    Excellent! Good solution, nice picture. (Only the unit V is missing from the vertical axis.)

    ehild
     
    Last edited by a moderator: May 6, 2017
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