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RC and RL(frequency and potential difference)

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data
    1.(RC in AC)What will happen to the potential difference on the capacitor as the frequency increases? In terms of the reactance of the capacitor, why might this happen?

    2.(RL in AC) What will happen to the potential difference on the inductor as the frequency increases? In terms of the reactance of the inductor, why might this happen?

    3. How might time constant and period determine the inductors potential difference?

    4. How might time constant and period determine the capacitors potential difference?

    2. Relevant equations
    XL= 2pif
    V=IR
    I= V/Z


    3. The attempt at a solution
    I do not know how to find the relationships between f and potential difference on both capacitor and inductor..What equations should I use?? Please help me answering these 4 questions! Thanks
     
  2. jcsd
  3. Dec 16, 2009 #2

    cepheid

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    Hi SAT2400

    For part 1, you know the impedance of a capacitor, right? You also know the impedance of a resistor (which is just its resistance). You know that they are in series (I assume?). If so, then you know that at any instant, the voltage drop across each has to add up to the voltage of the source (like in any series circuit). Does that help?
     
  4. Dec 17, 2009 #3
    hmmm,,not quite...

    Can you explain more in detail!?T_T

    and how do I know the impedence? is it the resistance??

    I have got values for Xc(ohm), Z(ohm), I(Amp), Vc(V), Vr(V) and Vs.
    THank you
     
  5. Dec 17, 2009 #4

    cepheid

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    Okay, well the impedance, Z, of a component is just a complex number that describes the relationship between the complex (or "phasor") voltage across that component and the complex current through it. In general, this complex impedance has a real part, R, and an imaginary part, X, such that:

    Z = R + jX

    The real part, R, is just the resistance of the comonent, and the imaginary part, X, is called the "reactance." In the case of a capacitor or an inductor, there is NO resistance (no real part). The impedance is purely imaginary (or purely "reactive", to use the EE jargon):

    ZC = jXC.

    The nice thing about impedances, is that they allow us to use the same circuit analysis techniques for AC circuits as we do for DC circuits. If two components are in series, the total impedance is just the sum of the individual impedances: Z = Z1 + Z2 etc. The fact that the voltages and currents are oscillatory functions of time (which could potentially make things complicated) is neatly handled by the complex numbers and the phase information they contain. If you are learning about this stuff, all of what I just said should be in your notes or your book somewhere, and you should have an expression for XC as a *function* of omega so that you know how the impedance of a capacitor changes with frequency.
     
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