Logarithmic decrement

  1. 1. The problem statement, all variables and given/known data

    Basically, I have LQ''(t) + RQ'(t) + (1/C)Q(t)=0, and I'm supposed to

    "Show that the ration of the charge Q between two successive maxima is given by exp(RTd/2L), where Td is the time between two successive maxima. The natural logarithm of this ration is called the logarithmic decrement.

    2. Relevant equations


    3. The attempt at a solution

    So I got a solution Q(t)=e(-Rt)/(2L) [ C1cos( (√(R2-4L/C) )/(2L)t) + C2sin( (√(R2-4L/C) )/(2L)t).

    But I can't figure out how to find Td. I mean, I could always find t when dQ/dt=0; but then I'd have to plug two values of t back into Q(t) and find the difference, and ............ So what's the right way to do this?
  2. jcsd
  3. lanedance

    lanedance 3,307
    Homework Helper

    First notice the sin & cos terms have the same argument & the choice of c1 & c2 will just choose an overall phase. So for this argument set c2 = 0.

    Then the maxima will just be where cos is maximum and successive maxima will occur where the argument of cos has changed by 2pi
  4. LCKurtz

    LCKurtz 8,346
    Homework Helper
    Gold Member

    Not exactly. The max's don't agree with the max's of the cosine, but the right idea. To the OP, just look at e-btcos(at+c).
  5. lanedance

    lanedance 3,307
    Homework Helper

    good pickup thanks - They will be pretty close when the natural frequency is much larger that the decay constant, but you do need to take the exponential into account
    Last edited: May 3, 2010
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