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Logarithmic decrement 
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#1
May210, 11:11 PM

P: 909

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(RT_{d}/2L), where T_{d} is the time between two successive maxima. The natural logarithm of this ration is called the logarithmic decrement. 2. Relevant equations Dunno 3. The attempt at a solution So I got a solution Q(t)=e^{(Rt)/(2L)} [ C1cos( (√(R^{2}4L/C) )/(2L)t) + C2sin( (√(R^{2}4L/C) )/(2L)t). But I can't figure out how to find T_{d}. 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
May210, 11:49 PM

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P: 3,307

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 


#3
May310, 12:01 AM

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Thanks
PF Gold
P: 7,583




#4
May310, 12:15 AM

HW Helper
P: 3,307

Logarithmic decrement
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



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