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Homework Help: Electromagnetic damping logarithmic decrement task

  1. Mar 14, 2014 #1
    1. The problem statement, all variables and given/known data

    The contour consists of a condenser with capacitance C = 2.22*10^-9 F and a coil of copper wire. The diameter of the wire 5*10^-4 m, the length of the coil 20*10^-2 m. Determine the damping logarithmic decrement Λ of the fluctuations?

    Given answer:0.018

    2. Relevant equations

    damping logarithmic decrement: Λ=2*Pi/sqrt((4L/CR^2)-1)
    self-inductance of a coil: L=4*Pi*10^-7*(N^2/lc)*Sc (N number of turns, lc length of coil, Sc area of coil section)
    resistance of the coil: R=(ρCu*lw)/(Pi*(d/2)^2) (ρCu=1.7*10^-8, lw-length of the wire,d-diameter of the wire)

    3. The attempt at a solution

    The answer I got by combining the three equations: ~2.79*10^-12 . Probably wrong because differs a lot from the given so i'm asking for help.
  2. jcsd
  3. Mar 14, 2014 #2
    Is the "length of the coil" the actual length of the wire used to make the inductor (i.e. if you unwrapped the whole thing), or is it the length measured across the outside of the coil? If it's the former, how many coils are there? If it's the latter, what is the cross-sectional area of the coil? Information is missing.
  4. Mar 15, 2014 #3
    Length of the coil is the length of the cylinder shape the coil makes. There is no information missing. You are supposed to use equations to eliminate the need for the missing information. The task is from a widely used book.
  5. Mar 15, 2014 #4
    Maybe I'm misunderstanding something. You know the length of the coil. You know the diameter of the wire. So you know the number of turns. But you don't know the total length of the wire, so you can't figure out the circumference of a single loop or its cross-sectional area. It could be anything and still satisfy the length of the coil and the number of turns. I don't believe you supplied this information.
  6. Mar 15, 2014 #5

    rude man

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    Not sure what is meant by "damping logarithmic decrement". The exponential term is R/2L, i.e. the envelope decays as exp(-Rt/2L).
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