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Homework Help: Ohmic Resistor modelling ?(Check my interpretation of question)

  1. May 1, 2012 #1
    http://dl.dropbox.com/u/33103477/Resistor.png [Broken]

    For the first bit first I solved the equation

    [tex] \frac{dI}{dt}=\frac{U-RI}{L} [/tex]

    [tex] I(t)=\frac{U-C_2e^{\frac{-t}{L}}}{R} [/tex]

    Now I put in T(0)=0 to work out the constant and I got

    [tex] I(t)=\frac{U-Ue^{\frac{-t}{L}}}{R} [/tex]

    Now here's the dodgy bit, I did not do physics in my final years at school so I no clue what an "Ohmic Resistor" is but here is my interpretation of the question

    [tex] I(t)=\frac{75U}{100R} [/tex]

    Working that out I got

    [tex] t = -L log(0.25) [/tex]

    Now for the final part:

    My interpretation was that the maximum saturation level was U/R=200/50=4

    So I worked out:

    [tex] I(t)= 3.5 [/tex]

    Which gave me t=90.31

    Does what I have done make sense cause I don't know much "higher level" physics so am just working from logic.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. May 1, 2012 #2


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    Homework Helper

    an ohmic resistor satisfies U=IR, so what you've done seems reasonable, though I haven't checked your numbers

    one thing though, its always good practice to differentiate you solution, in this case I(t), and make sure it satisfies the original DE. I would recommend that check as usually the time constant in the exponential would depend on both R and L

    Also you really need to know if they are in series or parallel, though it's implied by the DE..
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