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Thread: CRE/LRE Circuit Linear Equation Word Problem

  1. #1
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    CRE/LRE Circuit Linear Equation Word Problem

    An electromotive force of 100 volts is in series with a 2 henry inductor and a 50 ohm resister.
    (a) Determine the current in the system at any time t after the switch is closed.
    (b) What is the maximum current that can be obtained from the simple circuit described?
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    Re: CRE/LRE Circuit Linear Equation Word Problem

    use Kirchoff's Law for a series RL circuit, $E - V_R - V_L = 0$

    $E - IR - L \cdot \dfrac{dI}{dt} = 0$

    $E - IR = L \cdot \dfrac{dI}{dt}$

    $\dfrac{dt}{L} = \dfrac{dI}{E-IR}$

    $- \dfrac{R}{L} \, dt = -\dfrac{R}{E-IR} \, dI$

    $-\dfrac{Rt}{L} + C = \ln|E-IR|$

    at $t=0$, $I = 0$ $\implies C = \ln|E|$

    $-\dfrac{Rt}{L} = \ln|E-IR| - \ln|E| = \ln \bigg|\dfrac{E-IR}{E} \bigg|$

    $\dfrac{E-IR}{E} = e^{-Rt/L}$

    $I = \dfrac{E(1-e^{-Rt/L})}{R}$

    $\displaystyle \lim_{t \to \infty} \dfrac{E(1-e^{-Rt/L})}{R} = \dfrac{E}{R}$
    Thanks from topsquark
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