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Homework Help: RC Circuit with a voltage source

  1. Nov 30, 2016 #1
    1. The problem statement, all variables and given/known data
    A voltage source E_0 cos ωt is connected in series with a resistor R and a capacitor C. Write down the differential equation expressing Kirchhoff’s law. Then guess an exponential form for the current, and take the real part of your solution to find the actual current. Determine how the amplitude and phase of the current behave for very large and very small ω, and explain the results physically.

    2. Relevant equations

    3. The attempt at a solution

    I got the complex current to be I(t) = [(iωE0/R)/(1/RC + iω)] * e^(iωt)

    I just don't know how to convert this back to real current. I know e^(iωt) = cos(ωt)+isin(ωt), but how do I deal with the rest of the term?
  2. jcsd
  3. Nov 30, 2016 #2


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    Gold Member

    Plug that in and multiply out.
  4. Nov 30, 2016 #3


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    Homework Helper

    You have the current as the product I=G E0eiωt. G=|G|e. Find the magnitude |G| and the phase φ, then I=|G|E0ei(ωt+φ). Take the real part.
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