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Math Help - Another First Order-RC circuits

  1. #1
    Junior Member
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    Another First Order-RC circuits

    RC circuits-
    A 10 ohm resistor and 10 microfarad capacitor are attached in series to a 10 volt DC supply. If the current i(0)=1 ampere, what is the current after 0.1 milliseconds (0.0001 seconds)? Round in the thousandths place.

    So I worked through this one so carefully! and I was sure I had the correct answer, but didn't! so this is what I did:

    10microfarad=0.00001 farads

    Rdq/dt+1/Cq=V

    10dq/dt+1/10=10

    Dividing by R and substituing 0.00001 farads.....

    dq/dt+1000q=1

    using IF e^1000....I have e^1000t

    e^1000tdq=integral[e^-1000t x dt] = 1/1000 e^-1000t + C
    q=1/1000+Ce^-1000t
    1=1/1000+Ce^-1000(0)
    C=0.999

    q=1/1000+0.999e^-1000(0.0001)
    q=0.905

    Where did I go wrong?
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  2. #2
    MHF Contributor Calculus26's Avatar
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    10dq/dt+1/10=10
    should be 10dq/dt + 10^5 q = 10

    Once you fix this

    q=1/1000+Ce^-1000t
    even if this were correct

    dq/dt = i(t) use this to compute C from i(0) = 1 q(0) is not 1
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