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Math Help - Find t (in seconds)

  1. #1
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    Find t (in seconds)

    The equation governing the amount of current I (in amperes) after time t (in seconds) in a simple RL circuit consisting of a resistance R (in ohms), an inductance L (in henrys) and an electromotive force E (in volts) is given by

    I = (E/R)[1 - e^(-(R/L))t]

    If E = 12 volts, R = 10 ohms and L = 5 henrys, how long does it take to obtain a current of 0.5 ampere?
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  2. #2
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    Quote Originally Posted by Air View Post
    Rearrange to make time the subject and then substitute the values give.

    Rearranging the equation for time, I obtained:
    t = \frac{E-IR}{e^{-\frac{R}{L}}E}
    Unfortunately Air, despite appearances to the contrary, the expression is meant to be I = \frac{E}{R}\left[1 - e^{-(R/L)t}\right].

    Since my algebra is terrible, I'd substitute the given values first and then re-arrange. I find it much easier to manipulate concrete numbers.

    Do this and you should be able to get to

    \frac{5}{12} = 1 - e^{-2t}.

    From here the path leading to t shouldn't be too slippery.
    Last edited by mr fantastic; August 4th 2008 at 03:03 AM.
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    Thanks

    I thank you for taking me half way.
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