R-L circuit exponential growth - need help

Discussion in 'Homework Help' started by _holmesss, Dec 7, 2015.

  1. _holmesss

    Thread Starter New Member

    Dec 7, 2015
    1
    0
    The current in an R-L circuit grows by the following equation:


    i = 5 [ 1 - e ^(R/L t ) ]


    Where I is the current, R the resistance, L the inductance and t the time.



    Determine the resistance R, given that the inductance L is 0.4 H and the current I is 4.5 amperes at t = 0.115 s.
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Start by substituting the known values into your equation, then solve for R.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    Did you drop a minus sign in the exponent, or make some other error? As the time t →∞, -e^((R/L)*t) grows without bound and approaches -∞. This is not physically possible.
     
  4. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Good catch. The exponent should show a minus sign.

    We dont always have to show the minus sign as in the more general:
    1-e^(at)

    but when we see R and L or R and C in there we really have to show the minus sign because we normally dont work with negative R or negative C and t itself wont be negative either.
     
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