inductance

Discussion in 'Homework Help' started by liam, Apr 22, 2008.

  1. liam

    Thread Starter New Member

    Apr 22, 2008
    9
    0
    hi guys could any body send me in the right direction with this question because im completely baffled " A 2H coil of negligible resistance while connected to a power supply carries 5A. If it is disconnected from its power supply and connected to a 120 Ω discharge resistor calculate the time taken for the current to fall to 0.25A."
     
  2. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    Current_Final = Current_Initial*e^x, where x = -t/(time_constant)
    time_constant = L/R.

    So, L/R = 2H/120Ohms = 0.0167sec.
    0.25 = 5 * e^x
    0.05 = e^x
    ln(0.05) = x = -t/(0.0167)
    t = -0.0167*ln(0.05) = 50e-3 = 50ms.

    Look here: http://www.allaboutcircuits.com/vol_1/chpt_16/4.html
     
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