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."

 

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