I was trying to solve this problem the other day for my physics class and I keep getting the wrong answer. The problem is as follows: An ideal solenoid is 18.5 cm long, has a circular cross-section 2.20 cm in diameter, and contains 545 equally spaced thin windings. This solenoid is connected in a series circuit with a 15 ohm resistor, a battery of internal resistance ohms and open-circuit terminal voltage of 25 V, and an open switch. (Note u = 4*(pi)*10-7 T *m/A) How long after closing the switch will it take for the stored energy in the solenoid to reach 1/2 of its maximum value? So this is what I did: L = (u0N2A)/l where L = inductance in Henry N = number of turns A = area of cross-section l = length in meters L = (4*(pi)*10-7 )(5452)(3.8*10-4) .185 L = 7.66 *10-4 H Then I found the current I: I = emf/R where emf = electromotive force/voltage R = resistance I = 25 Volts/(15 ohms + 5 ohms) = 1.25 Amps Next I used the energy equation to find the maximum energy: U = 0.5*L*I2 where U = energy L = inductance I = current U = 0.5*(7.66 *10-4)*(1.252) U = 5.99*10-4 J Then I don't know where to go from there to find time. I already tried an equation I found in my textbook.... U = U0e-2*(R/L)*t t = -ln(.5)*L/(2R) t = -ln(.5)*(7.66 *10-4)/(2*(15+5)) t = 1.32*10-5 sec and solved for t that way, but I keep getting 1.32*10-5 sec, when the answer should be t = 4.71 *10-5. I know t = 4.71 *10-5 is the correct answer because it came off of the answer sheet for a review. Can anyone shed some light on what I am doing right/wrong?