1. The problem statement, all variables and given/known data Hello. I'm afraid it's quite a simple and unexciting problem I have. Basically, I can't remember how to rearrange the equation for an exponential curve: The capacitor has a capacitance of 0.63 mF and the resistance in the discharge circuit is 2.4 kΩ. (i) Calculate the time constant of the discharge circuit. This I can do: -t = RC. (ii) Show that it takes about 3 s to discharge the capacitor from 120 V to 15 V. 2. Relevant equations V = Vo e^-t/RC 3. The attempt at a solution I remember it's the above equation I need to use. And I remember it's to do with the "ln" button on my calculator. But I can't remember how, when or where I use it. I know it's meant to "get rid of" the e and make my -t/RC "un-square" themselves so I can rearrange the equation. Apologies for my terrible lack of physics vocabulary :P. I've tried a combination of things, enclosing the whole equation in ln( ) but I can't get the right answer (which is 3.14 seconds) as I am not rearranging it properly. I would appreciate any help anyone has.