Physics---Capacitance3

3a) When the rheostat is set to maximum resistance, the discharge time constant is give by:
CR = 10 x 106 x 580 x 10-12 = 5.8 ms
The period of a discharge half cycle is 1/800 = 1.25 ms
Therefore, at the end of each discharge half cycle, the p.d. across the capacitor is given by:
V = 25e-1.25/5.8 = 20.15 V
(b) Initial current = 25/10 x 106 = 2.5 μA
So at that moment, current = 2.5e-1.25/5.8 = 2.015 μA
(c) To give a 99% discharge, time constant CR has to be adjusted such that e-1.25 x 10^-3/CR <= 0.01
-1.25 x 10-3/CR <= ln 0.01
CR <= 2.714 x 10-4
R <= 468 kΩ
(4) From the given, the maximum current that the galvanometer can stand is 25/400k = 62.5 μA
Now, by formula I = fCV where I is the current, we have:
I = 1000 x 580 x 10-12 x 25 = 14.5 μA
Therefore, a frequency of 1 kHz is still practical.