maths
posted by Rachael .
Given the the instantaneous voltage across a capacitor is VC=40(1e t/CR) Volts When being charged by a 40 volt DC supply, find the time, t for VC to reach 18 volts after the charging process has started if C = 100ìF and R = 20kÙ

R = 20K Ohms.
C = 100uF.
R*C = 20*10^3 * 100*10^6 = 2000*10^3=
2 s.
E/e^(t/RC) = EVc.
40/e^(t/2) = 4018 = 22
e^(t/2) = 40/22 = 1.8182.
t/2*lne = ln1.8182
(t/2)*1 = 0.59785
t = 1.2 s.