Homework Help: maths

Posted by Rachael on Friday, March 22, 2013 at 9:13am.

Given the the instantaneous voltage across a capacitor is VC=40(1-e -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

maths - Henry, Saturday, March 23, 2013 at 5:28pm
R = 20K Ohms.
C = 100uF.

R*C = 20*10^3 * 100*10^-6 = 2000*10^-3=
2 s.

E/e^(t/RC) = E-Vc.
40/e^(t/2) = 40-18 = 22
e^(t/2) = 40/22 = 1.8182.
t/2*lne = ln1.8182
(t/2)*1 = 0.59785
t = 1.2 s.

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