A $4\  \mu F$ capacitor, a resistance of $2.5 \,MW$$\Omega$ is in series with $12\, V$ battery. Find the time after which the potential difference across the capacitor is $3$ times the potential difference across the resistor.......$s$  [Given $ln(2)= 0.693$]
IIT 2005, Diffcult
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(a) ${V_R} = \frac{{{V_0}}}{4}$

$=$ ${V_0}{e^{ - \frac{t}{{RC}}}}$

$==>$ $\frac{1}{4}$ $=$ ${e^{ - \frac{t}{{10}}}}$
$==>$ $4 = {e^{\frac{t}{{10}}}}$

$==>$ ${\log _e}4 = \frac{t}{{10}}$

$==>$ $t = 10\log 4 = 13.86 \,s$

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