Question

A condenser of $20\mu F$ is charged to a certain voltage and then discharged through a $4M\Omega$ resistor. The time taken for the condenser to lose $63\%$ of its charge is:

• A. $5s$
• B. $80s$
• C. $8s$
• D. $50s$

Answer 1

The correct option is B $80s$

In an RC circuit, a condenser loses $63\%$ of its charge in 1 time constant.
For a given RC circuit, 1 time constant is given by,
$\tau =RC$
or, $\tau =20\times {10}^{-6}\times 4\times {10}^6=80s$