How many time constants will elapse before the charge on a capacitor falls to 0.1% of its maximum value in a discharging RC circuit ?
q=Qe−trc
q=0.1% Q=1×10−3Q
So,1×10−3Q=Q×e−trc
⇒e−trc=10−3
⇒trC=−(−6.9)=6.9