A resistor R and `2muF` capacitor in series is connected through a switch to 200 V direct supply. Across the capacitor is a neon bulb that light up at 120 V. Find the value of R to make the bulb light up 5 s after the switch is closed at t = 0.

Explanation

The voltage across R-C circuit is given by

`V = 200 (1-e^(-t//RC))`
`implies 120 = 200(1-e^(-t//RC))`
`impliese^(-t//RC)= (200-120)/200 = 80/200`
`= -t/(RC) = log (2.5) = 0.4`
`implies R= 5/((0.4 xx2 xx10^(-6)))= 625 xx10^6`