1.

A resistor R and 2 muF capacitor in series are connected through a 200 V direct supply. Across the capacitor is a neon bulb that lights up at 120 V. Find the value of R to make the bulb light up 5 s after the switch has been closed. (Take "log"_10 2.5=0.4)

Answer»

`1.7xx10^5 Omega`
`2.7xx10^6 Omega`
`3.3xx10^7 Omega`
`1.3 XX 10^4 Omega`

Solution :As `V_C=epsilon(1-e^(-t//RC))`
`1-e^(-t//RC)=V_C/epsilon=120/200=3/5`
`e^(t//RC)=2.5` or LOG `e^(t//RC)=log_e` 2.5
`t/(RC)=2.3026 log_102.5 =0.92`
`R=t/"0.92 C"=5/(0.92xx2xx10^(-6))=2.7xx10^6 Omega`


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