A resistor 'R' and 2μF capacitor in series is connected through a switch to 20 V direct supply. Across the capacitor is a neon bulb that lights up at 120 V. Calculate the value of R to make the bulb light up 5s after the switch has been closed. (log_{10}2.5 = 0.4)

- 2.7 x 10
^{6}Ω - 1.3 x 10
^{4}Ω - 1.7 x 10
^{5}Ω - 3.3 x 10
^{7}Ω

**Answer**

v = 200(1 - e^{-t/τ})

120 = 200(1 - e^{-t/τ})

e^{-t/τ} = (200 - 120)/200 = 80/200

-t/τ = log(2.5) = 0.4

5 = 0.4 x R x 2 x 10^{-6}

R = 5/(0.6 x 2 x 10^{-6})

R = 2.7 x 10^{6}

**The correct option is A.**