The figure below shows the circuit of a flashing lamp, like those attached to ba

Question

The figure below shows the circuit of a flashing lamp, like those attached to barrels at highway construction sites. The fluorescent lamp L (of negligible capacitance) is connected in parallel across the capacitor C of an RC circuit. There is a current through the lamp only when the potential difference across it reaches the breakdown voltage VL; then the capacitor discharges completely through the lamp and the lamp flashes briefly. For a lamp with breakdown voltage VL = 72.0 V wired to a 90.0 V ideal battery, and a 0.150

The figure below shows the circuit of a flashing lamp, like those attached to barrels at highway construction sites. The fluorescent lamp L (of negligible capacitance) is connected in parallel across the capacitor C of an RC circuit. There is a current through the lamp only when the potential difference across it reaches the breakdown voltage VL; then the capacitor discharges completely through the lamp and the lamp flashes briefly. For a lamp with breakdown voltage VL = 72.0 V wired to a 90.0 V ideal battery, and a 0.150 µF capacitor, what resistance R is needed for two flashes per second. ?

Explanation / Answer

voltage variation for dischargng in RC circuit is given by V= V0(1-e^-t/RC)

where t = time taken, RC = time constant

so in our problem,

1-e^-t/RC = 72/90 =0.8

e^-t/RC = 1-0.8=0.2

-t/RC = ln(0.2)

-t/RC = -1.609

time t= 0.5 secs

RC = 0.5/1.609

RC =0.301

R =0.301/0.150uF

R =2.07*10^6 ohms

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