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

Question

The figure 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 = 78.4 V, wired to a 96.7 V ideal battery and a 0.115 mF capacitor, what resistance R is needed for 3 flashes per second?

fig27_53.gif

The figure 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 = 78.4 V, wired to a 96.7 V ideal battery and a 0.115 mF capacitor, what resistance R is needed for 3 flashes per second?

Explanation / Answer

t=1/3 =0.33 s

VL=E(1-e^(-t/T)]

78.4=96.7(1-e^(-0.33/T))

e^(-0.33/T)=0.1892

-0.33/T =ln(0.1892)

-0.33/T =-1.665

T=0.2 sec

since T=RC

R =T/C =0.2/(0.115*10^-3)

R =1.739 Kohms or 1739.13 ohms

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