Figure 27-65 shows the circuit of a flashing lamp, like thoseattached to barrels

Question

Figure 27-65 shows the circuit of a flashing lamp, like thoseattached to barrels at highway construction sites. The fluorescentlamp L (of negligible capacitance) is connected inparallel across the capacitor C of an RC circuit.There is a current through the lamp only when the potentialdifference across it reaches the breakdown voltageVL; then the capacitor discharges completelythrough the lamp and the lamp flashes briefly. For a lamp withbreakdown voltage VL = 69.1 V, wired to a 92.4V ideal battery and a 0.121 mF capacitor, what resistanceR is needed for 4 flashes per second?

Explanation / Answer

. 1.   Vc = V*exp[-t/(RC)] 2.   t = 0.25 seconds; 4 flashes persecond 3.   Vc = 69.1 volts every 0.25seconds 4.   69.1 volts = (92.4volts)*exp[-(0.25)/(R*121E-6) 5.   Solve "2" for R 6.   R = (-0.25)/[(121E-6)*ln(69.1/92.4)] =7.11 . .

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