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 = 65.0 V, wired to a 95.0 V ideal battery and a 0.160 F capacitor, what resistance R is needed for two flashes per second?

Explanation / Answer

The time it takes for the voltage difference across the capacitor to reachVLis given by

VL = e ( 1- e^-t/RC)

R = t/ C ln ( e/( e-VL)

=0.5/0.160* 10^-6 ln( 95/95-65)

=2.71 * 10^6 ohm

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