Physics - Capacitor Question with Solution | TestHub

A highway emergency flasher uses a 120 volt battery, a 1 MΩ resistor, a 1 μF capacitor, and a neon flash lamp in the circuit shown in the figure. The flash lamp has a resistance of more than $10^{10}$ Ω when the voltage across it is less than 110 V. Above 110 V, the neon gas ionizes, the lamp’s resistance drops to 10 Ω, and the capacitor discharges completely. Until the capacitor voltage reaches the breakdown voltage $V_b=110$ V, the large resistance of the flash lamp ensures that it draws a negligible current. The capacitor charges as if the lamp were absent. At $V_b$, however, the lamp resistance quickly becomes negligible, and the capacitor discharges through the lamp as if the battery and the series resistor were absent. The time between the flashes is the time for the capacitor to charge to $V_b$. The flash duration is roughly the time for the capacitor to discharge through the lamp, or about 3 time constants of the capacitor–lamp circuit. The flash energy is the stored energy in the capacitor at 110 V.