A portable photographic flash unit consists of a 9 ,V battery
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A portable photographic flash unit consists of a 9\,V battery, a resistor R, a capacitor C, a neon indicator lamp that fires when the voltage across it reaches Vfire, and a momentary switch S that is kept open except during charging. The capacitor can be adjusted by turning a dial that changes the plate separation, thereby varying its capacitance from Cmin to Cmax.
Assume ideal components, negligible internal resistance of the battery, and that the neon lamp draws no current until it fires.
(a) Describe qualitatively how increasing the capacitance affects the time required for the capacitor to reach the firing voltage after the switch S is closed.
(b) Derive an expression for the time tfire required for the capacitor to reach the firing voltage Vfire in terms of the battery emf E, the resistance R, the capacitance C, and Vfire. Clearly state any assumptions you make.
(c) Using your expression from part (b), determine by what factor tfire changes if the capacitance is doubled while all other elements remain unchanged. Explain the physical significance of your result in the context of the flash-charging process.