A camera-flash unit contains a charging circuit that connects a fully discharged…
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A camera-flash unit contains a charging circuit that connects a fully discharged capacitor of capacitance 120μF to a high-voltage DC supply through a variable resistor. The manufacturer specifies that, after the moment the circuit is activated, the capacitor must reach at least 63 percent of its final charge in no more than 2.5ms. Assume the supply voltage remains constant during the charging interval.
(a) Describe, in words, the physical significance of a capacitor reaching 63 percent of its final charge during the charging process of an RC circuit.
(b) Using the manufacturer’s timing specification, determine the largest allowable numerical value of the circuit’s time constant. Express your answer in milliseconds.
(c) Explain qualitatively how increasing the resistance in the charging path would affect (i) the time constant and (ii) the initial charging current, and discuss one practical reason a designer might intentionally choose a resistance larger than the minimum required.