PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
In an emergency beacon, a 47μF capacitor is discharged through a single resistor after power is removed. Design requirements state that the capacitor’s charge must fall to 37 percent of its initial value within 200ms so that the beacon’s indicator LED extinguishes rapidly. You may select only one resistor from the standard E12 series in the decade range 1.0 kΩ – 10 kΩ.
(a) Calculate the ideal resistance that would satisfy the 200ms requirement exactly.
(b) From the E12 values in the specified decade, choose the single resistor that most closely matches the ideal resistance.
(c) For the resistor selected in part (b), determine the actual fraction of the initial charge remaining on the capacitor after 200ms. Show your reasoning.
(d) State whether the design requirement is met with the chosen resistor and justify your conclusion quantitatively.