Answer to Question 2

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A capacitor of 470 mF is charged up through a 150 k resistor, with a starting voltage of 12 V?

(a) What is the time constant?

(b) What is the time needed to get to 6 V?

(c) What is the time needed to get to 9 V?

(d) What is the voltage across the capacitor after RC seconds?

(e) How long does it take for the capacitor completely to charge?

(a) t = 150 000 W 470 10-6 F = 70.5 s = 71 s (2 s.f.)

(b) t 1/2 = 0.693 RC = 0.693 70.5 s = 49 s

(c) 2 half-lives: t = 2 49 s = 98 s

(d) V = 12 - (0.368 12 V ) = 7.6 V

(e) 5 RC = 5 49 = 245 s