The capacitor in an RC circuit with a time constant of 15 ms is charged to 10 V.

Question

The capacitor in an RC circuit with a time constant of 15 ms is charged to 10 V. The capacitor begins to discharge at t = 0 s. At what time will the charge on the capacitor be reduced to half its initial value? At what time will the energy stored in the capacitor be reduced to half its initial value? The capacitor in Figure P23.72 is initially uncharged and the switch, in position c, is not connected to either side of the circuit. The switch is now flipped to position a for 10 ms, then to position b for 10 ms, and then brought back a position c. What is the final potential difference across the capacitor? What value resistor will discharge a 1.0 mu F capacitor to 10% of its initial charge in 2.0 ms? The charging circuit for the flash system of a camera uses a 100 mu F capacitor that is charged from a 250 V power supply. What is the most resistance that can be in series with the capacitor if the capacitor is to charge to at least 87% of its final voltage in no more than 8.0 s? A capacitor is discharged through a 100 Ohm resistor. The discharge current decreasas to 25% of its initial value in 2.5 ms. What is the value of the capacitor? to 30 V is discharged

Explanation / Answer

For connection at a

Capacitor charges

q = CV(1- e-t/RC)

q = 40*10-6*12(1- e-10ms/150*40uF) = 3.89*10-4 C

When switch moves to b

Capacitor discharges

q = qoe-t/RC = 3.89*10-4e-10ms/150*40uF = 7.35*10-5 C

Now q =CV

V = q/C = 1.84 V

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