9) a) What is in volts, the voltage across the capacitor the instant switch S 1

Question

9)

a) What is in volts, the voltage across the capacitor the instant switch S1 was closed?

b) What is in volts, the voltage across the capacitor when it is fully charged?

c) What is the charge time constant for the circuit? Answer in units of seconds.

d) What is the discharge time constant for the circuit? Answer in units of seconds.

10)

a) What is the charge time constant for the circuit? Answer in units of seconds.

b) What is the discharge time constant for the circuit? Answer in units of seconds.

After fully discharging the circuit, we close S2 and then close S1. The capacitor will charge.

c) What is in micro-amps the current through R2 once the capacitor is fully charged?

d) What is in micro-amps the current through R1 once the capacitor is fully charged?

e) What is the voltage across the capacitor then?

11)

Explanation / Answer

a. I = 12/440 = 27.3 mA

b. calculate the current in R2 with C replaced by a short
R2/3 = 110*240/350 = 75.4
in series with R1 = 405.4
total current = 12/405.4 = 29.6mA
voltage across R2/3 = 2.23v
current through R2 = 2.23/240 = 9.3 mA

c. calculate the current with C replaced by an open.
same (a)

d. 1. calculate initial voltage across C, which is same as voltage across R3
V3 = 110 x 27.3 mA = 3 volts
2. calculate what voltage across C would be for I2 = 1 ma
voltage across R2 at that point is 1 ma x 240 = 0.240 volts.
voltage across R3 at that point is 0.11 volts
that means voltage across cap is 0.35 volts.
So you want the time it takes for the voltage across the cap to decay from 3 volts to 0.35 volts.

3. time constant is ? =RC = 350 x 4.7 mF = 1645 mS or 1.65s

4. voltage on a cap, discharging
v = v?e^(

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