1. Consider the circuit shown in Figure 6. a. If the resistance is 120 k and the

ID: 1549181 • Letter: 1

Question

1. Consider the circuit shown in Figure 6.

a. If the resistance is 120 k and the capacitance is 25 F, what is the time constant?

b. If the power supply voltage is 8 Volts, how long does it take the capacitor to charge to VC = 5 Volts? (See below for the algebra.)

c. If a 50 F capacitor is placed in parallel to the first, what is the new time constant?

d. If a 50 F capacitor is placed in series to the first, what is the new time constant?

Switch Figure 6. RC circuit diagram for measuring a single capacitor.

(1)

formula for time constant

T = RC = 120 * 10^3 ( 25 * 10^-6) = 3 s

(2)

V= Vo e^-t/T

5= 8 e^-t/3

-t/3= ln ( 5/8)

t= 1.41 s

(3)

If a 50 F capacitor is placed in parallel to the first, what is the new time constant

C= C1 + C2 = 25 uF + 50 uF = 75 uF

T = RC = 120 * 10^3 ( 75 * 10^-6) = 9s

(4)

1/C = 1/25 + 1/50

C = 16.66uF

T = RC = 120 * 10^3 ( 16.66 * 10^-6) = 2s

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