To left-leg electrode Recorder To right-leg electrode (ground) AGURE 19-68 To ar

Question

To left-leg electrode Recorder To right-leg electrode (ground) AGURE 19-68 To arm electrode m 53 15 5 m In Fig. 19-69 (same as Fig, 19-20a), the total resistance 5k, and the battery's emf is 24.0 V. If the time con- is measured to be 18.0 ?s, calculate (a) the total capacitance of the circuit and (b)the time it takes for the voltage across the resistor to reach 16.0 V after the switch C? is closed FIGURE 19-69 Problem 54 61 55. (11) Two 38-?F capacitors, two 22-k? resistors, and a source are connected in series. Starting from the uncharged state, how long does it take for the current to drop from its initial value to 1.50 mA? RI)TheRCcircuit of Fig. 19-70 (same as Fig.19-21a) has K 8.7 k? and C-30pF. capacitoris at voltage Vort 0when the switch is _ww? 63 osed. How long does it take

Explanation / Answer

54

a)

T = Time constant = 18 x 10-6 sec

R = Total resistance = 15000 ohm

C = capacitance = ?

Time constant is given as

T = RC

18 x 10-6 = (15000) C

C = 1.2 x 10-9 F

b)

Vr(t) = Voltage across the resistor at time "t" = 16 volts

E = battery emf = 24 Volts

Vc(t) = battery Voltage - Voltage across capacitor at time "t" = E - Vr(t) =  24 - 16 = 8 Volts

using the equation

Vc(t) = E (1 - e-t/T)

8 = 24 (1 - e-t/(18 x 10-6))

t = 7.3 x 10-6 sec

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