1.

In the circuit shown, VC is zero at t=0 s. For t>0, the capacitor current IC(t), where t is in second, is ___________(a) 0.50 e^-25t mA(b) 0.25 e^-25t mA(c) 0.50 e^-12.5t mA(d) 0.25 e^-6.25t mAI had been asked this question in an interview.My enquiry is from Advanced Problems on Network Theorems in section Useful Theorems in Circuit Analysis of Network Theory

Answer»

Correct choice is (a) 0.50 e^-25T mA

Easiest explanation: The CAPACITOR voltage VC (t) = VC (∞) – [VC (∞)-VC (0)]e^-t/RC

R = 20 || 20 = \(\FRAC{20×20}{20+20} = \frac{400}{40}\) = 10 kΩ

VC (∞) = 10 × \(\frac{20}{20+20}\) = 5 V

Given, VC (0) = 0

∴ VC (t) = 5 – (5-0)e^-t/10×4×10^(-6)×10^3

= 5(1 – e^-25t)

IC (t) = C\(\frac{dV_C (t)}{dt} = 4×10^{-6} \frac{d}{dt}5(1-e^{-25t})\)

= 4 × 10^-6 × 5 × 25e^-25t

∴ IL (t) = 0.50e^-2.5t mA.



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