Question

The switch in the circuit shown was on position X for a long time. The switch is then moved to position Y at time t=0. The current I(t) for t>0 is ________

(a) 0.2e^-125tu(t) mA

(b) 20e^-1250tu(t) mA

(c) 0.2e^-1250tu(t) mA

(d) 20e^-1000tu(t) mA

This question was posed to me in a job interview.

This interesting question is from Dot Convention in Magnetically Coupled Circuits in section Resonance & Magnetically Coupled Circuit of Network Theory

Answer 1

The correct choice is (c) 0.2e^-1250tu(t) mA

The best explanation: CEQ = $\frac{0.8 × 0.2}{0.8+0.2}$ = 0.16

VC (t=0^–) = 100 V

At t≥0,

The discharging current I (t) = $\frac{V_O}{R} e^{-\frac{t}{RC}}$

= $\frac{100}{5000} e^{- \frac{t}{5×10^3×0.16×10^{-6}}}$

= 0.2e^-1250tu(t) mA.