Question

A 10 μF capacitor is charged from a 5 volt source through a resistance of 10 kΩ. The charging current offer 35 m sec. If the initial voltage on C is – 3 V is ___________

(a) 0.56 mA

(b) 5.6 mA

(c) 6 mA

(d) 5 μA

This question was posed to me in an interview for internship.

Enquiry is from Relation between Hybrid Parameters with Short Circuit Admittance and Open Circuit Impedance Parameters topic in section Two-Port Networks of Network Theory

Answer 1

Right answer is (a) 0.56 mA

Easy explanation: Initial current immediately after charging is given by,

\(\frac{V}{R} = \frac{5+3}{10000}\)

= 0.8 mA

Now, i = i0e^-t/RC

= 0.8 mA x \(e^{\frac{t}{10k X 10 X 10^{-6}}}\)

= 0.8 X 10^-3 X \(e^{\frac{35 X 10^{-3}}{10^{-1}}}\)

= 0.56 mA.