Question

A coil of inductance 10 H, resistance 40 Ω is connected as shown in figure. The switch S connected with point 1 for a very long time, is moved to point 2 at t=0. For the value of R obtained, the time taken for 95 % of the stored energy dissipated is ________

(a) 0.10 s

(b) 0.15 s

(c) 0.50 s

(d) 1.0 s

This question was addressed to me in an interview.

The question is from Calculation of I(0+), di/dt(0+), d2I/dt2(0+) in Circuits Involving both Capacitor and Inductor topic in chapter Application of the Laplace Transform in Circuit Analysis of Network Theory

Answer 1

Correct option is (c) 0.50 s

For explanation: For source free circuit,

I (t) = Io\(e^{-\frac{R}{T} t}\)

∴ I (t) = 0.05 = 2 × \(e^{-\frac{60}{10} t}\)

Or, t = 0.61 ≈ 0.5 s.