Question

What is the expression for voltage in the given circuit?

(a) V=60e^-0.5t

(b) V=30e^-0.5t

(c) V=60e^-2t

(d) V=30e^-2t

The question was posed to me by my college director while I was bunking the class.

My doubt is from Transients in LR Networks in chapter Inductance in a DC Circuit of Basic Electrical Engineering

Answer 1

The correct choice is (c) V=60e^-2t

To elaborate: Applying KVL in above circuit, we get

60-30i-15di/dt = 0

i=2(1-e^-2t)A

di/dt = 4e^-2t

V=Ldi/dt=15*4e-^-2t=60e^-2t.