Question 1

What percentage does a charging capacitor reach at one time constant?

Accepted Answer

63.2% of its final value. A decaying quantity falls to 36.8% at one time constant. Practically full charge is reached at 5τ (99.3%).

Question 2

What is the condition for critical damping in a series RLC circuit?

Accepted Answer

R = 2√(L/C), equivalently ζ = 1. This gives the fastest response without overshoot.

Question 3

Why can't inductor current change instantaneously?

Accepted Answer

Because v = L di/dt — an instantaneous current change would require infinite voltage. Equivalently, the stored energy ½Li² cannot change in zero time.

Question 4

How do you find the time constant of a complex RC circuit?

Accepted Answer

Kill all independent sources and find the Thevenin resistance seen by the capacitor. Then τ = Rth × C. For RL circuits, τ = L / Rth.

TGPSC AEE Electrical Ch 1.3: Transient Response of DC & AC Networks – RL, RC, RLC Interactive Notes

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