A.C Fundamentals, Circuits And Circuit Theory MCQ Questions & Answers | Electrical Engineering

Time constant (τ) of an inductive circuit is defined as the time required for the current to reach approximately 63.2% of its final steady value after a sudden change in voltage.

The formula for the time constant in an RL (Resistor–Inductor) circuit is:

τ = L / R

Where:
L = Inductance in Henry (H)
R = Resistance in Ohms (Ω)

From the formula, we can see that:
1. If inductance (L) increases, the time constant increases.
2. If resistance (R) decreases, the time constant increases.

Therefore, the time constant increases with increase of inductance and decrease of resistance.

Correct Answer: Option A

Imagine a simple circuit with a resistor (R) and an inductor (L) connected in parallel. When an AC voltage is applied, both components allow current to flow, but in different ways.

The resistor always offers the same opposition (resistance) to current, regardless of the frequency of the AC signal.

However, the inductor's opposition (reactance, denoted as X_L) changes with frequency. At low frequencies, the inductor's reactance is low, meaning it allows a significant amount of current to flow through it.

As the frequency increases, the inductor's reactance increases. This means that less and less current flows through the inductor.

At very high frequencies, the inductor's reactance becomes so large that it almost completely blocks the current. The circuit's behavior is then almost entirely determined by the resistor, and therefore, the circuit appears purely resistive.

Therefore, the correct answer is D: Very high. At very high frequencies, the inductive reactance dominates, and the parallel combination acts almost entirely as a resistive circuit.