Which of the following is not ac waveform?

Question

Examoogle · Accepted Answer

constant

Explanation: An Alternating Current (AC) waveform is defined as a quantity that changes its magnitude and periodically reverses its direction over time. A constant signal, by definition, does not change with time; therefore, it is classified as a Direct Current (DC) signal, not an AC waveform.

Working Principle: AC waveforms are functions of time $f(t)$ that satisfy the periodic condition $f(t) = f(t + T)$, where $T$ is the period, and have a net average value of zero over one full cycle. In contrast, a constant signal is represented by $f(t) = C$, which maintains a steady state regardless of time. Since it never crosses the zero axis or reverses polarity, it lacks the defining characteristics of an alternating wave.

Key Formulas: $f(t) = V_m \sin(\omega t)$ — General equation for a sinusoidal AC waveform; $I(t) = I_{DC}$ — Constant current DC signal

Key Points: AC waveforms must exhibit periodic reversal in direction.; A constant signal ($V = constant$) has a frequency of 0 Hz.; Sinusoidal, square, and triangular waves are all periodic functions that swing across a reference axis.; The average value of a symmetric AC waveform over one complete cycle is zero.

Advantages: AC is easily transformed to higher/lower voltages; AC machines are generally more robust and maintenance-free

Disadvantages: Constant DC is required for electronics and battery storage; DC transmission has lower dielectric losses over very long distances

Applications: Power distribution grids; Induction motors and transformers

Additional Info: Option A (Sinusoidal), Option B (Square), and Option D (Triangular) are all examples of time-varying, periodic signals that periodically change polarity, fitting the criteria for AC.. Constant signals are strictly associated with stationary DC power sources like electrochemical batteries.

Answer: C is correct — A constant signal does not change its magnitude or direction over time, making it a DC signal, not an AC waveform.

Exam Tip: Remember that any signal with a non-zero time derivative ($d/dt 
eq 0$) is dynamic; only signals where $d/dt = 0$ are considered truly 'constant' or steady-state DC.

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