Which of the following is not ac waveform?

Question

Examoogle · Accepted Answer

Constant

Explanation: An Alternating Current (AC) waveform is defined as a current or voltage that periodically reverses direction and changes its magnitude over time, typically characterized by an average value of zero over one full cycle. A 'constant' signal, by definition, does not change its magnitude or direction over time; therefore, it is classified as a Direct Current (DC) signal, not an AC waveform.

Working Principle: AC waveforms are functions of time $f(t)$ where the integral over one period $T$ is zero (i.e., $\frac{1}{T} \int_{0}^{T} f(t) dt = 0$). Constant signals represent a stationary state where $f(t) = C$, resulting in a non-zero average value equal to the magnitude $C$, which violates the fundamental periodic reversal requirement of AC signals.

Key Formulas: $v(t) = V_m \sin(\omega t + \phi)$ — Sinusoidal AC voltage expression; $V_{avg} = \frac{1}{T} \int_{0}^{T} v(t) dt = 0$ — Defining property of AC waveform

Key Points: AC waveforms must be periodic and change polarity.; A constant signal (DC) has a constant magnitude and direction.; Sinusoidal, square, and triangular waveforms are common periodic AC types.; The average value of a pure AC wave is always zero.

Applications: Power distribution systems; Signal processing; Audio engineering

Additional Info: Constant signals (DC) are characterized by zero frequency (f = 0).. Option A, B, and D represent signals that periodically vary in magnitude or polarity, satisfying AC criteria.

Answer: C is correct — A constant signal does not change over time, meaning it lacks the periodic reversal of polarity required to be classified as an AC waveform.

Exam Tip: Remember that any waveform with a non-zero DC component (offset) is technically a composite of AC and DC; a purely 'constant' signal is strictly DC.

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