1. Amplitude of an AC waveform is:
Correct Answer: a) The maximum value of the waveform
Explanation: The amplitude of an AC waveform is its maximum displacement from zero (either positive or negative peak). For a sine wave v(t) = Vmsin(ωt), Vm is the amplitude. This is different from RMS (root mean square) value which is Vm/√2 ≈ 0.707Vm for sine waves.
2. The phase angle in an AC circuit determines:
Correct Answer: a) The time shift between two waveforms
Explanation: Phase angle (φ) represents the angular difference between two waveforms of the same frequency. It indicates how much one waveform is shifted in time relative to another. A phase difference of 360° means waveforms are perfectly aligned (in phase), while 180° means they are completely opposite (out of phase).
3. If a sine wave leads another sine wave by 90°, it means:
Correct Answer: a) The first wave is ahead in time
Explanation: When Wave A leads Wave B by 90°, it means Wave A reaches its peak value a quarter cycle (90°) before Wave B. This corresponds to a time difference of T/4, where T is the period. In a 50Hz system (T=20ms), 90° phase difference equals 5ms time difference.
4. Phase difference between voltage and current in a purely inductive circuit is:
Correct Answer: c) 90°
Explanation: In a purely inductive circuit, current lags voltage by 90°. This is because the inductor opposes changes in current (V = L di/dt). The voltage reaches its peak value a quarter cycle before the current does. The power factor (cosφ) in such a circuit is zero, meaning no real power is consumed.
5. In a purely resistive AC circuit, the phase difference between voltage and current is:
Correct Answer: a) 0°
Explanation: In a purely resistive circuit, voltage and current are in phase (0° phase difference). This means they reach their peak values at the same instant. The power factor is 1 (cos0° = 1), indicating all power delivered is real power (no reactive power).
6. The RMS value of an AC signal is given by:
Correct Answer: b) 0.707 × Peak Value
Explanation: For a sinusoidal waveform: RMS Value = Peak Value/√2 ≈ 0.707 × Peak Value This relationship comes from the root mean square calculation: √(1/T ∫[v(t)]²dt) over one period. The RMS value represents the equivalent DC value that would produce the same heating effect in a resistor.
7. The average value of a complete sine wave over a full cycle is:
Correct Answer: a) Zero
Explanation: The average value of a complete sine wave over one full cycle (0°-360°) is zero because the positive and negative halves cancel each other out. For half-cycle averages, it's 2/π × Peak Value ≈ 0.636 × Peak Value. This is why we typically use RMS values rather than averages for AC measurements.
8. The form factor of a sine wave is:
Correct Answer: a) 1.11
Explanation: Form Factor = RMS Value / Average Value (for half cycle) For sine wave: (0.707Vm) / (0.636Vm) ≈ 1.11 This ratio indicates how "peaky" a waveform is compared to its average value. Square waves have form factor 1, while more peaked waveforms have higher form factors.
9. The peak factor of a sine wave is:
Correct Answer: b) 1.41
Explanation: Peak Factor (Crest Factor) = Peak Value / RMS Value For sine wave: Vm / (0.707Vm) ≈ 1.414 This ratio is important for determining how much higher than RMS the peaks are. Equipment must be rated to handle peak voltages, not just RMS values.
10. The RMS value of a voltage waveform represents:
Correct Answer: b) The effective voltage that produces the same heating effect as DC
Explanation: RMS (Root Mean Square) value is the DC equivalent value that would produce the same power dissipation in a resistive load. For example, 120V RMS AC across a resistor produces the same heat as 120V DC. This is why AC voltages are typically specified as RMS values (e.g., 120V household voltage is 120V RMS, with peaks of about 170V).