Class 12 > Unit # 20:AC Circuits > Alternating Current (AC) and its Phases

Talha's Physics Academy

Alternating Current, AC Terminologies, and Phase Relationships

Video Lecture

Watch the complete video lecture below to thoroughly understand Alternating Current, key AC terminologies, phase concepts, and phasor diagrams.

Define Alternating Current and Terminologies Used in AC

Alternating Current (AC)

"An alternating current (AC) is an electrical current that periodically reverses direction and changes its magnitude continuously with time, following a regular sinusoidal pattern."

Unlike direct current (DC), which flows in a single constant direction, alternating current cycles between positive and negative values. As shown in graphical representations, during one half-cycle the current flows in one direction, and during the opposite half-cycle it reverses direction, oscillating back and forth continuously.

Key Terminologies in AC

1. Cycle:

   One complete set of both positive and negative values of an alternating quantity (one full wave pattern consisting of a positive half-cycle and a negative half-cycle).

2. Time Period ($T$):

   The time duration required to complete one full cycle of an alternating current or voltage wave. It is denoted by $T$ and related to frequency by the formula:

   $$T = \frac{1}{f}$$

3. Frequency ($f$):

   The number of complete cycles executed by the current or voltage waveform in one second, measured in hertz ($\text{Hz}$). It determines how rapidly the current reverses direction:

   $$f = \frac{1}{T}$$

4. Instantaneous Peak Value ($I_{\text{peak}}$ or $V_{\text{peak}}$):

   The maximum positive or negative amplitude reached by an alternating current or voltage waveform during a cycle.

5. Root Mean Square (rms) Value:

   Because the average value of a symmetrical AC cycle over time is zero, the rms value is used in power calculations. It represents the equivalent direct current (DC) voltage that delivers the same average electrical power. For a sinusoidal wave, the rms value is approximately $70\%$ or $0.707$ times the peak value ($V_{\text{rms}} = 0.707 V_{\text{peak}}$).

Describe the Phase of AC and How Phase Lags and Leads in AC Circuits

"In alternating current circuits, phase describes the relative position and timing relationship between two or more sinusoidal waveforms of the same frequency as time progresses."

1. In-Phase

When two waveforms have the identical frequency and reach their peak and zero values simultaneously, they are said to be in-phase (phase difference is zero).

2. Phase Lag

If one waveform reaches its peak or zero value after another waveform, there is a phase difference, and the second waveform is said to lag the first one (e.g., lagging by $90^\circ$ or $\frac{\pi}{2}\text{ radians}$).

3. Phase Lead

Conversely, if a waveform reaches its peak or zero value before another waveform, it is said to lead the first waveform.

Fig: Graphical representation of AC waveforms showing phase alignment.

Phasor Diagrams

A phasor diagram is a graphical representation in AC circuits that uses rotating vectors (phasors) to depict the magnitudes and phase relationships between different voltage and current quantities, simplifying calculations and wave analysis.

Fig: Graphical representation of AC waveforms showing phase alignment.