20.4 PHASE OF A.C

The phase of an alternating current (A.C.) describes the timing relationship between the alternating voltage and the alternating current in a circuit. In circuits containing components like capacitors or inductors, the voltage and current waveforms do not necessarily peak or cross the zero-axis at the same instant.

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Key Concepts

• Phase Difference ($\Phi$):

  • It is the angular separation between the zero points of the alternating voltage and current waveforms as they pass through in the same direction.
  • It represents the fraction of a time period difference between the peaks of the two waveforms.
  • Phase difference is measured in degrees or radians.

• Leading and Lagging:

  • Leading: The quantity (voltage or current) that passes through its zero point earlier than the other is said to be "leading".
  • Lagging: The quantity that passes through its zero point later than the other is said to be "lagging".
  • Since both alternating quantities share the same frequency, the phase difference between them remains constant.

• Measurement and Convention:

  • The phase difference is always less than or equal to $90^{\circ}$ ($\pi/2$ radians).
  • By convention, phase difference is usually described as the angle by which the voltage leads the current.

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Summary

• In AC circuits with capacitors or inductors, voltage and current are often out of sync. This timing difference is called the phase difference.
• The phase difference, denoted by $\Phi$, is the angle between the zero-crossing points of the voltage and current waveforms.
• The waveform that reaches its zero point first is said to be leading, while the other is lagging.
• Phase difference is measured in degrees or radians and, for simple circuits, does not exceed $90^{\circ}$.
• The standard convention is to describe the phase difference in terms of how much the voltage leads the current.

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References

(Derived from FBISE textbook)

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