**AC Phase :**

Things start to get complicated when we need to relate two or more AC voltages or currents that are out of step with each other. By "out of step," I mean that the two waveforms are not synchronized: that their peaks and zero points do not match up at the same points in time. The following graph illustrates an example of this:

The two waves shown above (A versus B) are of the same amplitude and frequency, but they are out of step with each other. In technical terms, this is called a phase shift.

A sampling of different phase shifts is given in the following graphs to better illustrate this concept:

- Phase shift is where two or more waveforms are out of step with each other.
- The amount of phase shift between two waves can be expressed in terms of degrees, as defined by the degree units on the horizontal axis of the waveform graph used in plotting the trigonometric sine function.
- A leading waveform is defined as one waveform that is ahead of another in its evolution. A lagging waveform is one that is behind another.
- Calculations for AC circuit analysis must take into consideration both amplitude and phase shift of voltage and current waveforms to be completely accurate. This requires the use of a mathematical system called complex numbers.

http://www.allaboutcircuits.com/vol_2/chpt_1/5.html

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