A very useful method of specifying an exact point in a periodic waveform, particularly with the sine wave, results from expressing the time at which it occurs, measured from the beginning of the period or cycle, as an angle (phase angle) related to the total angle of a circle. That is, the cycle is assumed to be equivalent to 360 degrees, and the fractional cycle is directly related to the equavelent fraction of 360 degrees. For example, a quarter cycle is 90 degrees, a half cycle is 180 degrees, 5/8 cycle is 225 degrees, and so on. The beginning of the cycle (or poriod) usually is assumed to be the point at which the wave goes through zero moving in the positive direction.

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