Radians of phaseAn angle of 1 radian is about 57.3 degrees. A complete circle is 6.28 radians around. Ifa wave has a frequency of f Hz, then the vector goes through 1 radian of phase every1/(57.3f) seconds. The number of radians per second for an ac wave is called the angular frequency.Radians are used mainly by physicists. Engineers and technicians generally use degrees when talking about phase, and Hertz when talking about frequency.Phase coincidenceWhen two sine waves have the same frequency, they can behave much differently iftheir cycles begin at different times. Whether or not the phase difference, often calledthe phase angle and specified in degrees, matters depends on the nature of the circuit.Phase angle can have meaning only when two waves have identical frequencies. Ifthe frequencies differ, even by just a little bit, the relative phase constantly changes, andyou can’t specify a single number. In the following discussions of phase angle, assumethat the two waves always have identical frequencies.Phase coincidence means that two waves begin at exactly the same moment.They are “lined up.” This is shown in Fig. 12-7 for two waves having different ampli-tudes. (If the amplitudes were the same, you would see only one wave.) The phase dif-ference in this case is 0 degrees. You might say it’s any multiple of 360 degrees, too, butengineers and technicians almost never speak of any phase angle of less than 0 or morethan 360 degrees.Phase coincidence22112-6The four points for the model of Fig. 12-5, shown on a standardamplitude-versus-time graph of a sine wave.