In an a.c. circuit, the frequency of the alternating current and alternating emf is same. However, it is not necessary that they both be in the same phase. Usually, when the current in the circuit in maximum, the emf is not maximum and vice-versa. The phase difference between the two, depends upon the type of circuit. Phase of a.c may be defined as the time period that has elapsed. Since the current (or emf) last passed its zero value in the positive direction. The study of a.c circuits is much simplified if we treat alternative current or alternating emf as (kind of) vectors. The angle between the ‘vectors’ representing current and voltage is taken equal to the phase difference between them. The current and emf ‘vectors’ are better called ‘phasors’. A diagram, representing alternating current and emf as ‘phasors’, showing the phase angle between them, is called a ‘Phasor Diagram’.
