I'm sorry for asking a question this simple but I can't understand it . Single phase ac has two wires , phase and neutral but I don't know which wire change voltage . Like neutral wire is always 0 v and phase wire changing +V to -V or phase and neutral changes direction ? (Like phase +V , neutral 0V then neutral +V , phase 0V ). Also what's the difference between pulse signal and ac ?
The neutral wire is connected to an Earth Ground somewhere, and is therefore defined to be "Zero Volts". The voltage on the phase (or "live" or "line") wire varies/alternates between positive and negative relative to the Neutral wire.
AC normally implies a regular alternating voltage (50 or 60Hz for Mains Power, but any frequency for other uses).
A pulse signal is normally at one voltage (often, but not necessarily, zero), but occasionally changes to another voltage for a short time.