3. Two typical gauges To reduce arbitrariness of potential, we give some constraint-- Gauge fiⅸing。 Symmetry or explicit physical interpretation ● Coulomb gauge condition V·A=0 transverse (横场),V0 longitudinal(纵场)。 o is determined by instantaneous distribution of charge density (similar to static coulomb field)l Coulomb gauge condition  A = 0  3)．Two typical gauges transverse (横场)， longitudinal (纵场)。 is determined by instantaneous distribution of charge density (similar to static coulomb field) A    To reduce arbitrariness of potential, we give some constraint --- Gauge fixing。 Symmetry or explicit physical interpretation