Practical electrician technical exchange: what is alternating current?

Alternating polarity of voltage and current in two or more lines is called alternating current, such as single-phase alternating current flowing in live wire and neutral wire or three-phase alternating current flowing between three live wires, audio alternating current flowing in loudspeaker or high-frequency alternating current in switching power supply. Ordinary people say alternating current generally refers to 220 volts or 380 volts of strong electricity. Take 220 volts as an example, because the polarity of N and S magnetic fields of the rotating magnet of the engine alternately sweeps across the generator coil, and because the polarity of the magnet determines the direction of coil current, the current starts from the live wire, flows through electrical appliances (such as light bulbs), flows back to the zero line, then flows out from the zero line and flows back to the live wire through the light bulbs. In this way, the conversion rate of positive and negative directions is the same as that of the generator at 50 Hz. To put it bluntly, the live wire flows into the zero line through electrical appliances, and the power is cut off. The neutral wire flows into the fire line through the electrical appliance, and then the power supply is cut off. This cycle is endless. Within 1 s, the direction of in-line current changes alternately for 50 cycles, the in-line voltage disappears 100 times, and the current is cut off 100 times (the in-line voltage disappears because the generator magnet rotates. The positive and negative polarities of other alternating currents are also alternating, and the current is intermittent, but the alternating frequency and amplitude waveforms are different. The frequency of 220 volts is 50 Hz, which is called sine wave alternating current. Others have different frequencies and waveforms, such as audio frequency of 20-20000 Hz, and the amplitude is not fixed.