Where do I need alternating current and direct current? Why?

Direct current: the direction of current is always the same. Current flows from the positive electrode to the negative electrode through wires and loads, and the direction of current is always the same, so we will output this power supply with fixed current direction, which is called "DC power supply". The abbreviation is dc, such as dry battery and lead battery.

Alternating current: The direction and magnitude of current will change with time. The generator in the power plant uses electricity to make the coil in the generator run, and the direction of the generator output current will change once every turn 180, so the current will change regularly with time. This power supply is called "AC power supply". The abbreviation is ac, such as: household 220V power supply.

Note that AC and DC only refer to the direction of current, regardless of the size. Direct current may also be a current with a constant direction, but its magnitude always changes.

Alternating current can be converted into direct current through rectification (the simplest thing is to change the size of direct current with diodes), and direct current can also be converted into alternating current through an oscillating circuit.