Microphones generate an electrical signal when you move their diaphragms, but not all of them can do this without their own source of power. Dynamic mics, due to the way they're built, don't need an external power supply, but condensers do. You have to charge the plate that the diaphragm moves towards when it gets hit with sound pressure.

Each of the three primary types of microphones—dynamic microphones, condenser microphones, and ribbon microphones—has a different method for converting sound into electrical signals.