974. How does a stereo convert 110 volt electric current into the positive and negative current that is sent to power the speakers? — JF
A stereo contains a power supply that converts 110-volt alternating current into lower-voltage direct current. This direct current is ultimately when powers the speakers. The stereo's power supply first lowers the voltage with the help of a transformer. Alternating current from the power line flows back and forth through a coil of wire in this transformer, the primary coil, and causes that coil to become magnetic. Since the coil's magnetism reverses 120 times a second (60 full cycles of reversal each second), along with the alternating current, it produces an electric field—changing magnetic fields always produce electric fields. This electric field pushes current through a second coil of wire in the transformer, the secondary coil, and transfers power to that current. There are fewer turns of wire in the secondary coil than in the primary coil, so charges flowing in the secondary coil never reach the full 120 volts of the primary coil. Instead, more current flows in the secondary coil than in the primary coil, but that secondary current involves less energy per charge—less voltage. In this manner, power is transferred from a modest current of high voltage charges in the primary coil to a large current of low voltage charges in the secondary coil.
Having used the transformer to produce lower voltage alternating current, the power supply than converts this alternating current into direct current with the help of four diodes and some capacitors. Diodes are one-way devices for electric current and, with four of them, it's possible to arrange it so that the alternating current leaving the transformer always flows in the same direction through the circuit beyond the diodes. The diodes act as switches, always directing the current in the same direction around the rest of the circuit. The capacitors are added to this circuit to store separated electric charge for the times while the alternating current is reversing and the diodes receive no current from the transformer. The capacitors store separated charge while there is plenty of it coming from the transformer and provide current while the alternating current is reversing. Overall, the stereo's power supply is a steady source of direct current.