Voltage Sources

For current to flow, there must have voltage.

Direct Current (DC) is a fixed steady current flow from one direction to the other.  The voltage can be positive or negative, measure relative to a point called ground.

Alternate Current (AC) flows in two directions over the conductor.  In other words, electron flows in one direction for a moment and then reverse to flow in the opposite direction later, and so on.  The AC signals that comes into home are in the shape of sine wave.  The frequency refers to the number of reversion per second.  In the case of AC signal, it is generally at a frequency of 60Hz.

AC is represented by a symbol of circle with a varying wave in the middle.