If a conductive or semiconductive path is provided between the two points having a relative potential difference, an electric current flows.
Voltage can be either direct or alternating.
A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency , which is measured in hertz . An example of pure direct voltage is the EMF between the terminals of an electrochemical cell. The output of a power-supply rectifier, prior to filtering, is an example of pulsating direct voltage.