Voltage, electromotive force, or electric potential difference is the difference in electrical potential between two points, such as the positive and negative poles of a battery or other device that generates a flow of electrons, or between a point in a device and an electrical ground.
Electrical potential (V) is energy (W) divided by positive charge (Q), or energy per unit of electric charge. Thus, V = W/Q. Since the joule is a unit of energy and the coulomb is a unit of electric charge, the joule per coulomb is a unit of electromotive force or of electric potential difference. A volt equals one joule per coulomb. The units of electrical potential in the SI and most other systems of units are volts.
Electrical potential difference can be analogized by comparing to gravitational potential difference, the difference in potential energy per unit of mass between two objects at different elevations in a gravitational field. It can also, perhaps more usefully, be compared to pressure differences in hydraulic systems, with current flow analogized as the quantity of water flow, resistance as the size and roughness of the pipe or channel.