Voltage, electrical potential difference, electric tension or electric pressure is the electric potential difference between two points, or the difference in electric potential energy of a unit charge transported between two points. Voltage is equal to the work done per unit charge against a static electric field to move the charge between two points. A voltage may represent either a source of energy, or lost, used, or stored energy. A voltmeter can be used to measure the voltage between two points in a system; usually a common reference potential such as the ground of the system is used as one of the points. Voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three.