Voltage, otherwise known as electrical potential difference or electric tension (denoted ∆V and measured in units of electric potential: volts, or joules per coulomb), is the electric potential difference between two points — or the difference in electric potential energy of a unit test charge transported between two points. Voltage is equal to the work which would have to be 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 (electromotive force), or it may represent lost, used, or stored energy (potential drop). A voltmeter can be used to measure the voltage (or potential difference) 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 a combination of all three.