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The potential difference between two points of a steady-state electric or gravitational field is defined as the work done by the forces of the field when a unit positive charge or unit mass, respectively, is moved from the point with the higher potential to the point with the lower potential. If Φ1 and Φ2 are the potentials at the initial and the final points of the trajectory of the charge or mass being moved, then the potential difference is u = Φ1 - Φ2, and the change in the potential is ΔΦ = Φ2 - Φ1 = - u.
The work done by an arbitrary electric field in moving a positive unit charge from one point to another is called the voltage between the points. In the case of a steady-state field, the voltage is the same as the potential difference.