4. Potential Difference and EMF

Electric Potential and Potential Difference

Electric potential at a point is the amount of work done in bringing a unit positive charge from infinity to that point against the electric field. What matters in circuits, however, is usually the potential difference between two points, defined as the work done in moving a unit positive charge from one point to another. Potential difference is what drives current through a conductor — just as water flows from a region of higher pressure to lower pressure, conventional current flows from a point of higher potential to a point of lower potential when a conducting path is provided. The SI unit of potential difference is the volt (V), named after the Italian physicist Alessandro Volta, who invented the first chemical battery (the voltaic pile). One volt is defined as the potential difference between two points such that one joule of work is done in moving one coulomb of charge between them (1 V = 1 J/C). Potential difference is measured using a voltmeter, which is always connected in parallel across the component whose potential difference is to be measured.

Electromotive Force (EMF)

Electromotive force, or EMF, is the total energy supplied by a source (such as a cell or battery) per unit charge, driving the charge around the complete circuit, including through the source itself. Although its name suggests a force, EMF is actually measured in the same unit as potential difference, the volt, because it is energy per unit charge, not a force in the mechanical sense. A subtle but important distinction frequently tested in exams is that EMF is the potential difference across the terminals of a cell when no current is being drawn (an 'open circuit'), whereas the terminal voltage is slightly less than the EMF when current is flowing, because some energy is lost overcoming the cell's own internal resistance. Thus EMF is, in a sense, the cause, while potential difference/terminal voltage is the practical effect available to the external circuit.