Electromotive Force

Electromotive Force or emf, the potential difference between the terminals of an electric cell, battery, or other source of electrical energy on open circuit (“open circuit” means that no current is being taken). Potential difference is a measure of the energy available to make electricity flow: 1 joule of energy is absorbed or given out for each coulomb of electricity that is moved through 1 volt. The emf of a cell therefore indicates the maximum energy per coulomb available from the cell. Emf is measured in volts (V), and is the quantity shown on the packaging of a cell; for example, a 1.5-V cell has an emf of 1.5 V. In practice, when the cell is in use, the potential difference, measured across the terminals is less than this, because as soon as a current flows the potential difference drops. Modern digital voltmeters have a very high resistance, and are therefore ideal for measuring emf directly, because negligible current can flow through them.

See Electricity: Current Electricity.