Question

Explain the term EMF.

Answer 1

EMF is the abbreviation of the electromotive force. It is defined as the energy per unit electric charge supplied by an energy source like a battery or a generator to move the charge through the complete circuit. So conversion of energy takes place in a battery or a generator.

Complete answer:
EMF or electromotive force of a cell is essentially a characteristic of the cell (or energy source) which is capable of driving an electric charge around the circuit.

In a battery or a generator, energy is converted from one form to another since the battery or generator works on the electric charge being transferred within the battery or the generator. One terminal of the device will become positively charged while the other terminal becomes negatively charged. So electromotive force is essentially the work done on a unit electric charge to move it through the given complete circuit. It is denoted by $\varepsilon$ and its S.I units are volt.

Suppose $10{\text{J}}$ of energy is supplied to $2{\text{C}}$ of charge to move it through a complete circuit. Then the emf will be $\varepsilon = \dfrac{{10}}{2} = 5{\text{V}}$ .

EMF can also be defined as the maximum potential difference between the positive electrode and the negative electrode of a cell in an open circuit and can be produced by an electrochemical cell or by changing the magnetic field.

Note:
Open circuit refers to the scenario when no current flows through the cell. Though EMF is considered as the maximum potential difference in a cell, it is different from the potential difference between two points in a circuit. Potential difference is defined as the energy dissipated when the unit charge passes through the components in a circuit. EMF of a cell is constant but the potential difference in a circuit is not constant. This is because EMF is independent of the circuit resistance while the potential difference depends on the resistance between two ends during measurement. The electromotive force is not a force.