Emf is an extremely important term associated with a cell. When we talk about any circuit, certain components in the circuit insert energy into the circuit while certain components withdraw energy. The device which puts energy into the circuit is said to be providing an electromotive force while those withdrawing energy from the circuit are said to be exhibiting potential difference across them. Potential difference is abbreviated as ‘pd,’ and emf is an abbreviation for electromotive force. Both these terms are measured in volts and they illustrate the quantity of energy inserted or drawn-out per coulomb of charge passing through the specific portion of the circuit.

Emf of a cell can also be defined as the work performed or required for transforming a unit positive charge from negative to positive terminal within the cell. Though its nature resembles that of a force, it should not be confused with force in a literal sense. In simple scientific words, we may define emf and pd as:

Potential difference – It is also abbreviated as pd and is the quantity of electrical energy that is transformed into some other forms of energy per unit of charge.

Emf – It is the amount of energy in any form which is transformed into electrical energy per coulomb of charge.

Sources of emf – Emf is generally generated in nature whenever some kinds of variations occur along a surface. Some of the major sources of emf include:cell, battery (a combination of cells), solar cell, voltaic cells, generator, dynamo, thermocouple, transformers.

When a conductor is placed in a varying magnetic field, it results in the production of emf in the system. This kind of emf is called as induced emf.

Figure shows a source of currents that is, a cell connected to a conductor of resistance R through a key K. A voltmeter V is connected across the two terminals of the cell.

When the key K is open, the reading of the voltmeter gives the electromotive force (emf) E of the cell.

The electromotive force E of a cell is defined as the difference of potential between its terminals when there is no current in the external circuit. That is when the cell is in an open circuit.

When the key is closed, an electric current i flows from positive to negative terminal in the external circuit and from negative terminal to positive terminal within the cell. Thus, a potential drop ‘r’ takes place across the internal resistance ‘r’ (shown separately) of the cell. The direction of ‘ir’ is opposite to the e.m.f. ‘E’ of the cell. The reading of the voltmeter now decreases and gives the potential difference V between the terminals when it is in a closed circuit.

\( S_{O}, V=E-i r \)
\( O r, E=V-i r \)

Hence, E is greater than V by an amount equal to the drop of potential within the cell due to its internal resistance.