Electromotive Force

When electrons flow through a wire, or ions flow through a solution, they generate electricity. This flow of electrons, or, more specifically, electric charge, is defined as an electric current. But what causes the electrons to flow?

Consider a redox reaction between copper and zinc. When a zinc electrode is placed into a copper ion solution, electrons are transferred from one substance to another.

Zinc, having a greater tendency to lose electrons, is oxidized to zinc ions, while copper ions are reduced to solid copper. In this reaction, electrons flow from zinc to copper, but this reaction does not generate electricity.

Now, consider that the reactants, zinc and copper, are physically separated and connected via an external conductor such as a wire. The reactants’ tendency to gain or lose electrons still persists, driving the electrons to flow through the wire, which connects the two half-reactions.

This electron flow constitutes an electrical current and can power electrical appliances such as a lightbulb.

Electrical current is measured in amperes. One ampere equals the flow of one coulomb of electrical charge per second, corresponding to 6.24 × 10¹⁸ electrons per second.

The flow of electrical current is similar to water flowing down a waterfall. The water is driven by the difference in gravitational potential energy, while the flow of electrons is driven by the difference of the electrical potential energy between the reactants.

This difference in electrical potential energy is described by the terms potential difference, electromotive force, or cell potential. The cell potential is a measure of the driving force between two reactants and the tendency for electron transfer.

The cell potential of two isolated reactants is measured with a voltmeter, which is read in cell voltage. One volt correlates to one joule of potential energy per one coulomb of electrical charge.

A high cell potential indicates a large driving force and greater ease of electron transfer. Lastly, the cell potential, or electromotive force, depends on the reactants’ nature, reaction temperature, and the concentration of ions present in the reaction.