18.2: Electromotive Force

Electricity is generated by either electrons or ions flowing through a solution or a conducting medium. This flow of electrons or specifically electrical charge is defined as an electric current. When electrons move through a wire, they generate an electric current. It can be recalled that in a redox reaction, electrons are lost and gained. In the spontaneous redox reaction of zinc with copper, when zinc is immersed in a copper ion solution, a transfer of electrons from one substance to another occurs.

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

Electrical Current and How Electrons Flow

Electron transfer occurs directly from a reducing agent to an oxidizing agent in a solution. Even if the components of half-reactions are physically isolated in separate vessels and connected via an external conductor such as a wire, the tendency to lose and gain electrons by the reactants still persists. However, now, electrons are forced to flow through the wire connecting the two half-reactions. This electron flow through the wire constitutes an electric current and can power electronic appliances, such as a light-bulb. Electric current is measured in amperes. One ampere is equal to the flow of one coulomb of electrical charge per second and is equal to 6.24 × 10⁻¹⁸ electrons per second.

Since an electron has a charge of 1.602 × 10⁻¹⁹ C, 1 ampere correlates to the flow of 6.242 × 10¹⁸ electrons per second.

Driving Force for Electrical Current, Potential Difference, and Emf

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 either by the terms potential difference, electromotive force (emf), or cell potential. The emf is a measure of the driving force between two reactants and the tendency for electron transfer.

Some redox reactions are spontaneous, while others are not. For example, a copper wire undergoes spontaneous oxidation by silver(I) ions, but fails to yield any reaction when immersed in a solution of lead(II) ions. This is due to the difference in the redox activity of the two species, Ag⁺ (aq) and Pb²⁺ (aq), towards copper: the silver ion spontaneously oxidizes copper, but the lead ion does not. This difference in redox reactivity in electrochemistry can be quantified using the term ‘cell potential’; also commonly known as ‘voltage’.

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 electromotive force, or cell potential, depends on the reactants’ nature, the reaction temperature, and the concentration of ions present in the reaction.

This text is adapted from OpenStax, Chemistry 2e, Section 17.3: Electrode and Cell Potentials.

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.

• Electricity – The flow of electrons or ions through a conducting solution or medium.
• Electric Current – The flow of electrical charge, typically via electrons moving through a wire.
• Electron Transfer – The movement of electrons from one substance to another in a redox reaction.
• Electromotive force (emf) – Describes the difference in electrical potential energy between reactants.
• Cell Potential – A measure of the redox reactivity in electrochemistry, also commonly known as 'voltage'.

• Define Electricity – Explain how it is generated by electron or ion flow (e.g., electricity).
• Contrast Electric Current and Electron Transfer – Explain their roles in generating electricity (e.g., electric current).
• Explore Emf and Cell Potential – Describe how they drive the flow of electrons (e.g., emf).
• Explain the Mechanism of Electron Flow – Demonstrates how electrons flow through a wire to generate electricity.
• Apply in Context – Illustrate how understanding electron flow and electrical potential difference can be used in everyday applications.

• [Question 1] What is electricity and how does it involve electron or ion flow?
• [Question 2] How does the concept of emf and cell potential drive the flow of electrons?
• [Question 3] How does understanding electron flow and electrical potential difference apply to everyday life?

• Students – Understanding of electricity and its generation supports academic learning in physics and engineering.
• Educators – Offers a clear framework for teaching the concepts of electric current, emf, and cell potential.
• Researchers – Relevant for studies in electrical engineering, physics, and energy science.
• Science Enthusiasts – Provides insights into the generation of electricity, stimulating curiosity in science.