Understanding the Differences Between Potential Difference and Electromotive Force

When discussing electrical circuits, terms like electromotive force (EMF) and potential difference (PD) are often used. However, the distinction between these two concepts can be easily misunderstood. This article aims to clarify the differences, providing a clear understanding of how they apply in circuit analysis.

What is Electromotive Force (EMF)?

Electromotive force (EMF) is a concept that has two distinct applications in circuit analysis. The first use is in describing the terminal voltage of a battery, which can vary as the current being delivered changes. This is because every 'real' battery has an internal resistance that cannot be eliminated. As the current increases, the terminal voltage decreases by an amount given by the product of internal resistance (Rinternal) and the current delivered. At a very low current draw, the 'ideal' voltage of the battery can be approximated. This 'ideal' voltage, often referred to as the EMF of the cell, is determined by the chemical reaction occurring in the cell.

The chemical EMF is a potential difference (PD), which represents the electrical potential energy difference between two points in a circuit. It can be thought of as the chemical phenomena that generate a voltage difference. A real cell is often assumed to have this ideal EMF in series with a fixed amount of internal resistance.

Faraday’s Law and Electromotive Force (EMF)

EMF has another, entirely unrelated use, referring to Faraday’s law of electromagnetic induction. According to Faraday’s law, the EMF induced around a closed loop through which a changing magnetic flux exists is equal to the rate of change of magnetic flux through the given loop. This EMF, however, cannot be expressed as a potential difference. Instead, it represents the driving force that induces an electric current in a conductor.

The key distinction here is that the EMF in the context of Faraday’s law is a measure of the changing magnetic flux and the induced electric field, not the potential difference. Transformers, which rely on this principle, are described in terms of the Faraday EMF.

Common Misunderstandings

There is a common misconception that voltage and EMF are always the same thing. While both are measures of potential difference, they are not interchangeable in all contexts. EMF is often referred to as the force that generates potential difference, whereas voltage is the potential difference itself.

It is also important to note that EMF is sometimes referred to as E.M.F. in old texts and can be written in capital letters. However, in modern use, it is generally written in lowercase. Similarly, potential difference is often simply referred to as voltage, with the distinction between the two made when necessary.

Back EMF (Back E.M.F.), on the other hand, is a specific type of EMF generated in motors, stating that the induced voltage opposes the direction of current. This is a unique phenomenon and does not pertain to the general concept of EMF and potential difference.

Conclusion

In summary, while both potential difference and EMF are measures of electrical potential, the terms have distinct applications and meanings in circuit analysis. Understanding these differences is crucial for accurate circuit design and analysis. Whether you are working with batteries, transformers, or motors, it is important to differentiate between these concepts to avoid confusion.

The key takeaways are:

EMF can refer to the chemical phenomena generating a voltage (battery EMF) or the induced electric field due to changing magnetic flux (Faraday’s law). Potential difference (PD) is the difference in electrical potential between two points. Back EMF is specific to motor systems and is used to explain the opposition to current flow.

By understanding these distinctions, you can improve your ability to work with electrical systems and circuits effectively.