Differences Between Electromotive Force and Potential Difference

Potential difference (PD) and electromotive force (EMF) are energy types measured in volts, but the two parameters are very different. 

We notice two potential differences between the battery terminals when working with cells and batteries in real life. These potential differences are caused by the battery’s non-ideal nature, which increases the resistance inside the battery. Such variations are measured using a potentiometer. 

These various values are called EMF and a cell’s potential difference. To comprehend why this happens, we must examine the resistance provided by the circuit and the battery. Let’s take a closer look at these ideas.

What is electromotive force?

Electromotive force is the electrical potential difference supplied by an energy source, such as a battery. According to Michael Faraday’s law, varying magnetic fields can also produce an EMF. Although EMF is a voltage measured in volts (V), it is all about creating a potential difference. An EMF is required for an electric circuit to drive current through it. It functions similarly to a charge pump.

According to Kirchhoff’s second law, when an electric circuit is run with an EMF, the sum of the potential drops in that circuit equals the EMF. Solar cells, fuel cells and thermocouples are examples of EMF generators and batteries that use electrochemical energies.

Potential difference

Potential is a term found in electric, magnetic and gravitational fields. Potential is a function of location and the potential difference between points A and B is calculated by subtracting A’s potential from B’s potential. 

In other words, the gravitational potential difference between points A and B represents the amount of work required to move a unit mass (1 kg) from point B to point A; the amount of work required to move a unit charge (+1 Coulomb) from B to A in an electric field. 

The difference in gravitational potential is measured in J/kg, whereas the difference in electric potential is measured in volts. Current flows from the higher potential to the lower potential in an electrical circuit.

However, “potential difference” refers to electrical potential differences in common parlance. As a result, we must use this term with caution to avoid misunderstandings.

Key differences between EMF and PD

• The electromotive force is the quantity of energy that each coulomb of charge receives, whereas the potential difference is the amount of energy used by 1 coulomb of charge.
• The symbol E represents electromotive force, whereas the symbol V represents potential difference.
• The electromotive force is autonomous of the circuit’s internal resistance, whereas the potential difference is proportional to the resistance.
• The electromotive force transfers energy throughout the circuit. The potential difference is the portion of energy between any two points on a circuit.
• The EMF increases the electrical energy in the circuit, whereas the potential difference decreases it.

Example of EMF and PD

To illustrate the distinction between EMF and PD, consider a simple battery used to power a circuit. For example, the voltage is usually stated on the battery’s outer package: 1.5 volts for single-cell alkaline cells. However, it has been discovered that the voltage drops when the battery is in use. This is mainly when larger loads are applied and used as the battery ages. Even if the cell or battery is brand new, there will be some voltage drop when it supplies current. This is because there is some resistance within the cell.

The EMF can thus be defined as the driving potential within any electrical or electronic source, regardless of internal resistance. When measured with very high impedance and no load, the voltage is effectively seen on the source’s output, i.e. battery, etc. It is the cell’s internal voltage.

The potential difference represents the actual potential seen at any point in a circuit. It is not reliant on the absence of a load. When no load is applied to the source, the EMF and potential difference are the same. The potential difference decreases when a load is applied, but the EMF remains constant.

Conclusion

Electromotive force (EMF) is the portion of energy per unit charge produced by an electric or non-electric field. It is a cause determined by electrochemical cells and magnetic field variations. It is a constant form encountered in a circuit even when there is no current flow. It is abbreviated as E and has the SI unit of volts.

The potential difference is the energy difference between two points in a circuit. Potential differences can only be observed in electric sources and they are caused by free electrons passing through a conductor as current flows in a circuit. The potential difference is denoted as PD and is measured in the same SI unit, volts.