Electromotive Force EMF, Potential Difference & Voltage

Electromotive force, EMF, potential difference and voltage are all terms associated with electric potential and measured in volts, but they have major differences in what they are.

Voltage Includes:
What is voltage     Electric field     Voltage / potential-divider     Electromotive force    

There can be a lot of confusion between electromotive force, EMF and the voltage or potential difference, PD, at a point in an electrical or electronic circuit.

Both EMF and potential difference are measured in volts, but the two parameters are very different in what they are.

The difference between electro-motive force and potential difference - EMF vs potential difference

These differences can be important in some aspects of electrical and electronic circuit design

It helps to have an understanding about what they are so that confusion can be reduced and the correct terms and terminology used where it is needed.

We'll take a look at the three terms in turn to see exactly what they are and how they apply to different areas of electronic theory, electronic circuit design and the like.

What is electromotive force, EMF

It helps to define what the electromotive force is before looking further.

Electromotive force definition:

Electromotive force is defined as the characteristic of any energy source capable of driving electric charge around a circuit - it is the force within a voltage source that drives the current around a circuit. It is abbreviated E in the international metric system but the abbreviation EMF is also widely used.

From this it can be seen that the EMF is the driving force in a generator used to push the current around the circuit. The electromotive force is the basic electrical force that actually drives the current around the circuit.

What is potential difference

The potential difference at any point in an electrical or electronic circuit is the voltage at any given point with respect to another point in the circuit.

It is possible to have a more rigorous definition of the potential difference, and although many definitions of potential difference exist, the one below can give a good idea.

Potential difference definition:

The potential difference between two points in an electrical or electronic circuit represents the work involved or the energy released in the transfer of a unit quantity of electricity from one point to the other.

Effectively, the potential difference is the difference in the electric potential between two points. Energy is released when a charge is moved from a point of higher potential to a point of lower potential. Often this is in the form of heat.

Take the example of a current flowing through a resistor where some heat is released as the current flows from the point of a higher potential to a point of lower potential.

This means that the potential is the voltage at a given point in a circuit, not the source of the force to move it around the circuit.

The key point to remember is that EMF is the cause, i.e. it is the driving force, whereas potential difference is the result of the EMF.

What is voltage

The voltage at a certain point in an electrical or electronic circuit is the potential difference at that point measured in volts.

The volt is the universal unit of the measurement of potential difference and EMF.

Multimeters and all test equipment is calculated in volts for the measurement of the potential difference or voltage at a certain point.

Typically the voltage of a point usually, but not always, refers to the potential difference between a point and the ground or zero volt line of a circuit.

It is worth considering exactly what a volt is.

The volt definition:

The standard unit of voltage or potential difference and electromotive force in the International System of Units (SI), is formally defined to be the difference of electric potential between two points of a conductor carrying a constant current of one ampere, when the power dissipated between these points is equal to one watt.

Example of EMF and PD

To explain the difference between EMF and PD, take the example of a simple battery used to power a circuit. Normally on the outer package of the battery itself the voltage will be stated: often 1.5 volts for single cell alkaline cells and the like.

However it is found that when the battery is in use the output potential difference or voltage will fall, especially when larger loads are applied and as it becomes older and it has been used.

Even when the cell or battery is new, some voltage drop will be seen when it supplies current. The reason for this is that there is some resistance inside the cell.

From this it is possible to define the EMF as the the driving potential within any electrical or electronic source regardless of any internal resistance.

It is effectively the voltage seen on the output of the source, i.e. battery, etc when it is measured with a very high impedance and no load. It is the internal voltage of the cell.

Voltage and electromotive force, EMF
Potential difference and electromotive force, EMF

The potential difference at any point is the actual potential see at any given point in a circuit. It is not dependent upon there being no load. In fact the EMF or a source and the potential difference are the same when no load is applied to the source. As a load is applied the potential difference will fall, but the EMF remains the same.

Typically the output potential difference will be measured in volts and can be termed the output voltage.

Summary of the similarities & differences between EMF & PD

It is worth tabulating the main differences between electromotive force and potential difference as this highlights the similarities and differences.

As the terms are often used incorrectly and interchangeably, it is good to have a sound understanding of what each term means, what exactly it means and where it can be used.


Similarities and differences of EMF and Potential Difference
 
Electromotive force (EMF) Potential Difference (PD)
EMF is the driving electric force from a cell or generator. Potential difference results from the current passing through a resistance within a circuit.
EMF is the cause. Potential difference is the effect.
The EMF is also present even when no current is drawn through the battery. Potential difference across the conductor is zero in the absence of current.
The unit of EMF is the volt. The unit of potential difference is the volt.
EMF remains constant. Potential difference does not remain constant - it depends upon the circuit conditions.
Its symbol is E. Its symbol is V.
It does not depend on circuit resistance. It depends on the resistance between two points of measurement.

EMF and potential difference have many similarities, but they also have some significant differences. Essentially the EMF is the driving force ina circuit, whereas the potential difference is the result of the EMF within a circuit to which the source is connected.

Understanding where to correctly use each of the terms is of great benefit as it can give weight to any spoken or written material. If used incorrectly it can indicate that the terms are not correctly understood and may lessen the impact of anything else that is said or written.

Ian Poole   Written by Ian Poole .
  Experienced electronics engineer and author.




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