Misconception 3: Potential, Potential Difference, Voltage, and Electromotive Force

©2019 L A Waygood

Potential

Potential, which is measured in volts, exists at a specified point within a circuit, and is always measured with respect to a fixed reference point, usually earth (ground). That potential will change if a different reference point is chosen.

We can compare ‘potential’ with ‘height’. For example, the height of a mountain is normally measured with respect to sea level. However, the height of the same mountain will change if the reference point is changed to, for example, the base of that mountain or, say, from the peak of another mountain.

The potential at any point in a circuit is entirely dependent upon the point of reference in exactly the same way that the height of a mountain is entirely dependent upon its point of reference.

So the potential difference of any given point in a circuit isn’t absolute; it depends entirely on its point of reference.

We normally label a potential as being either more ‘positive’ or more ‘negative’ with respect to the reference point.

Potential Difference (‘Voltage’)

‘Potential difference‘ and ‘voltage’ are synonymous. They mean exactly the same thing, and are also measured in volts.

‘Potential’ and ‘voltage’, on the other hand, are not synonymous. Potential exists at one point; voltage, or potential difference, exists between two points.

If the potentials of two different points in a circuit are at, say, +10 V and +30 V with respect to earth, then the potential difference, or voltage, between them is 20 V.

If the potentials of two points in a circuit are at, say, +10 V and −30 V with respect to earth, then the potential difference, or voltage, between them is 40 V.

As potential differences are not reliant upon a point of reference (such as earth), they are absolute.

We do not allocate ‘positive’ or ‘negative’ (in the sense of charge polarity) to potential differences (voltages).

We can, however, allocated the terms ‘positive’ or ‘negative’ to potential differences if we wish to indicate the directions in which they are acting. For example, if a battery’s potential difference acts to drive current in a clockwise direction around a circuit, we could arbitrarily describe that direction as being ‘positive’, while the potential difference of a battery that acts in the opposite direction could then be allocated as being ‘negative’.

Electromotive Force (E.m.f.)

When a battery or a generator supplies a load current, as well as passing through the load itself, that same current also passes through the battery or generator. As all batteries and generators have internal resistance, an internal voltage drop occurs within that battery or generator, resulting in a drop in the potential difference appearing across its terminals compared to that when no load current is being supplied.

The terminal potential difference (voltage) of an unloaded battery or generator is always higher than a loaded battery or generator. This higher (‘open circuit’) terminal potential difference is known as battery or generator’s ‘electromotive force‘ which, of course, is also measured in volts.

Another way of defining electromotive force is to say that it is equal to the sum of all the voltage drops, including the internal voltage drop, around a circuit.

Symbols

The symbol for potential or potential difference (voltage) is either U or V. The symbol, U, is more commonly used in the UK and European countries, while the symbol, V, is more commonly used in North America.

The symbol for electromotive force is usually E.

Terminology

It is correct to say:

‘The potential of the line conductor is 230 V with respect to the neutral conductor.’

It is incorrect to say:

‘The potential of the line conductor is 230 V.’ (because its point of reference must be specified.)

It is incorrect to say:

‘The potential difference (or voltage) of a line conductor is 230 V.’ (potential difference exists between two points, not one)

It is incorrect to say:

‘The potential difference (or voltage) of a line conductor is 230 V with respect to the neutral conductor.’

It is correct to say:

‘The potential difference (or voltage) between the line conductor and the neutral conductor is 230 V.’

 

3 thoughts on “Misconception 3: Potential, Potential Difference, Voltage, and Electromotive Force”

  1. Can you write more like this? It’s proven extremely helpful for people.

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