The difference between electromotive force and voltage
Article By : John Dunn
Electromotive force and voltage share the same unit, but they are conceptually different entities.
I once read this very strongly-written essay about the difference between electromotive force (EMF) and voltage. The author seemed like he was on some kind of holy crusade and was intensely determined to set his readers straight on fundamental truths that were NOT to be dismissed by infidels. I read his words, but for all of the vitriol, I didn’t understand him at all. Recently, I decided to take a closer look at the issue.
EMF is given in units of joules per coulomb. The frame of reference for this definition is for any device that uses a non-electrical source of energy to impart electrical energy to a unit of charge, which if given the chance, will flow somewhere (a current) and do something. Examples of non-electrical energy sources would include batteries that use chemical energy, generators that use rotational kinetic energy, photo diodes that use light energy, or perhaps a well-fed squirrel in a wire cage using acorn energy.
Dimensional analysis yields the following result:
Figure 1 A dimensional analysis of EMF yields these equations.
The dimensional analysis reasoning is as follows:
- One watt of power is the consumption of one joule of energy per second. Therefore, one joule equals one watt second.
- One watt of power equals one volt times one ampere.
- One ampere is the current flow of one coulomb per second.
- Do the two indicated cancellations and the unit of EMF is found to be the volt.
The distinction between an EMF and a voltage can now be illustrated with a simple series circuit.
Figure 2 This series circuit shows the distinction between EMF and voltage.
The energetic squirrel in Figure 2 generates the EMF. With the Thevenin resistance dropping V1 and the three series connected resistors dropping V2, V3, and V4, the numerical sum around the loop is:
EMF – V1 – V2 – V3 – V4 = Zero.
Thus, the voltages V1 thru V4 arise as a consequence of the EMF and their sum numerically equals the EMF, but since those V1 thru V4 arise as the result of electrical excitation, V1 thru V4 are by definition not EMFs.
EMF and voltage share the same unit, the volt, but they are conceptually different entities. Sometimes though, I get tired of semantics.
This article was originally published on EDN.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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