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What is Electromotive force (EMF)? How is it related to potential difference? Is it the creation of potential difference in any conductor??Is it a process?Why is it called force?

Does writing emf instead of voltage make a difference in AC circuits...I find some books using emf while others voltage(whle studying Alternating current)

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See "Electromotive Force: A Guide for the Perplexed" arxiv.org/abs/1211.6463 –  Alfred Centauri Mar 6 '14 at 16:31
@Uzair- You ask questions but never accept any answers. Please don't do this if you want your questions to be taken seriously by the community. –  Parth Vader Apr 6 '14 at 13:05

6 Answers 6

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The link you have provided answers pretty much all your answers, but still I'll quote and explain a bit more.

First The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts.

It is neither the creation of potential, nor it is a process and it is not even a force. It came to be known as a force because wrongful interpretation in the past made it seem as if a force from the battery pushed the particles in a circuit. This analysis was obviously discarded later but the name prevails!

In electric circuits emf and potential have a significant difference. While the emf is the potential difference between the terminals of a source in open circuit, the potential is the potential difference betwren the terminals in a closed one.

The potential is defined as $ V = \epsilon - Ir $. Here $\epsilon $ is the emf of the source and $ Ir $ is tue potential drop in the internal resistance. It is pretty clear that replacing potential with emf in any sort of circuit is not a very wise decision, however sometimes the potential drop in internal resistance is negligible.

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Electromotive force, abbreviated as E.M.F and denoted by $\varepsilon$, is not a force. It is defined as the energy utilized in assembling a charge on the electrode of a battery when the circuit is open.Simply, it is the work done per unit charge which is the potential difference between the electrodes of the battery measured in volts. Mathematically, $\textbf{V} = \frac{\textbf{W}}{\textbf{q}}$.

Initially, energy is available in the form of chemical energy. This energy is utilized to take a charge say $+q$ to the anode by overcoming the electrostatic force of attraction due to the the negative charges on the cathode and the electrostatic force of repulsion due to the positive charges on the anode. The chemical energy then gets transformed into electrostatic potential energy present in the electric field between the electrodes of the battery.

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I dislike the term EMF (Electromotive force) as it is very confusing.

Electromotive force, also called emf (denoted $\mathcal{E}$ and measured in volts), is the voltage developed by any source of electrical energy such as a battery or dynamo.

Which means that all EMF are voltages but not all voltages are EMF. A voltage is only an EMF if it is a source of energy.

Kind of like the distinction between luminescent light (from a light bulb) and reflected light (from your desk) if you measure it there is no physical measurable difference. The only difference is that one is a source and the other is not.

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I think that definition is too vague, although probably correct. I'd like it better if it pointed out that voltages developed by static charges are not developed by any "source of electrical energy" –  garyp Mar 6 '14 at 14:19

There is no creation of anything, but it can be assumed that a circuit creates a voltage when the power, combination of that voltage and any current which would flow from it, has been gained from "outside the circuit" - e.g. through chemical processes (batteries), or electromagnetic processes (dynamo that converts mechanical power to electrical). This very particular voltage is your electromotive force in your circuits, for example the back-emf of the windings of a motor. It's a concept which exists also in magnetic circuits (for the big picture), see magnetomotive force.

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It may be pointed out that the word electromotive force is a misnomer. It does not represent force on the carriers of electricity. Instead, it represents the potential difference between the two poles in an open circuit (when no current is drawn from the cell).

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The relationship between emf and potential is exactly the same as the relationship between work and potential energy. The emf from a circuit element is the work per unit charge done on the charges in the system. Often that emf is associated with a difference of potential (potential energy per unit charge), in which case the two are interchangeable. Both can be measured in units of volts, and so can be called a "voltage". A resistor applies an emf of -IR to the charges that move through it, and so the potential difference across the resistor is IR (specifically, a drop of IR if following the current).

One case where emf is NOT associated with a potential difference is when it is induced by a change in magnetic flux. In a circuit with induced current, if you add up the potential differences around a circuit you won't get zero, even though you end at the same place you began. That's because the changing magnetic flux through the circuit is doing work (per unit charge) from outside, but there are no two points in the circuit you could put a voltmeter across to measure that directly. Induced emf can still be measured in volts, and so is sometimes called "loop voltage", but that's a voltage without a difference! ;-)

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