• [Wikipedia]: an electromotive force (not to be confused with electromagnetic field) is the electrical action produced by a non-electrical source. In electronics and electromagnetism, an electromotive force (abbreviation: emf) is denoted and measured in volts. Devices (known as transducers) provide an emf by converting other forms of energy into electrical energy, such as batteries (which convert chemical energy) or generators (which convert mechanical energy). Sometimes an analogy to water pressure is used to describe electromotive force (the word “force” in this case is not used to mean forces of interaction between bodies).

In electromagnetic induction, emf can be defined around a closed loop of conductor as the electromagnetic work that would be done on an electric charge if it travels once around the loop. For a time-varying magnetic flux linking a loop, the electric potential’s scalar field is not defined due to a circulating electric vector field, but an emf nevertheless does work which can be measured as a virtual electric potential around the loop.

In the case of a two-terminal device (such as an electrochemical cell) which is modeled as a Thévenin’s equivalent circuit, the equivalent emf can be measured as the open-circuit potential difference, or voltage, between the two terminals. This potential difference can drive an electric current if an external circuit is attached to the terminals, in which case the device becomes the voltage source of that circuit.

Overview: Devices which can provide emf include electrochemical cells, thermoelectric devices, solar cells, photodiodes, electrical generators, transformers and even Van de Graaff generators. In nature, emf is generated when magnetic field fluctuations occur through a surface. For example, the shifting of the Earth’s magnetic field during a geomagnetic storm induces currents in an electrical grid as the lines of the magnetic field are shifted about and cut across the conductors.

In a battery, the charge separation which gives rise to a voltage difference between the terminals is accomplished by chemical reactions at the electrodes which convert chemical potential energy into electromagnetic potential energy. In an electrical generator, a time-varying magnetic field inside the generator creates an electric field via electromagnetic induction, which creates a voltage difference between the generator terminals. Charge separation takes place within the generator because the electric charge flows away from one terminal toward the other, until, in the open-circuit case, an electric field is developed which makes further charge separation impossible. The emf is countered by the electrical voltage due to charge separation. If a load is attached, this voltage can drive a current. The general principle governing the emf in such electrical machines is Michael Faraday’s “Law of Induction”.

History: Around 1830, Michael Faraday established that chemical reactions at each of two electrode-electrolyte interfaces provide the “seat of emf” for the voltaic cell. That is, these reactions drive the current and are not an endless source of energy as was initially thought. In the open-circuit case, charge separation continues until the electrical field from the separated charges is sufficient to arrest the reactions. Years earlier, Alessandro Volta, who had measured a contact potential difference at the metal-metal (electrode-electrode) interface of his cells, held the incorrect opinion that contact alone (without taking into account a chemical reaction) was the origin of the emf.

Voltage Difference: An electrical voltage difference is sometimes called an emf. The points below illustrate the more formal usage, in terms of the distinction between emf and the voltage it generates:

1. For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, electrical voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (...). The emf is due solely to the chemistry in the battery whch causes charge separation, which in turn creates an electrical voltage that drives the current.

2. For a circuit consisting of an electrical generator which drives current through a resistor, the emf is due solely to a time-varying magnetic field within the generator that generates an electrical voltage which in turn drives the current. (...).

3. A transformer coupling two circuits may be considered a source of emf for one of the circuits, just as if it were caused by an electrical generator; this example illustrates the origin of the term “transformer emf”.

4. A photodiode or solar cell may be considered as a source of emf, similar to a battery, resulting in an electrical voltage generated by charge separation driven by light rather than chemical reaction.

5. Other devices which produce emf are fuel cells, thermocouples, and thermopiles. In the case of an open circuit, the electric charge that has been separated by the mechanism generating the emf creates an electric field opposing the separation mechanism. For example, the chemical reaction in a voltaic cell stops when the opposing electric field at each electrode is strong enough to arrest the reactions. A larger opposing field can reverse the reactions in what are called reversible cells.

The electric charge which has been separated creates an electric potential difference which can be measured with a voltmeter between the terminals of the device. The magnitude of the emf for the battery (or other source) is the value of this ‘open circuit’ voltage. When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source. (...).

Generation.

Chemical Sources: The question of how batteries (galvanic cells) generate an emf occupied scientists for most of the nineteenth century. The “seat of the electromotive force” was eventually determined in 1889 by Walther Nernst to be primarily at the interfaces between the electrodes and the electrolyte. (...). In batteries, coupled half-reactions, often involving metals and their ions, occur in tandem, with a gain (termed “reduction”) by one conductive electrode and loss (termed “oxidation”) by another (reduction-oxidation or redox reactions). The spontaneous overall reaction can only occur if the electric charge moves through an external wire between the electrodes. The electrical energy given off is the free energy lost by the chemical reaction system.

As an example, a Daniell Cell consists of a zinc anode (a collector) which is oxidised as it dissolves into a zinc sulphate solution. (...). If the cathode and anode are connected by an external conductor, the electric charge passes through that external circuit (light bulb in figure), while ions pass through the salt bridge to maintain charge balance until the anode and cathode reach electrical equilibrium of zero volts as chemical equilibrium is reached in the cell. In the process the zinc anode is dissolved while the copper electrode is plated with copper. The so-called “salt bridge” has to close the electrical circuit while preventing the copper ions from moving to the zinc electrode and being reduced there without generating an external current. It is not made of salt but of material able to wick {to draw off (liquid) by capillary action⁽*⁾} cations and anions (a dissociated salt) into the solutions. The flow of positively charged cations along the “bridge” is equivalent to the same number of negative charges flowing in the opposite direction. If the light bulb is removed (open circuit) the emf between the electrodes is opposed by the electric field due to the charge separation, and the reactions stop.

> ⁽*⁾wick (n.): 1. any piece of cord which conveys liquid by capillary action; [e.g.]: “The physician put a wick in the wound to drain it”; (related word): cord (a line made of twisted fibres or threads; [e.g.]: “A wick is a cord or band of loosely twisted or woven fibres, as in a candle, cigarette lighter, etc, which supplies fuel to a flame by capillary action”)

> 2. a loosely woven cord (in a candle or oil lamp) which draws fuel by capillary action up into the flame; [e.g.]: “The kerosene, {gradually wicking up through the loosely woven cord, fuelled the flame”; (synonym): taper; (related words): candle, wax light, taper (stick of wax with a wick in the middle); candlewick (the wick of a candle); [e.g.]: “The candlewick, a strand of loosely woven, twisted, or braided fibres, draws up fuel to the flame by capillary action”; cord (a line made of twisted fibres or threads; [e.g.]: “The cord, a twist or braid of soft threads in a lamp, etc., serves to draw up the flammable liquid to be burned”); kerosene lamp, kerosene lamp, oil lamp (a lamp which burns oil,} as kerosene, for light). [curly-bracketed inserts added] ~ (Princeton’s WordNet 3.0).

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Voltaic cells: Alessandro Volta developed the voltaic cell about 1792, and presented his work March 20, 1800. He correctly identified the role of dissimilar electrodes in producing the voltage, but incorrectly dismissed any role for the electrolyte. He ordered the metals in a ‘tension series’, “that is to say in an order such that any one in the list becomes positive when in contact with any one that succeeds, but negative by contact with any one that precedes it”. A typical symbolic convention in a schematic of this circuit ( –||– ) would have a long electrode 1 and a short electrode 2, to indicate that electrode 1 dominates. Alessandro Volta’s law about opposing electrode emfs implies that, given ten electrodes (for example, zinc and nine other materials), forty-five unique combinations of voltaic cells (10 × 9/2) can be created.

Values: The electromotive force produced by primary (single-use) and secondary (rechargeable) cells is usually of the order of a few volts.

(...)

Electromagnetic Induction: the production of a circulating electric field by a time-dependent magnetic field. A time-dependent magnetic field can be produced either by motion of a magnet relative to a circuit, by motion of a circuit relative to another circuit (at least one of these must be carrying an electric current), or by changing the electric current in a fixed circuit. The effect on the circuit itself, of changing the electric current, is known as self-induction; the effect on another circuit is known as mutual induction.

For a given circuit, the electromagnetically induced emf is determined purely by the rate of change of the magnetic flux through the circuit according to Michael Faraday’s “Law of Induction”.

An emf is induced in a coil or conductor whenever there is change in the flux linkages. Depending on the way in which the changes are brought about, there are two types: When the conductor is moved in a stationary magnetic field to procure a change in the flux linkage, the emf is statically induced. The electromotive force generated by motion is often referred to as motional emf. When the change in flux linkage arises from a change in the magnetic field around the stationary conductor, the emf is dynamically induced. The electromotive force generated by a time-varying magnetic field is often referred to as transformer emf.

Contact Potentials: When solids of two different materials are in contact, thermodynamic equilibrium requires that one of the solids assume a higher electrical potential than the other. This is called the contact potential. (...). The transfer causes a charge separation, with one body gaining electrons and the other losing electrons. This charge transfer causes a potential difference between the bodies, which partly cancels the potential originating from the contact, and eventually equilibrium is reached. (...). The contact potential cannot drive steady current through a load attached to its terminals because that current would involve a charge transfer. No mechanism exists to continue such transfer and, hence, maintain a current, once equilibrium is attained. (...).

Solar Cell: Light, of sufficient energy (greater than the bandgap of the material), creates mobile electron-hole pairs in a semiconductor. Charge separation occurs because of a pre-existing electric field associated with the p-n junction {a p-n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor} in thermal equilibrium (this electric field is created from a built-in potential, which arises from the contact potential between the two different materials in the junction).

The charge separation between positive and negative across a p-n junction {the p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy, i.e., by growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant; this allows electrical current to pass through the junction only in one direction}, i.e., a diode, yields a forward voltage, the photo voltage, between the illuminated diode terminals, which drives current through any attached load. The photo voltage is sometimes referred to as the photo emf, distinguishing between the effect and the cause. (...). [curly-bracketed inserts added] ~ (2012 Wikipedia Encyclopaedia ).

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