Electromotive force (emf, 
) is a term used to characterize electrical devices, such as voltaic cells, thermoelectric devices, electrical generators and transformers, and even resistors. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. Classical electromagnetism (or classical electrodynamics) is a theory of Electromagnetism that was developed over the course of the 19th century most prominently In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes In Physics, the Lorentz force is the Force on a Point charge due to Electromagnetic fields It is given by the following equation Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Displacement current is a quantity that arises in a changing electric field In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric The electromagnetic field is a physical field produced by electrically charged objects. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. The Liénard-Wiechert potential describes the electromagnetic effect of a moving Electric charge. The Maxwell Stress Tensor (also known as Maxwell's Stress Tensor is used to calculate the stresses on objects in magnetic or electrical fields An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in The thermoelectric effect is the direct conversion of temperature differences to electric Voltage and vice versa A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | For a given device, if a charge Q passes through that device, and gains an energy U, the net emf for that device is the energy gained per unit charge, or U/Q. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός This has units of volts, or newton meters per coulomb, and hence can be thought of as a voltage induced by the device in question. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Since force has the unit of the newton, emf is a misnomer, but one that over time has resisted change. In Physics, a force is whatever can cause an object with Mass to Accelerate.
In most circuits current is driven by a so-called "source of emf", which usually is a voltaic cell (or battery, which consists of voltaic cells in series and/or in parallel) or the power company. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. For a voltaic cell the source of emf is the chemical reactions that occur at each of the electrode-electrolyte interfaces, so that a voltaic cell can be thought of as two "surface pumps" of atomic dimension. The reactions at the electrode-electrolyte interfaces provide the "seat" of emf for the voltaic cell. For the power company, the source of emf is electromagnetic induction, which is more extended than an atomic size, but nevertheless is confined to the power generation building, usually many miles from the user.
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Sources and unit of measurement
Sources of electromotive force include electric generators (both alternating current and continuous current types), batteries, and thermocouples (in a heat gradient). In Electricity generation, an electrical generator is a device that converts Mechanical energy to Electrical energy, generally using Electromagnetic An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant Direct current ( DC) is the unidirectional flow of Electric charge. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy In Electrical engineering and industry thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal Potential [1] Electromotive force is often denoted by or ℰ (script capital E).
Electromotive force is measured in volts (in the International System of Units equal in amount to a joule per coulomb of electric charge). The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. Electromotive force in electrostatic units is the statvolt (in the centimeter gram second system of units equal in amount to an erg per electrostatic unit of charge). The statcoulomb ( statC) or franklin ( Fr) or electrostatic unit of charge ( esu) is the physical unit for Electrical The statvolt is the unit of Voltage and Electrical potential used in the Cgs system of units The centimetre-gram-second system ( CGS) is a system of physical units. An erg is the unit of Energy and Mechanical work in the centimetre-gram-second (CGS system of units symbol "erg" Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction.
Terminology
The term electromotive force is due to Alessandro Volta (1745–1827), who invented the battery, or voltaic pile. Count Alessandro Giuseppe Antonio Anastasio Volta was a Lombard physicist known especially for the development of the first electric cell in A voltaic pile is a set of individual Voltaic cells placed in series "Electromotive force" originally referred to the 'force' with which positive and negative charges could be separated (i. In Physics, a force is whatever can cause an object with Mass to Accelerate. e. moved, hence "electromotive"), and was also called "electromotive power" (although it is not a power in the modern sense). In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for Maxwell's 1865 explanation of what are now called Maxwell's equations used the term "electromotive force" for what is now called the electric field strength. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can [2][3]
Formal definition of electromotive force
If the vector field f represents the force per unit charge on a charge carrier, the emf around a circuit C is
This formal definition is not very helpful for a voltaic cell; there f due to the chemical reactions is either very large but not calculable (at the electrode-electrolyte interfaces), or zero (everywhere else). In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. However, this definition is quite helpful for emfs generated by a time-dependent magnetic field (Faraday's Law of electromagnetic induction). Note that the electrostatic potential does not contribute to the net emf around a circuit (although it does contribute over parts of a circuit). Like the electric potential at a point and the voltage between two points, the emf around a loop is measured in volts. At a point in space the electric potential is the Potential energy per unit of charge that is associated with a static (time-invariant Electric field The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force.
The emf is sensitive to non-electrostatic forces, since the force f can include magnetic, chemical, mechanical, and gravitational components. [5] In practice, the power sources for the non-electrostatic forces in a voltaic cell are the chemicals that react at the electrode-electrolyte interfaces; for the power company they are the moving rotors that produce a non-electrostatic field by Faraday's Law of electromagnetic induction; and for a thermoelectric device they are the heaters and coolers that maintain a temperature difference across the device. The EMF of a source (electromagnetic, chemical, thermal or otherwise) may be defined as the work done by an external agent, per unit charge, with sign reversed, in bringing a test charge once around a circuit that contains the source and no other source. Such a source if often described as a seat of EMF.
Another term for emf is electromotance.
Electromotive force in thermodynamics
When multiplied by an amount of charge de the emf ℰ yields a thermodynamic work term ℰde that is used in the formulism for the change in Gibbs free energy when charge is passed in a battery:
-
- dG = -SdT + VdP + ℰde
The combination ℰ. e is an example of a conjugate pair of variables. In Thermodynamics, the Internal energy of a system is expressed in terms of pairs of conjugate variables such as temperature/entropy or pressure/volume At constant pressure the above relationship produces a Maxwell relation that links the change in open cell voltage with temperature (a measurable quantity) to the change in entropy when charge is passed isothermally and isobarically. Maxwell's relations are a set of equations in Thermodynamics which are derivable from the definitions of the Thermodynamic potentials. An isothermal process is a Thermodynamic process in which the Temperature of the System stays Constant: &Delta T = 0 An isobaric process is a Thermodynamic process in which the pressure stays constant \Delta p = 0 The term derives from the Greek isos "equal" The latter is closely related to the reaction entropy ΔrS of the electrochemical reaction that lends the battery its power. In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy
Electromotive force and voltage difference
According to Maxwell, even a voltage difference can have the same effect as an emf. Nevertheless, normal usage does not consider a voltage difference as a source of emf.
- For a resistor the voltage difference across its ends serves as the sole source of emf.
- For a voltaic cell the net emf is the sum of the chemical emf, which always tends to drive current so as to discharge the cell, and the voltage difference emf across its terminals. The combination of the two emfs can drive current in either direction, thus permitting both charge and discharge; in equilibrium, where there is zero current, these two emfs cancel.
- For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (See Kirchoff's Law)
- For a circuit consisting of a capacitor that discharges through a resistor, the emf that drives current is solely due to the voltage difference across the resistor, and due to the capacitor.
If a source of emf is not connected to an external resistor, then an electric current cannot flow through that resistor (Ohm's Law). In this case, between the terminals of the source there must exist a true electric field that produces a voltage difference that exactly cancels the emf of the source.
The source of this true electric field is the electric charge that has been separated by the mechanism generating the emf [6]. For example, the chemical reaction in a voltaic cell stops when the electric field across each electrode is strong enough to stop the reactions at each electrode.
This electric field between the terminals of the battery creates an electric potential difference that can be measured with a voltmeter. In Physics, the potential difference or pd between two points is the difference of the points' Scalar potential, equivalent to the line integral A voltmeter is an instrument used for measuring the Electrical potential difference between two points in an electric circuit The polarity of this measured potential difference is always opposite to that of the generated emf. The value 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. It can, however, be inferred from a measurement of the current I and voltage difference V, provided that the internal resistance has already been measured: I=( -V)/r.
One of Volta's great contributions to science was to recognize that a voltaic cell has two sources of emf, the chemical reactions at each electrode. He showed that they provide distinct emfs 
and 
that oppose one another, so that two identical electrodes give no net emf, but that two different electrodes give a net emf of 
, which we assume is positive. A schematic of this circuit would have a long electrode 1 and a short electrode 2, to indicate that electrode 1 dominates. Volta's law about opposing electrode emfs means that, given ten electrodes (e. g. zinc and nine other materials), which can be used to produce 45 types of voltaic cells (10*9/2), only nine relative measurements (e. g. copper and each of the nine others) are needed to get all 45 possible emfs that these ten electrodes can produce.
Electromotive force generation
Besides voltaic cells, which utilize electrochemical reactions, other devices that produce chemical emfs are fuel cells, where there is no electrolyte, but chemicals are introduced directly at the two electrodes. Electrochemistry is a branch of Chemistry that studies Chemical reactions which take place in a Solution at the interface of an electron conductor A fuel cell is an electrochemical conversion device It produces electricity from Fuel (on the Anode side and an oxidant (on the Radiant and thermal energy (e. Radiant energy is the Energy of Electromagnetic waves The quantity of radiant energy may be calculated by integrating Radiant flux (or power Thermal energy is the sum of the sensible energy and latent energy. g. , a solar cell or a thermocouple) can also produce emfs. Some other sources of emf include thermocouples, thermopiles, and photodiodes. A thermopile is an electronic device that converts Thermal energy into Electrical energy. A photodiode is a type of Photodetector capable of converting Light into either current or Voltage, depending upon the mode of operation
Dissimilar metals in contact also produce what is known as a contact electromotive force or contact potential (eg. Contact electrification is an Obsolete scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at , the volta effect). Contact electrification is an Obsolete scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at However, this is a truly electrostatic effect, and does not affect the overall emf of a circuit.
The principle of electromagnetic induction, noted above, states that a time-dependent magnetic field can produce a circulating electric field. Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of 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 a current), or by changing the current in a fixed circuit. The effect on the circuit itself, of changing the current, is known as self-induction; the effect on another circuit is known as mutual induction. The electromotive force generated by motion is often referred to as motional electromotive force
For a given circuit, the electromagnetically induced electric field is determined purely by the geometry and the rate of change of the magnetic flux through the circuit, by Faraday's law of induction. Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of However, the accompanying electrostatic field does depend on the details of the circuit, since the emf across a resistor will have contributions from both the electromagnetic and electrostatic fields, and their detailed form will depend on the value and shape of the resistor.
If an electric circuit has self-inductance L, and carries current i, then by Faraday's Law
. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting
Given this emf and the resistance of the circuit, the instantaneous current can be computed with Ohm's Law, for example, or more generally by solving the differential equations that arise out of Kirchhoff's laws. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Ohm's law applies to Electrical circuits it states that the current through a conductor between two points is directly proportional to the For other laws named after Gustav Kirchhoff, see Kirchhoff's laws.
Classification of induced emfs
An emf is induced in a coil or conductor whenever there is change in the flux linkages. Flux linkage is a property of a coil of conducting wire and the magnetic field through which it passes Depending on the way in which the changes are brought about, there 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. When the change in flux linkage arises from a change in the magnetic field around the stationary conductor, the emf is dynamically induced.
Electromotive force of cells
The electromotive force produced by primary and secondary cells is usually of the order of a few volts. The figures quoted below are nominal, because emf varies according to the size of the load and the state of exhaustion of the cell.
| Emf | Cell chemistry |
|---|---|
| 1. 2 V | Nickel-cadmium |
| 1. The nickel-cadmium battery (commonly abbreviated NiCd and ˈnɑɪˌkæd "nye-cad" is a type of Rechargeable battery using Nickel oxide hydroxide 2 V | Nickel Metal-Hydride |
| 1. A nickel-metal hydride battery, abbreviated NiMH, is a type of Rechargeable battery similar to a nickel-cadmium ( Ni[[Cadmium Cd]] battery 5 V | Zinc-carbon |
| 2. A Zinc-carbon Dry cell or battery is packaged in a Zinc can that serves as both a container and anode 1 V | Lead-acid |
| 3. Lead-acid batteries, invented in 1859 by French Physicist Gaston Planté, are the oldest type of Rechargeable battery. 6 V to 3. 7 V | Lithium-Ion |
References
- General
- Griffiths, David (1999). Lithium-ion batteries (sometimes abbreviated Li-ion batteries) are a type of Rechargeable battery in which a Lithium ion moves between the Anode Introduction to Electrodynamics, 3e, Prentice-Hall. ISBN 0-13-805326-X.
- Saslow, Wayne M. (2002). Electricity, Magnetism, and Light. Toronto: Thomson Learning. ISBN 0-12-619455-6. Ch. 8 (especially pp. 302-315) discusses emfs for voltaic cells. Ch. 12 discusses electromagnetically induced emfs; especially see example 12. 13 and Fig. 12. 16(b), where a distributed electromagnetic emf is discussed and shown in a circuit.
- Citations
- ^ John S. Rigden, (editor in chief), Macmillan encyclopedia of physics. New York : Macmillan, 1996.
- ^ Edward J. Rothwell and Michael J. Cloud, Electromagnetics. CRC Press. Pg 22. ISBN 0-8493-1397-X
- ^ James Clerk Maxwell (W. Garnett; P Pesic) (1888). An elementary treatise on electricity. Mineola, NY: Dover Publications, Chapter IX, pp. 96 ff. . ISBN 0486438848.
- ^ Griffiths, Introduction to Electrodynamics, p. 293
- ^ Griffiths, Introduction to Electrodynamics, p. 285; ". . . or trained ants with tiny harnesses. "
- ^ Roberts, Dana: "How batteries work: A gravitational analog", Am. J. Phys. , 51,829 (1983)
Ohm's Law (PDF in German)
Further reading
- Andrew Gray, "Absolute Measurements in Electricity and Magnetism", Electromotive force. Macmillan and co. , 1884.
- Charles Albert Perkins, "Outlines of Electricity and Magnetism", Measurement of Electromotive Force. Henry Holt and co. , 1896.
- John Livingston Rutgers Morgan, "The Elements of Physical Chemistry", Electromotive force. J. Wiley, 1899.
- George F. Barker, "On the measurement of electromotive force". Proceedings of the American Philosophical Society Held at Philadelphia for Promoting Useful Knowledge, American Philosophical Society. January 19, 1883.
- "Abhandlungen zur Thermodynamik, von H. Helmholtz. Hrsg. von Max Planck". (Tr. "Papers to thermodynamics, on H. Helmholtz. Hrsg. by Max Planck". ) Leipzig, W. Engelmann, Of Ostwald classical author of the accurate sciences series. New consequence. No. 124, 1902.
- Nabendu S. Choudhury, "Electromotive force measurements on cells involving [beta]-alumina solid electrolyte". NASA technical note, D-7322.
- Henry S. Carhart, "Thermo-electromotive force in electric cells, the thermo-electromotive force between a metal and a solution of one of its salts". New York, D. Van Nostrand company, 1920. LCCN 20020413
- Hazel Rossotti, "Chemical applications of potentiometry". London, Princeton, N. J. , Van Nostrand, 1969. ISBN 0-442-07048-9 LCCN 69011985 //r88
- Theodore William Richards and Gustavus Edward Behr, jr. , "The electromotive force of iron under varying conditions, and the effect of occluded hydrogen". Carnegie Institution of Washington publication series , 1906. LCCN 07003935 //r88
- G. W. Burns, et al. , "Temperature-electromotive force reference functions and tables for the letter-designated thermocouple types based on the ITS-90". Gaithersburg, MD : U. S. Dept. of Commerce, National Institute of Standards and Technology, Washington, Supt. of Docs. , U. S. G. P. O. , 1993.
External articles
- Doug Gingrich, "Physics lecture notes, electronics", Direct Current Circuits, Electromotive Force (EMF). University of Alberta, Department of Physics, 1999.
- Advanced Physics lecture notes, "Electromagnetism", Faraday’s Law—Electromagnetic Induction. Electromotive Force". Semiconductor Physics Group, Department of Physics, University of Cambridge, 2006. (PDF)
External links
- Electromotive Force in Inductors - Interactive Java Tutorial National High Magnetic Field Laboratory
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