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| {{distinguish|Electromagnetic field}}
| | Methods like massage therapy, yoga, breathing exercises, and meditation can help to manage stress. Vaginal herpes pictures also show the dorsal nerve root where the virus resides. Genital herpes pictures show the disease on the victim's legs, arms, or torso and the common name of this disease is shingles. Mohels were directed to ask parents to sign the form, which spells out the risks of MBP before it is performed. When scientists injected CMV into half of each group and then after several weeks took the blood pressure of the mice, the blood pressure rose only for the CMV-infected mice. <br><br>Although the idea of that happening is extremely unlikely because of the weakness of the virus itself. The bathroom floor gets cleaned several times a day with disinfectant. In most cases of HSV, there are no symptoms at the initial stages and this in turn does not make the sufferer aware of the condition. As stated above, the simplex virus occurs with HS1 and HS2 are combined. Don't forget to drink loads as this could enable if you are obtaining difficulty passing urine because of to pain. <br><br>Other medicines reduce the virus and make transferring the virus to others less likely. oral L-lysine monohydrochloride for the prevention and treatment of. Currently there are several problems which are preventing gene therapy from becoming a common way of treating genetic diseases. A virus is essentially a very simple particle with nucleic acid at its core and a few essential proteins, such as its protein coat. Sometimes, it is that the sores will not be present on human body. <br><br>This is truly safe sex as far as herpes is concerned. Simplex 1: Herpes pictures of this type of Herpes show the full form of the disease on the face. The outbreak will get cleared after about two weeks of treatment. Mucous membranes in mouth, vagina, urethra or open wounds facilitate the virus invasion due to their moistness. If someone has been infected with the virus, their immune system generates antibodies specific to that virus. <br><br>There are a few reasons why women are more susceptible. We're going to consider some time to converse about what herpes search like and examine wherever health professionals are on herpes cures. When you can feel an outbreak coming, do not involve yourself in activities that might transmit the infection to other people as the virus can be easily transmitted by just simple skin contact. Use herpes as an excuse to be vigilant about having protected sex. You can get herpes not only through sexual intercourse.<br><br>If you enjoyed this article and you would such as to receive even more info regarding is herpes curable [[http://www.sohen.info/sitemap/ www.sohen.info]] kindly check out our own web site. |
| {{Electromagnetism|cTopic=[[Classical electromagnetism|Electrodynamics]]}}
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| '''Electromotive force''', also called '''emf'''<ref>emf. (1992). ''American Heritage Dictionary of the English Language'' 3rd ed. Boston:Houghton Mifflin.</ref> (denoted <math>\mathcal{E} </math> and measured in [[volts]]), is the voltage developed by any source of electrical energy such as a [[Battery (electricity)|battery]] or [[dynamo]].<ref>
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| {{cite journal
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| | journal = Transactions of the American Electrochemical Society
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| | title = The Relation Between Contact Potentials and Electrochemical Action
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| | author = Irving Langmuir
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| | volume = 29
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| | issue =
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| | publisher = The Society
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| | pages = 125–182
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| | year = 1916
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| | url = http://books.google.com/?id=OW0SAAAAYAAJ&pg=PA172&dq=%22electromotive+force+is+that%22&q=%22electromotive%20force%20is%20that%22
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| }}</ref>
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| | |
| The word "force" in this case is not used to mean mechanical [[force]], measured in [[newton (unit)|newtons]], but a potential, or energy per unit of charge, measured in [[volt]]s.
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| | |
| In electromagnetic induction, emf can be defined around a closed loop as the electromagnetic [[work (physics)|work]] that would be transferred to a unit of [[electric charge|charge]] if it travels once around that loop.<ref name=Cook>
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| {{cite book
| |
| | title = The Theory of the Electromagnetic Field
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| | author = David M. Cook
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| | publisher = Courier Dover
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| | year = 2003
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| | isbn = 978-0-486-42567-2
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| | page = 157
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| | url = http://books.google.com/?id=bI-ZmZWeyhkC&pg=PA157
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| }}</ref>
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| (While the charge travels around the loop, it can simultaneously lose the energy via resistance into thermal energy.) For a time-varying magnetic flux impinging a loop, the [[electric potential]] scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop.<ref name=Lerner>
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| {{cite book
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| | title=Physics for scientists and engineers
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| | author=Lawrence M Lerner
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| | url=http://books.google.com/?id=Nv5GAyAdijoC&pg=PA727
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| | publisher=Jones & Bartlett Publishers
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| | isbn=0-7637-0460-1
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| | pages=724–727
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| | year=1997}}</ref>
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| In a two-terminal device (such as an electrochemical cell or electromagnetic generator), the emf can be measured as voltage across the two open-circuited terminals. The created electrical potential difference drives current flow if a circuit is attached to the source of emf. When current flows, however, the voltage across the terminals of the source of emf is no longer the open-circuit value, due to voltage drops inside the device due to its [[internal resistance]].
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| Devices that can provide emf include [[electrochemical cell]]s, [[Thermoelectric effect|thermoelectric device]]s, [[solar cells]] and [[photodiode]]s, [[electrical generator]]s, [[transformer]]s, and even [[Van de Graaff generator]]s.<ref name=Lerner/><ref name=Tipler>
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| {{cite book
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| | title=Physics for Scientists and Engineers
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| | author=Paul A. Tipler and Gene Mosca
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| | url=http://books.google.com/?id=BMVR37-8Jh0C&pg=PA850
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| | page=850
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| | isbn=1-4292-0124-X
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| | year=2007
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| | edition=6
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| | publisher=Macmillan
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| }}</ref> In nature, emf is generated whenever magnetic field fluctuations occur through a surface. An example for this is the variation in the [[Earth's magnetic field]] during a [[geomagnetic storm]], acting on anything on the surface of the planet, like an extended [[electrical grid]].
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| | |
| In the case of a battery, charge separation that gives rise to a voltage difference is accomplished by chemical reactions at the electrodes.<ref name=Schaum/>
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| Chemically, by separating positive and negative charges, an electric field can be produced, leading to an electric potential difference.<ref name=Schaum>
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| {{cite book
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| |title=Schaum's outline of theory and problems of beginning physics II
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| |author=Alvin M. Halpern, Erich Erlbach
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| |url=http://books.google.com/?id=vN2chIay624C&pg=PA138
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| |page=138 |year=1998
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| |publisher=McGraw-Hill Professional
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| |isbn=0-07-025707-8
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| }}</ref><ref>
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| {{cite book
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| | title = Physics the easy way
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| | author = Robert L. Lehrman
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| | publisher = Barron's Educational Series
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| | year = 1998
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| | isbn = 978-0-7641-0236-3
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| | page = 274
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| | url = http://books.google.com/?id=wMhCxOsPNE8C&pg=PA274&dq=emf++separated+charge+reaction+potential
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| }}</ref>
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| A voltaic cell can be thought of as having a "charge pump" of atomic dimensions at each electrode, that is:<ref name=Singh>
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| {{cite book |title= Basic Physics |page=152 |chapter=§3.16 EMF of a source
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| |url=http://www.flipkart.com/basic-physics-kongbam-chandramani-singh/8120337085-iu23f9qdih |isbn=81-203-3708-5 |author=Kongbam Chandramani Singh |publisher=Prentice Hall India Pvt Ltd |year=2009 }}
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| </ref>
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| <blockquote>A source of emf can be thought of as a kind of ''charge pump'' that acts to move positive charge from a point of low potential through its interior to a point of high potential. … By chemical, mechanical or other means, the source of emf performs work ''dW'' on that charge to move it to the high potential terminal. The emf ''ℰ'' of the source is defined as the work ''dW'' done per charge ''dq'': ''ℰ'' = ''dW/dq''.
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| </blockquote>
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| Around 1830 Faraday established that the reactions at each of the two electrode–electrolyte interfaces provide the "seat of emf" for the voltaic cell, that is, these reactions drive the current.<ref name=cajori/> In the open-circuit case, charge separation continues until the electrical field from the separated charges is sufficient to arrest the reactions. Years earlier, Volta, who had measured a contact potential difference at the metal-metal (electrode-electrode) interface of his cells, held the incorrect opinion that this contact potential was the origin of the seat of emf.
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| In the case of an electrical generator, a time-varying magnetic field inside the generator creates an electric field via [[electromagnetic induction]], which in turn creates an energy difference between generator terminals. Charge separation takes place within the generator, with electrons flowing away from one terminal and toward the other, until, in the open-circuit case, sufficient electric field builds up to make further movement unfavorable. Again 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 [[Faraday's law of induction]].
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| == Notation and units of measurement ==
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| Electromotive force is often denoted by <math>\mathcal{E}</math> or ''ℰ'' (script capital E, Unicode U+2130).
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| | |
| In a device without internal resistance, if an [[electric charge]] ''Q'' passes through that device, and gains an [[energy]] ''W'', the net emf for that device is the energy gained per unit [[electric charge|charge]], or ''W''/''Q''. Like other measures of energy per charge, emf has [[International System of Units|SI]] units of [[volt]]s, equivalent to [[joule]]s per [[coulomb]].<ref>
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| {{cite book
| |
| | title = Basic Electricity
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| | author = Van Valkenburgh
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| | publisher = Cengage Learning
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| | year = 1995
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| | isbn = 978-0-7906-1041-2
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| | pages = 1–46
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| | url = http://books.google.com/?id=vmg1UKsTntAC&pg=PT67&dq=electromotive-force+joules-per-coulomb+volts+charge+energy
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| }}</ref>
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| Electromotive force in [[electrostatic unit]]s is the [[statvolt]] (in the [[centimeter gram second system of units]] equal in amount to an [[erg]] per electrostatic unit of [[electric charge|charge]]).
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| ==Formal definitions of electromotive force==
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| ''Inside'' a source of emf that is open-circuited, the conservative electrostatic field created by separation of charge exactly cancels the forces producing the emf. Thus, the emf has the same value but opposite sign as the integral of the [[electric field]] aligned with an internal path between two terminals ''A'' and ''B'' of a source of emf in open-circuit condition (the path is taken from the negative terminal to the positive terminal to yield a positive emf, indicating work done on the electrons moving in the circuit).<ref name=Griffiths>
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| {{cite book
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| | title = Introduction to Electrodynamics
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| | author = David J Griffiths
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| | publisher = Pearson/Adisson Wesley
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| | year = 1999
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| | isbn = 0-13-805326-X
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| | page = 293
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| | edition=3rd
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| | url = http://www.amazon.com/gp/product/013805326X
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| }}</ref>
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| Mathematically:
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| :<math>\mathcal{E} = -\int_{A}^{B} \boldsymbol{E_{cs} \cdot } d \boldsymbol{ \ell } \ ,</math>
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| where '''''E<sub>cs</sub>''''' is the conservative electrostatic field created by the charge separation associated with the emf, ''d'''''ℓ''' is an element of the path from terminal ''A'' to terminal ''B'', and ‘<big>'''·'''</big>’ denotes the vector [[dot product]].<ref name =diode>
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| | |
| Only the electric field due to the charge separation caused by the emf is counted. In a solar cell, for example, an electric field is present related to the contact potential that results from thermodynamic equilibrium (discussed later), and this electric field component is not included in the integral. Rather, only the electric field due to the particular portion of charge separation that causes the photo voltage is included.
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| </ref> This equation applies only to locations ''A'' and ''B'' that are terminals, and does not apply to paths between points ''A'' and ''B'' with portions outside the source of emf. This equation involves the electrostatic electric field due to charge separation '''''E<sub>cs</sub>''''' and does not involve (for example) any non-conservative component of electric field due to Faraday's law of induction.
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| | |
| In the case of a closed path in the presence of a varying [[magnetic field]], the integral of the [[electric field]] around a closed loop may be nonzero; one common application of the concept of emf, known as "''induced emf''" is the voltage induced in a such a loop.<ref>{{cite book
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| | title = Beyond the mechanical universe: from electricity to modern physics
| |
| | author = Richard P. Olenick, Tom M. Apostol and David L. Goodstein
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| | publisher = Cambridge University Press
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| | year = 1986
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| | isbn = 978-0-521-30430-6
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| | page = 245
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| | url = http://books.google.com/?id=Ht4T7C7AXZIC&pg=RA1-PA245&dq=define+electromotive-force+around-a-closed-path
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| }}</ref> The "''induced emf''" around a stationary closed path ''C'' is:
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| :<math>\mathcal{E}=\oint_{C} \boldsymbol{E \cdot } d \boldsymbol{ \ell } \ ,</math>
| |
| | |
| where now '''''E''''' is the entire electric field, conservative and non-conservative, and the integral is around an arbitrary but stationary closed curve ''C'' through which there is a varying magnetic field. Note that the electrostatic field does not contribute to the net emf around a circuit because the electrostatic portion of the electric field is [[Conservative force|conservative]] (that is, the work done against the field around a closed path is zero).
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| | |
| This definition can be extended to arbitrary sources of emf and moving paths ''C'':<ref name=Cook2>{{cite book
| |
| | title = The Theory of the Electromagnetic Field
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| | author = David M. Cook
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| | publisher = Courier Dover
| |
| | year = 2003
| |
| | isbn = 978-0-486-42567-2
| |
| | page = 158
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| | url = http://books.google.com/?id=bI-ZmZWeyhkC&pg=PA158
| |
| }}
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| </ref>
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| | |
| :<math>\mathcal{E}=\oint_{C}\boldsymbol{ \left[E + v \times B \right] \cdot } d \boldsymbol{ \ell } \ </math>
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| :::<math> +\frac{1}{q}\oint_{C}\mathrm {\mathbf{effective \ chemical \ forces \ \cdot}} \ d \boldsymbol{ \ell } \ </math>
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| ::::<math> +\frac{1}{q}\oint_{C}\mathrm {\mathbf { effective \ thermal \ forces\ \cdot}}\ d \boldsymbol{ \ell } \ ,</math>
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| | |
| which is a conceptual equation mainly, because the determination of the "effective forces" is difficult.
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| ==Electromotive force in thermodynamics==
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| When multiplied by an amount of charge ''dZ'' the emf ℰ yields a thermodynamic work term ℰ''dZ'' that is used in the formalism for the change in [[Gibbs free energy]] when charge is passed in a battery:
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| :: <math>dG = -SdT + VdP + \mathcal{E}dZ\ , </math>
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| | |
| where ''G'' is the Gibb's free energy, ''S'' is the [[entropy]], ''V'' is the system volume, ''P'' is its pressure and ''T'' is its [[absolute temperature]].
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| | |
| The combination ( ℰ, ''Z'' ) is an example of a [[conjugate variables (thermodynamics)|conjugate pair of variables]]. At constant pressure the above relationship produces a [[Maxwell relation]] that links the change in open cell voltage with temperature ''T'' (a measurable quantity) to the change in entropy ''S'' when charge is passed [[isothermally]] and [[isobarically]]. The latter is closely related to the reaction [[entropy]] of the electrochemical reaction that lends the battery its power. This Maxwell relation is:<ref name=Finn/> | |
| | |
| :<math>
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| \left(\frac{\partial \mathcal{E}}{\partial T}\right)_Z=
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| -\left(\frac{\partial S}{\partial Z}\right)_T
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| </math>
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| If a mole of ions goes into solution (for example, in a Daniell cell, as discussed below) the charge through the external circuit is:
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| :<math> \Delta Z = -n_0F_0 \ , </math>
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| where ''n''<sub>0</sub> is the number of electrons/ion, and ''F''<sub>0</sub> is the [[Faraday constant]] and the minus sign indicates discharge of the cell. Assuming constant pressure and volume, the thermodynamic properties of the cell are related strictly to the behavior of its emf by:<ref name=Finn/>
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| :<math>\Delta H = -n_0 F_0 \left( \mathcal{E} - T \frac {d\mathcal{E}}{dT}\right) \ , </math>
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| where Δ''H'' is the [[heat of reaction]]. The quantities on the right all are directly measurable.
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| == Electromotive force and voltage difference ==
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| An electrical voltage difference is sometimes called an emf.<ref name=Fogiel>
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| {{cite book |title=Basic Electricity
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| |page=76
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| |url=http://books.google.com/?id=_DapslzANfwC&pg=PA76
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| |author= M. Fogiel
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| |isbn=0-87891-420-X
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| |year=2002
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| |publisher= Research & Education Association
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| }}</ref><ref name=" Halliday">
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| {{cite book
| |
| |title=Fundamentals of Physics
| |
| |edition=6th
| |
| |author = David Halliday, Robert Resnick, and Jearl Walker
| |
| |url=http://books.google.com/?id=VXlEQlznCO0C&pg=PA638
| |
| |page=638
| |
| |isbn= 978-0-471-75801-3
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| |publisher=Wiley
| |
| |year=2008
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| }}</ref><ref name=Freeman>
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| {{cite book
| |
| |title=Fundamentals of Telecommunications
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| |author=Roger L Freeman
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| |url=http://books.google.com/?id=6_yQ-dEGc5wC&pg=PA576
| |
| |page=576 |isbn=0-471-71045-8
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| |publisher=Wiley
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| |year=2005
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| |edition=2nd
| |
| }}</ref><ref name=Croft>
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| {{cite book
| |
| |title=Practical Electricity
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| |year=1917
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| |author=Terrell Croft
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| |page=533
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| |publisher=McGraw-Hill
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| |url=http://books.google.com/?id=zuZMAAAAMAAJ&pg=PA533
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| }}</ref><ref name=Loeb>
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| {{cite book
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| |title=Fundamentals of Electricity and Magnetism
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| |author=Leonard B Loeb
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| |page =86
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| |url=http://books.google.com/?id=zw-3icfx9qAC&pg=PA86
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| |isbn=1-4067-0733-3
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| |publisher=Read Books
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| |edition=Reprint of Wiley 1947 3rd
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| |year=2007
| |
| }}</ref> The points below illustrate the more formal usage, in terms of the distinction between emf and the voltage it generates:
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| # 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 ohmic ''IR'' drop plus the applied electrical voltage is zero. See [[Kirchhoff's circuit laws|Kirchhoff's Law]]). The emf is due solely to the chemistry in the battery that causes charge separation, which in turn creates an electrical voltage that drives the current.
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| # For a circuit consisting of an electrical generator that drives current through a resistor, the emf is due solely to a time-varying magnetic field that generates an electrical voltage that in turn drives the current. (The ohmic ''IR'' drop plus the applied electrical voltage again is zero. See [[Kirchhoff's circuit laws|Kirchhoff's Law]])
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| # 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".
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| #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.<ref name=Nelson>
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| {{cite book |title=The Physics of Solar Cells
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| |author=Jenny Nelson
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| |url=http://books.google.com/?id=s5NN34HLWO8C&pg=PA6
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| |isbn=1-86094-349-7
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| |year=2003
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| |publisher=Imperial College Press
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| |page=6
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| }}</ref>
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| #Other devices that produce emf are [[fuel cell]]s, [[thermocouple]]s, and [[thermopiles]].<ref>John S. Rigden, (editor in chief), ''Macmillan encyclopedia of physics''. New York : Macmillan, 1996.</ref>
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| 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.<ref name=Peters>
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| {{cite book
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| |title=Concise Chemical Thermodynamics
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| |author=J. R. W. Warn, A. P. H. Peters
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| |page=123
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| |url=http://books.google.com/?id=oCTRVcJ1mqYC&pg=PA123
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| |isbn=0-7487-4445-2
| |
| |year=1996
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| |edition=2
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| |publisher=CRC Press
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| }}</ref><ref>
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| {{cite book
| |
| |title=Thermodynamics for Chemists
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| |author=Samuel Glasstone
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| |url=http://books.google.com/?id=oW5XqmTSXyEC&pg=RA1-PA301
| |
| |page=301
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| |isbn=1-4067-7322-0
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| |publisher=Read Books
| |
| |year=2007
| |
| |edition= Reprint of D. Van Nostrand Co (1964)
| |
| }}</ref>
| |
| | |
| The electric charge that has been separated creates an electric [[potential difference]] that 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.<ref name=" Halliday">
| |
| | |
| {{cite book |title=Fundamentals of Physics |edition=6th |author= David Halliday; Robert Resnick; Jearl Walker |url=http://books.google.com/books?id=VXlEQlznCO0C&pg=PA638 |page=638 |isbn= 9780471758013 |publisher=Wiley}}</ref> It can, however, be inferred from a measurement of the current ''I'' and voltage difference ''V'', provided that the internal resistance ''r'' already has been measured: ''ℰ'' = ''V'' + ''Ir''.
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| ==Electromotive force generation==
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| ===Chemical sources===
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| {{Main|Electrochemical cell}}
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| | |
| [[File:Reaction path.JPG|thumb|380px|A typical reaction path requires the initial reactants to cross an energy barrier, enter an intermediate state and finally emerge in a lower energy configuration. If charge separation is involved, this energy difference can result in an emf. See Bergmann ''et al.''<ref name=Bergmann>
| |
| | |
| {{cite book |title=Constituents of Matter: Atoms, Molecules, Nuclei, and Particles |author =Nikolaus Risch |chapter=Molecules - bonds and reactions |editor=L Bergmann ''et al.'' |isbn=0-8493-1202-7 |year=2002 |publisher=CRC Press |url=http://books.google.com/?id=mGj1y1WYflMC&printsec=frontcover#PPA374,M1}}
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| | |
| </ref> and [[Transition state]].]]
| |
| | |
| [[Image:Galvanic cell labeled.svg|thumb|380px|[[Galvanic cell]] using a [[salt bridge]]]]
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| | |
| The question of how batteries ([[galvanic cell]]s) generate an emf is one that occupied scientists for most of the 19th century. The "seat of the electromotive force" was eventually determined by [[Walther Nernst]] to be primarily at the interfaces between the [[electrode]]s and the [[electrolyte]].<ref name=cajori>{{cite book
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| | title = A History of Physics in Its Elementary Branches: Including the Evolution of Physical Laboratories
| |
| | author = Florian Cajori
| |
| | publisher = The Macmillan Company
| |
| | year = 1899
| |
| | pages = 218–219
| |
| | url = http://books.google.com/?id=ICASAAAAYAAJ&pg=PA219&dq=%22seat+of%22+%22electromotive+force%22
| |
| }}</ref>
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| | |
| Molecules are groups of atoms held together by [[chemical bond]]s, and these bonds consist of electrical forces between electrons (negative) and protons (positive). The molecule in isolation is a stable entity, but when different molecules are brought together, some types of molecules are able to steal electrons from others, resulting in charge separation. This redistribution of charge is accompanied by a change in energy of the system, and a reconfiguration of the atoms in the molecules.<ref name=reconfigure>
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| The brave reader can find an extensive discussion for organic electrochemistry in {{cite book |title=Organic electrochemistry |edition=4 |year=2000 |publisher=CRC Press |isbn=0-8247-0430-4 |editor=Henning Lund, Ole Hammerich |url=http://books.google.com/?id=tBxxZclgKyMC&pg=PA23 |author=Christian Amatore |chapter=Basic concepts}}
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| | |
| </ref> The gain of an electron is termed "reduction" and the loss of an electron is termed "oxidation". Reactions in which such electron exchange occurs (which are the basis for batteries) are called [[redox|reduction-oxidation reactions]] or [[redox|redox reactions]]. In a battery, one electrode is composed of material that gains electrons from the solute, and the other electrode loses electrons, because of these fundamental molecular attributes. The same behavior can be seen in atoms themselves, and their ability to steal electrons is referred to as their [[electronegativity]].<ref name=electronegativity>
| |
| | |
| The idea of electronegativity has been extended to include the concept of ''electronegativity equalization'', the notion that when molecules are brought together the electrons rearrange to achieve an equilibrium where there is no net force upon them. See, for example, {{cite book |title=Advanced organic chemistry |author= Francis A. Carey, Richard J. Sundberg |isbn=0-387-68346-1 |edition=5 |year=2007 |publisher=Springer |page=11 |url=http://books.google.com/?id=g5dYyJMBhCoC&pg=PA11}}
| |
| | |
| </ref>
| |
| | |
| As an example, a [[Daniell cell]] consists of a zinc anode (an electron collector), which dissolves into a zinc sulfate solution, the dissolving zinc leaving behind its electrons in the electrode according to the oxidation reaction (''s'' = solid electrode; ''aq'' = aqueous solution):
| |
| | |
| :<math>\mathrm{Zn_{(s)} \rightarrow Zn^{2+}_{(aq)} + 2 e ^- \ }. </math>
| |
| | |
| The zinc sulfate is an [[electrolyte]], that is, a solution in which the components consist of ions, in this case zinc ions <math>\mathrm{Zn}_{} ^{2+}</math>, and sulfate ions <math>\mathrm{SO}_4^{2-}\ </math>. | |
| | |
| At the cathode, the copper ions in a copper sulfate electrolyte adopt electrons from the electrode by the reduction reaction:
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| | |
| :<math> \mathrm{Cu^{2+}_{(aq)} + 2 e^- \rightarrow Cu_{(s)}\ }, </math>
| |
| | |
| and the thus-neutralized copper plates onto the electrode. (A detailed discussion of the microscopic process of electron transfer between an electrode and the ions in an electrolyte may be found in Conway.)<ref name=Conway>
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| {{cite book |title=Electrochemical supercapacitors |author=BE Conway |chapter=Energy factors in relation to electrode potential |page=37 |url=http://books.google.com/?id=8yvzlr9TqI0C&pg=PA37 |isbn=0-306-45736-9 |year=1999 |publisher=Springer}}
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| | |
| </ref>
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| | |
| The electrons pass through the external circuit (light bulb in figure), while the ions pass through the salt bridge to maintain charge balance. In the process the zinc anode is dissolved while the copper electrode is plated with copper.<ref name= Tilley>
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| | |
| {{cite book |title=Understanding Solids |url=http://books.google.com/?id=ZVgOLCXNoMoC&pg=PA267 |page=267 |author=R. J. D. Tilley |isbn=0-470-85275-5 |year=2004 |publisher=Wiley}}
| |
| | |
| </ref> If the light bulb is removed (open circuit) the emf between the electrodes is opposed by the electric field due to charge separation, and the reactions stop.
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| | |
| At 273 K, the emf ''ℰ'' = 1.0934 V, with a temperature coefficient of d''ℰ''/d''T'' = −4.53×10<sup>−4</sup> V/K.<ref name= Finn>
| |
| | |
| {{cite book |title=Thermal Physics |author=Colin B P Finn |page=163 |url=http://books.google.com/?id=BTMPThGxXQ0C&pg=PA162 |isbn=0-7487-4379-0 |year=1992 |publisher=CRC Press}}
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| | |
| </ref>
| |
| | |
| ====Voltaic cells====
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| Volta developed the voltaic cell about 1792, and presented his work March 20, 1800.<ref name=Mottelay>
| |
| | |
| {{cite book |title=Bibliographical History of Electricity and Magnetism |author=Paul Fleury Mottelay |page=247 |url=http://books.google.com/?id=9vzti90Q8i0C&pg=RA1-PA247 |isbn=1-4437-2844-6 |publisher=Read Books |year=2008 |edition=Reprint of 1892}}
| |
| | |
| </ref> Volta correctly identified the role of dissimilar electrodes in producing the voltage, but incorrectly dismissed any role for the electrolyte.<ref name=Kragh>
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| {{cite journal |journal=Nuova Voltiana:Studies on Volta and his times |publisher=Università degli studi di Pavia |year=2000 |url=http://ppp.unipv.it/Collana/Pages/Libri/Saggi/NuovaVoltiana_PDF/sei.pdf |title=Confusion and Controversy: Nineteenth-century theories of the voltaic pile |author=Helge Kragh }}
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| | |
| </ref> Volta 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.”<ref name=Cumming>
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| | |
| {{cite book |title=An Introduction to the Theory of Electricity |author=Linnaus Cumming |url=http://books.google.com/?id=Nrb8723u4WEC&pg=PA118 |page=118 |isbn=0-559-20742-5 |publisher=BiblioBazaar |year=2008 |edition=Reprint of 1885}}
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| | |
| </ref> A typical symbolic convention in a schematic of this circuit ( –<big>|</big>'''<small>|</small>'''– ) 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 (for example, zinc and nine other materials), which can be used to produce 45 types of voltaic cells (10 × 9/2), only nine relative measurements (for example, copper and each of the nine others) are needed to get all 45 possible emfs that these ten electrodes can produce.{{Citation needed|date=June 2009}} | |
| | |
| ====Electromotive force of cells====
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| 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.
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| | |
| {| class=wikitable
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| ! Emf !! Cell chemistry
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| |-
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| | 1.2 V || [[Nickel–cadmium battery|nickel-cadmium]]
| |
| |-
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| | 1.2 V || [[Nickel–metal hydride battery|nickel–metal hydride]]
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| |-
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| | 1.5 V || [[Zinc–carbon cell|zinc–carbon]]
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| |-
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| | 2.1 V || [[Lead–acid battery|lead–acid]]
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| |-
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| | 3.6 V to 3.7 V || [[lithium-ion battery|lithium-ion]]
| |
| |}
| |
| | |
| ===Electromagnetic induction===
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| {{Main|Faraday's law of induction}}
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| | |
| The principle of [[electromagnetic induction]], noted above, states that a time-dependent magnetic field produces a circulating electric 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 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.
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| | |
| 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 [[Faraday's law of induction]].
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| An emf is induced in a coil or conductor whenever there is change in the [[flux linkage]]s. 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===
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| {{See also|Volta potential|Electrochemical potential}}
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| When two different solids are in contact, it is common that [[thermodynamic equilibrium]] requires one of the solids assume a higher electrical potential than the other, the ''contact potential''.<ref name=Trigg>
| |
| | |
| {{cite book |title=Landmark experiments in twentieth century physics |author=George L. Trigg |page=138 ''ff'' |url=http://books.google.com/?id=YOQ9fi5yQ4sC&pg=PA138 |isbn=0-486-28526-X |year=1995 |publisher=Courier Dover |edition=Reprint of Crane, Russak & Co 1975}}
| |
| | |
| </ref> For example, dissimilar metals in contact produce what is known also as a [[contact electromotive force]] or [[Galvani potential]]. The magnitude of this potential difference often is expressed as a difference in [[Fermi level]]s in the two solids at charge neutrality, where the Fermi level (a name for the [[chemical potential]] of an electron system<ref name=Rockett>
| |
| | |
| {{cite book |title=Materials science of semiconductors |author=Angus Rockett |chapter=Diffusion and drift of carriers |page=74 ''ff'' |url=http://books.google.com/?id=n5zMiMfw6ZUC&pg=PA74 |isbn=0-387-25653-9 |year=2007 |publisher=Springer Science |location=New York, NY}}
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| | |
| </ref><ref name=Kittel>
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| | |
| {{cite book |title=Elementary Statistical Physics |author=Charles Kittel |url=http://books.google.com/?id=5sd9SAoRjgQC&pg=PA67 |chapter= Chemical potential in external fields |page=67 |isbn=0-486-43514-8 |publisher=Courier Dover |year=2004 |edition=Reprint of Wiley 1958}}
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| | |
| </ref>) describes the energy necessary to remove an electron from the body to some common point (such as ground).<ref name=Hanson>
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| | |
| {{cite book |title=Fundamentals of Nanoelectronics |author=George W. Hanson |page=100 |url=http://books.google.com/?id=L7AUi7ltCksC&pg=PA100 |isbn=0-13-195708-2 |year=2007 |publisher=Prentice Hall}}
| |
| | |
| </ref> Evidently, if there is an energy advantage in taking an electron from one body to the other, then such a transfer will occur. 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, and therefore, charge transfer becomes more difficult as the charge separation increases. At thermodynamic equilibrium, the [[Fermi level]]s are equal (the electron removal energy is identical) and there is now a built-in electrostatic potential between the bodies.
| |
| The original difference in Fermi levels, before contact, is referred to as the emf.<ref name=Sato>
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| | |
| {{cite book |title=Electrochemistry at metal and semiconductor electrodes |author=Norio Sato |page=110 ''ff'' |url=http://books.google.com/?id=olQzaXNgM74C&pg=PA110 |isbn=0-444-82806-0 |year=1998 |publisher=Elsevier |edition=2nd |chapter= Semiconductor photoelectrodes}}
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| | |
| </ref>
| |
| 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.
| |
| | |
| One might inquire why the contact potential does not appear in [[Kirchhoff's circuit laws|Kirchhoff's law of voltages]] as one contribution to the sum of potential drops. The customary answer is that any circuit involves not only a particular diode or junction, but also all the contact potentials due to wiring and so forth around the entire circuit. The sum of ''all'' the contact potentials is zero, and so they may be ignored in Kirchhoff's law.<ref name=Quimby>
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| | |
| {{cite book |title=Photonics and lasers |author=Richard S. Quimby |page=176 |url=http://books.google.com/?id=82f-gIvtC7wC&pg=PA176 |isbn=0-471-71974-9 |publisher=Wiley |year=2006}}</ref><ref name=Neamen>
| |
| | |
| {{cite book |title=Semiconductor physics and devices |author=Donald A. Neamen |url=http://books.google.com/?id=9oEifMuMAVsC&pg=PA240 |page=240 |year=2002 |isbn=0-07-232107-5 |publisher=McGraw-Hill Professional |edition=3rd}}
| |
| | |
| </ref>
| |
| | |
| ===Solar cell===
| |
| {{Main|Theory of solar cells}}
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| | |
| [[Image:Solar cell equivalent circuit.svg|thumb|250px |The equivalent circuit of a solar cell; parasitic resistances are ignored in the discussion of the text.]]
| |
| | |
| [[File:Solar cell characterisitcs.JPG|thumb |250px |Solar cell voltage as a function of solar cell current delivered to a load for two light-induced currents ''I''<sub>L</sub>; currents as a ratio with reverse saturation current ''I''<sub>0</sub>. Compare with Fig. 1.4 in Nelson.<ref name=J_Nelson/>]]
| |
| | |
| Operation of a solar cell can be understood from the equivalent circuit at right. Light, if it includes [[photons]] of sufficient energy (greater than the [[bandgap]] of the material), creates mobile [[electron–hole pair]]s in a semiconductor. Charge separation occurs because of a pre-existing electric field associated with the p-n junction in thermal equilibrium (a [[Volta potential|contact potential]] creates the field). This charge separation between positive [[Electron hole|hole]]s and negative [[electron]]s across a [[p-n junction]] (a [[diode]]), yields a ''forward voltage'', the ''photo voltage'', between the illuminated diode terminals.<ref name=Dhir>
| |
| {{cite book
| |
| | title=Electronic Components and Materials: Principles, Manufacture and Maintenance
| |
| | author=S M Dhir
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| | url=http://books.google.com/?id=sGbwj4J76tEC&pg=PA283
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| | chapter=§3.1 Solar cells |publisher=Tata McGraw-Hill
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| | year=2000
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| | isbn=0-07-463082-2
| |
| }}</ref> As has been noted earlier in the [[#Terminology|terminology]] section, the photo ''voltage'' is sometimes referred to as the photo ''emf'', rather than distinguishing between the effect and the cause.
| |
| | |
| The light-induced charge separation creates a reverse current through the cell's junction (that is, not in the direction that a diode normally conducts current), and the charge separation causes a photo voltage that drives current through any attached load. However, a side effect of this voltage is that it tends to [[p-n junction#Forward bias|forward bias]] the junction. At high enough levels, this forward bias of the junction will cause a forward current in the diode that subtracts from the current created by the light. Consequently, the greatest current is obtained under short-circuit conditions, and is denoted as ''I''<sub>L</sub> (for light-induced current) in the equivalent circuit.<ref name=Lorenzo>
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| {{cite book
| |
| |title=Solar Electricity: Engineering of photovoltaic systems
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| |editor=Eduardo Lorenzo
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| |author= Gerardo L. Araújo
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| |url=http://books.google.com/?id=lYc53xZyxZQC&pg=PA74
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| |chapter=§2.5.1 Short-circuit current and open-circuit voltage
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| |isbn=84-86505-55-0
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| |year=1994
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| |page=74
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| |publisher=Progenza for Universidad Politechnica Madrid
| |
| }}</ref>
| |
| Approximately this same current is obtained for forward voltages up to the point where the diode conduction becomes significant.
| |
| | |
| With this notation, the current-voltage relation for the illuminated diode is:
| |
| | |
| :<math>I = I_L -I_0 \left( e^{qV/(mkT)} - 1 \right) \ , </math>
| |
| | |
| where ''I'' is the current delivered to the load, ''I''<sub>0</sub> is the reverse saturation current, and ''m'' the ideality factor, two parameters that depend on the solar cell construction and to some degree upon the voltage itself,<ref name= Lorenzo/> and where ''kT/q'' is the [[thermal voltage]] (about 0.026 V at room temperature). This relation is plotted in the figure using a fixed value ''m'' = 2.<ref name =params>
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| | |
| In practice, at low voltages ''m'' → 2, whereas at high voltages ''m'' → 1. See Araújo, ''op. cit.'' isbn = 84-86505-55-0. [http://books.google.com/books?id=lYc53xZyxZQC&pg=PA72 page 72]
| |
| | |
| </ref> Under open-circuit conditions (that is, as ''I'' → 0), the open-circuit voltage is the voltage at which forward bias of the junction is enough that the forward current completely balances the photocurrent. Rearrangement of the [[I–V curve|''I–V'' equation]] provides the open-circuit voltage as:
| |
| | |
| :<math>V_\text{oc} = m\ \frac{kT}{q}\ \ln \left( \frac{I_\text{L}}{I_0}+1 \right) \ , </math>
| |
| | |
| which is useful in indicating a logarithmic dependence of ''V''<sub>oc</sub> upon the light-induced current. Typically, the open-circuit voltage is not more than about 0.5 V.<ref name=Northrop>
| |
| {{cite book
| |
| | title=Introduction to Instrumentation and Measurements
| |
| | author=Robert B. Northrop
| |
| | page=176
| |
| | chapter=§6.3.2 Photovoltaic Cells
| |
| | url= http://books.google.com/?id=mcpcfpQfxB4C&pg=PA176
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| | isbn=0-8493-7898-2 |year=2005 |publisher=CRC Press
| |
| }}</ref>
| |
| | |
| The value of the photo voltage when driving a load is variable. As shown in the figure, for a load resistance ''R''<sub>L</sub>, the cell develops a voltage between the short-circuit value ''V'' = 0, ''I'' = ''I''<sub>L</sub> and the open-circuit value ''V''<sub>oc</sub>, ''I'' = 0, a value given by [[Ohm's law]] ''V = I R''<sub>L</sub>, where the current ''I'' is the difference between the short-circuit current and current due to forward bias of the junction, as indicated by the equivalent circuit (neglecting the [[Parasitic element (electrical networks)|parasitic resistances]]).<ref name=J_Nelson/>
| |
| | |
| In contrast to the battery, at current levels near ''I''<sub>L</sub>, the solar cell acts more like a ''current source'' rather than a voltage source.<ref name=J_Nelson>
| |
| {{cite book
| |
| |author=Jenny Nelson
| |
| |title=Solar cells
| |
| |url=http://books.google.com/?id=s5NN34HLWO8C&pg=PA8
| |
| |page=8
| |
| |isbn=1-86094-349-7
| |
| |year=2003
| |
| |publisher=Imperial College Press
| |
| }}</ref>
| |
| The current drawn is nearly fixed over a range of load voltages, at one electron per converted [[photon]]. The [[quantum efficiency]], or probability of getting an electron of photocurrent per incident photon, depends not only upon the solar cell itself, but upon the spectrum of the light.
| |
| | |
| The diode possesses a "[[p–n junction#Equilibrium (zero bias)|built-in potential]]" due to the contact potential difference between the two different materials on either side of the junction. This built-in potential is established when the junction is formed as a by-product of thermodynamic equilibrium. Once established, this potential difference cannot drive a current, however, as connecting a load does not upset this equilibrium. In contrast, the accumulation of excess electrons in one region and of excess holes in another due to illumination results in a photo voltage that does drive a current when a load is attached to the illuminated diode. As noted above, this photo voltage also forward biases the junction, and so ''reduces'' the pre-existing field in the [[depletion region]].
| |
| | |
| ==See also==
| |
| *[[Counter-electromotive force]]
| |
| *[[Electric battery]]
| |
| *[[Electrochemical cell]]
| |
| *[[Electrolytic cell]]
| |
| *[[Galvanic cell]]
| |
| *[[Voltaic pile]]
| |
| | |
| ==References==
| |
| {{reflist|2}}
| |
| | |
| ==Further reading==
| |
| * Andrew Gray, "Absolute Measurements in Electricity and Magnetism", [http://books.google.com/books?vid=0pkd5YYtaGRtjR6Oes&id=WxeFSg38JLQC&pg=PA41&dq= Electromotive force]. Macmillan and co., 1884.
| |
| *{{cite book |title=Modern Electrochemistry: An Introduction to an Interdisciplinary Area |author=John O'M. Bockris, Amulya K. N. Reddy |url=http://books.google.com/?id=5OGsg_v_7yoC&pg=PA647 |chapter=Electrodics |isbn=0-306-25002-0 |year=1973 |edition=2 |publisher=Springer}}
| |
| *{{cite journal |author=Roberts, Dana |title= How batteries work: A gravitational analog |journal= Am. J. Phys.|volume= 51 |page= 829 |year=1983 |doi=10.1119/1.13128|bibcode = 1983AmJPh..51..829R }}
| |
| * Charles Albert Perkins, "Outlines of Electricity and Magnetism", [http://books.google.com/books?vid=OCLC02316583&id=jd1vvFcD-MAC&pg=PA158&vq=&dq= Measurement of Electromotive Force]. Henry Holt and co., 1896.
| |
| * John Livingston Rutgers Morgan, "The Elements of Physical Chemistry", [http://books.google.com/books?vid=OCLC10759094&id=ovjORAvJZZkC&pg=PA235&dq= Electromotive force]. J. Wiley, 1899.
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| * George F. Barker, "[http://books.google.com/books?vid=0uowlltB5bCx84xQrO&id=zXAQ97d7YiIC&pg=PA649&lpg=PA650 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.
| |
| * {{cite book |author=Norio Sato |title=Electrochemistry at metal and semiconductor electrodes |page=326 ''ff'' |url=http://books.google.com/?id=olQzaXNgM74C&pg=PA328 |isbn=0-444-82806-0 |year=1998 |publisher=Elsevier |edition=2nd |chapter= Semiconductor photoelectrodes}}
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| | |
| ==External links==
| |
| * [http://www.magnet.fsu.edu/education/tutorials/java/backemf/index.html Electromotive Force in Inductors - Interactive Java Tutorial] National High Magnetic Field Laboratory
| |
| * {{cite journal|date=2009-03-08|title=Electromotive force and huge magnetoresistance in magnetic tunnel junctions|journal=[[Nature (journal)|Nature]]|url=http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature07879.html|accessdate=2009-03-10| doi= 10.1038/nature07879|last1=Hai|first1=Pham Nam|last2=Ohya|first2=Shinobu|last3=Tanaka|first3=Masaaki|last4=Barnes|first4=Stewart E.|last5=Maekawa|first5=Sadamichi|volume=458|pages=489|pmid=19270681|issue=7237|bibcode = 2009Natur.458..489H }}
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| {{DEFAULTSORT:Electromotive Force}}
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| [[Category:Electromagnetism]]
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| [[Category:Electrodynamics]]
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