Saturday, August 2, 2014

EMF: multiple definitions

Though EMF has multiple definitions as you will see below, I was interested primarily in the electrical definitions.

EMF

From Wikipedia, the free encyclopedia
EMF may stand for:

Music

Organizations

Science and technology

end quote from:
http://en.wikipedia.org/wiki/EMF

Here are the two definitions of EMF I was interested in:

Electromotive force, the origin of voltage in electrical devices

Electromagnetic field, a physical field

Electromotive force

From Wikipedia, the free encyclopedia
Not to be confused with Electromagnetic field.
Electromagnetism
Solenoid
Electromotive force, also called emf[1] (denoted \mathcal{E} and measured in volts), is the voltage developed by any source of electrical energy such as a battery or dynamo.[2]
The word "force" in this case is not used to mean mechanical force, measured in newtons, but a potential, or energy per unit of charge, measured in volts.
In electromagnetic induction, emf can be defined around a closed loop as the electromagnetic work that would be transferred to a unit of charge if it travels once around that loop.[3] (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.[4]
In a two-terminal device (such as an electrochemical cell or electromagnetic generator), the emf can be measured as the open-circuit potential difference across the two terminals. The potential difference thus created drives current flow if an external circuit is attached to the source of emf. When current flows, however, the potential difference across the terminals is no longer equal to the emf, but will be smaller because of the voltage drop within the device due to its internal resistance.
Devices that can provide emf include electrochemical cells, thermoelectric devices, solar cells and photodiodes, electrical generators, transformers, and even Van de Graaff generators.[4][5] 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.
In the case of a battery, charge separation that gives rise to a voltage difference is accomplished by chemical reactions at the electrodes.[6] Chemically, by separating positive and negative charges, an electric field can be produced, leading to an electric potential difference.[6][7] A voltaic cell can be thought of as having a "charge pump" of atomic dimensions at each electrode, that is:[8]
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.
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.[9] 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 the fact of contact alone (without taking into account a chemical reaction) was the origin of the emf.
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.

Notation and units of measurement

Electromotive force is often denoted by \mathcal{E} or ℰ (script capital E, Unicode U+2130).
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 charge, or W/Q. Like other measures of energy per charge, emf has SI units of volts, equivalent to joules per coulomb.[10]
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).

Formal definitions of electromotive force

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).[11] Mathematically:
\mathcal{E} = -\int_{A}^{B} \boldsymbol{E_{cs} \cdot } d \boldsymbol{ \ell } \ ,
where Ecs is the conservative electrostatic field created by the charge separation associated with the emf, dℓ is an element of the path from terminal A to terminal B, and ‘·’ denotes the vector dot product.[12] 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 Ecs and does not involve (for example) any non-conservative component of electric field due to Faraday's law of induction.
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.[13] The "induced emf" around a stationary closed path C is:
\mathcal{E}=\oint_{C} \boldsymbol{E \cdot } d \boldsymbol{ \ell } \ ,
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. The electrostatic field does not contribute to the net emf around a circuit because the electrostatic portion of the electric field is conservative (that is, the work done against the field around a closed path is zero).
This definition can be extended to arbitrary sources of emf and moving paths C:[14]
\mathcal{E}=\oint_{C}\boldsymbol{ \left[E + v \times B \right] \cdot } d \boldsymbol{ \ell } \
+\frac{1}{q}\oint_{C}\mathrm {\mathbf{effective \ chemical \ forces \ \cdot}} \ d \boldsymbol{ \ell } \
+\frac{1}{q}\oint_{C}\mathrm {\mathbf { effective \ thermal \ forces\ \cdot}}\ d \boldsymbol{ \ell } \ ,
which is a conceptual equation mainly, because the determination of the "effective forces" is difficult.

Electromotive force in thermodynamics

When multiplied by an amount of charge dZ the emf ℰ yields a thermodynamic work term ℰdZ that is used in the formalism for the change in Gibbs free energy when charge is passed in a battery:
dG = -SdT + VdP + \mathcal{E}dZ\ ,
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.
The combination ( ℰ, Z ) is an example of a 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:[15]
\left(\frac{\partial \mathcal{E}}{\partial T}\right)_Z= -\left(\frac{\partial S}{\partial Z}\right)_T
If a mole of ions goes into solution (for example, in a Daniell cell, as discussed below) the charge through the external circuit is:
\Delta Z = -n_0F_0 \ ,
where n0 is the number of electrons/ion, and F0 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:[15]
\Delta H = -n_0 F_0 \left( \mathcal{E} - T \frac {d\mathcal{E}}{dT}\right) \ ,
where ΔH is the heat of reaction. The quantities on the right all are directly measurable.

Electromotive force and voltage difference

An electrical voltage difference is sometimes called an emf.[16][17][18][19][20] 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 ohmic IR drop plus the applied electrical voltage is zero. See 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.
  2. 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 Law)
  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.[21]
  5. Other devices that produce emf are fuel cells, thermocouples, and thermopiles.[22]
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.[23][24]
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.[17] 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.

Electromotive force generation

Chemical sources

Main article: Electrochemical cell
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.[25] and Transition state.
Galvanic cell using a salt bridge
The question of how batteries (galvanic cells) 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 electrodes and the electrolyte.[9]
Molecules are groups of atoms held together by chemical bonds, 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.[26] 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 reduction-oxidation reactions or 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.[27]
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):
\mathrm{Zn_{(s)} \rightarrow Zn^{2+}_{(aq)} + 2 e ^- \ }
The zinc sulfate is an electrolyte, that is, a solution in which the components consist of ions, in this case zinc ions \mathrm{Zn}_{} ^{2+}, and sulfate ions \mathrm{SO}_4^{2-}\ .
At the cathode, the copper ions in a copper sulfate electrolyte adopt electrons from the electrode by the reduction reaction:
\mathrm{Cu^{2+}_{(aq)} + 2 e^- \rightarrow Cu_{(s)}\ }
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.)[28]
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.[29] 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.
At 273 K, the emf ℰ = 1.0934 V, with a temperature coefficient of dℰ/dT = −4.53×10−4 V/K.[15]

Voltaic cells

Volta developed the voltaic cell about 1792, and presented his work March 20, 1800.[30] Volta correctly identified the role of dissimilar electrodes in producing the voltage, but incorrectly dismissed any role for the electrolyte.[31] 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.”[32] 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. 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]

Electromotive force of cells

The electromotive force produced by primary (single-use) and secondary (rechargeable) 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
1.2 V nickel–metal hydride
1.5 V zinc–carbon
2.1 V lead–acid
3.6 V to 3.7 V lithium-ion

Electromagnetic induction

Main article: Faraday's law of induction
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.
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.
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

See also: Volta potential and Electrochemical potential
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.[33] 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 levels in the two solids at charge neutrality, where the Fermi level (a name for the chemical potential of an electron system[34][35]) describes the energy necessary to remove an electron from the body to some common point (such as ground).[36] 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, which partly cancels the potential coming from the contact, and therefore, charge transfer becomes more difficult as the charge separation increases. At thermodynamic equilibrium, the Fermi levels 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.[37] 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 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.[38][39]

Solar cell

Main article: Theory of solar cells
The equivalent circuit of a solar cell; parasitic resistances are ignored in the discussion of the text.
Solar cell voltage as a function of solar cell current delivered to a load for two light-induced currents IL; currents as a ratio with reverse saturation current I0. Compare with Fig. 1.4 in Nelson.[40]
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 pairs in a semiconductor. Charge separation occurs because of a pre-existing electric field associated with the p-n junction in thermal equilibrium (a contact potential creates the field). This charge separation between positive holes and negative electrons across a p-n junction (a diode), yields a forward voltage, the photo voltage, between the illuminated diode terminals.[41] As has been noted earlier in the 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 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 IL (for light-induced current) in the equivalent circuit.[42] 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:
I = I_L -I_0 \left( e^{qV/(mkT)} - 1 \right) \ ,
where I is the current delivered to the load, I0 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,[42] 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.[43] 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 equation provides the open-circuit voltage as:
V_\text{oc} = m\ \frac{kT}{q}\ \ln \left( \frac{I_\text{L}}{I_0}+1 \right) \ ,
which is useful in indicating a logarithmic dependence of Voc upon the light-induced current. Typically, the open-circuit voltage is not more than about 0.5 V.[44]
The value of the photo voltage when driving a load is variable. As shown in the figure, for a load resistance RL, the cell develops a voltage between the short-circuit value V = 0, I = IL and the open-circuit value Voc, I = 0, a value given by Ohm's law V = I RL, 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 resistances).[40]
In contrast to the battery, at current levels near IL, the solar cell acts more like a current source rather than a voltage source.[40] 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 "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

References

  1. emf. (1992). American Heritage Dictionary of the English Language 3rd ed. Boston:Houghton Mifflin.
  2. Irving Langmuir (1916). "The Relation Between Contact Potentials and Electrochemical Action". Transactions of the American Electrochemical Society (The Society) 29: 125–182.
  3. David M. Cook (2003). The Theory of the Electromagnetic Field. Courier Dover. p. 157. ISBN 978-0-486-42567-2.
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  12. 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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  26. The brave reader can find an extensive discussion for organic electrochemistry in Christian Amatore (2000). "Basic concepts". In Henning Lund, Ole Hammerich. Organic electrochemistry (4 ed.). CRC Press. ISBN 0-8247-0430-4.
  27. 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, Francis A. Carey, Richard J. Sundberg (2007). Advanced organic chemistry (5 ed.). Springer. p. 11. ISBN 0-387-68346-1.
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  31. Helge Kragh (2000). "Confusion and Controversy: Nineteenth-century theories of the voltaic pile". Nuova Voltiana:Studies on Volta and his times (Università degli studi di Pavia).
  32. Linnaus Cumming (2008). An Introduction to the Theory of Electricity (Reprint of 1885 ed.). BiblioBazaar. p. 118. ISBN 0-559-20742-5.
  33. George L. Trigg (1995). Landmark experiments in twentieth century physics (Reprint of Crane, Russak & Co 1975 ed.). Courier Dover. p. 138 ff. ISBN 0-486-28526-X.
  34. Angus Rockett (2007). "Diffusion and drift of carriers". Materials science of semiconductors. New York, NY: Springer Science. p. 74 ff. ISBN 0-387-25653-9.
  35. Charles Kittel (2004). "Chemical potential in external fields". Elementary Statistical Physics (Reprint of Wiley 1958 ed.). Courier Dover. p. 67. ISBN 0-486-43514-8.
  36. George W. Hanson (2007). Fundamentals of Nanoelectronics. Prentice Hall. p. 100. ISBN 0-13-195708-2.
  37. Norio Sato (1998). "Semiconductor photoelectrodes". Electrochemistry at metal and semiconductor electrodes (2nd ed.). Elsevier. p. 110 ff. ISBN 0-444-82806-0.
  38. Richard S. Quimby (2006). Photonics and lasers. Wiley. p. 176. ISBN 0-471-71974-9.
  39. Donald A. Neamen (2002). Semiconductor physics and devices (3rd ed.). McGraw-Hill Professional. p. 240. ISBN 0-07-232107-5.
  40. Jenny Nelson (2003). Solar cells. Imperial College Press. p. 8. ISBN 1-86094-349-7.
  41. S M Dhir (2000). "§3.1 Solar cells". Electronic Components and Materials: Principles, Manufacture and Maintenance. Tata McGraw-Hill. ISBN 0-07-463082-2.
  42. Gerardo L. Araújo (1994). "§2.5.1 Short-circuit current and open-circuit voltage". In Eduardo Lorenzo. Solar Electricity: Engineering of photovoltaic systems. Progenza for Universidad Politechnica Madrid. p. 74. ISBN 84-86505-55-0.
  43. In practice, at low voltages m → 2, whereas at high voltages m → 1. See Araújo, op. cit. isbn = 84-86505-55-0. page 72
  44. Robert B. Northrop (2005). "§6.3.2 Photovoltaic Cells". Introduction to Instrumentation and Measurements. CRC Press. p. 176. ISBN 0-8493-7898-2.

Further reading

  • Andrew Gray, "Absolute Measurements in Electricity and Magnetism", Electromotive force. Macmillan and co., 1884.
  • John O'M. Bockris, Amulya K. N. Reddy (1973). "Electrodics". Modern Electrochemistry: An Introduction to an Interdisciplinary Area (2 ed.). Springer. ISBN 0-306-25002-0.
  • Roberts, Dana (1983). "How batteries work: A gravitational analog". Am. J. Phys. 51: 829. Bibcode:1983AmJPh..51..829R. doi:10.1119/1.13128.
  • 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 20-20413
  • Hazel Rossotti, "Chemical applications of potentiometry". London, Princeton, N.J., Van Nostrand, 1969. ISBN 0-442-07048-9 LCCN 69-11985 //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 07-3935 //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.
  • Norio Sato (1998). "Semiconductor photoelectrodes". Electrochemistry at metal and semiconductor electrodes (2nd ed.). Elsevier. p. 326 ff. ISBN 0-444-82806-0.

External links

This page was last modified on 8 July 2014 at 18:27.
end quote from:
Electromotive force,

and


Electromagnetic field, a physical field

Electromagnetic field

From Wikipedia, the free encyclopedia
Electromagnetism
Solenoid
An electromagnetic field (also EMF or EM field) is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces of nature (the others are gravitation, weak interaction and strong interaction).
The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law.
From a classical perspective in the history of electromagnetism, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner; whereas from the perspective of quantum field theory, the field is seen as quantized, being composed of individual particles.[citation needed]

Structure of the electromagnetic field

The electromagnetic field may be viewed in two distinct ways: a continuous structure or a discrete structure.

Continuous structure

Classically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a 'smooth', continuous, wavelike fashion. In this case, energy is viewed as being transferred continuously through the electromagnetic field between any two locations. For instance, the metal atoms in a radio transmitter appear to transfer energy continuously. This view is useful to a certain extent (radiation of low frequency), but problems are found at high frequencies (see ultraviolet catastrophe).

Discrete structure

The electromagnetic field may be thought of in a more 'coarse' way. Experiments reveal that in some circumstances electromagnetic energy transfer is better described as being carried in the form of packets called quanta (in this case, photons) with a fixed frequency. Planck's relation links the energy E of a photon to its frequency ν through the equation:[1]
E= \, h \, \nu
where h is Planck's constant, named in honor of Max Planck, and ν is the frequency of the photon . Although modern quantum optics tells us that there also is a semi-classical explanation of the photoelectric effect—the emission of electrons from metallic surfaces subjected to electromagnetic radiation—the photon was historically (although not strictly necessarily) used to explain certain observations. It is found that increasing the intensity of the incident radiation (so long as one remains in the linear regime) increases only the number of electrons ejected, and has almost no effect on the energy distribution of their ejection. Only the frequency of the radiation is relevant to the energy of the ejected electrons.
This quantum picture of the electromagnetic field (which treats it as analogous to harmonic oscillators) has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter. It also gives rise to quantum optics, which is different from quantum electrodynamics in that the matter itself is modelled using quantum mechanics rather than quantum field theory.

Dynamics of the electromagnetic field

In the past, electrically charged objects were thought to produce two different, unrelated types of field associated with their charge property. An electric field is produced when the charge is stationary with respect to an observer measuring the properties of the charge, and a magnetic field (as well as an electric field) is produced when the charge moves (creating an electric current) with respect to this observer. Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole — the electromagnetic field. Recall that "until 1831 electricity and magnetism had been viewed as unrelated phenomena. In 1831, Michael Faraday, one of the great thinkers of his time, made the seminal observation that time-varying magnetic fields could induce electric currents and then, in 1864, James Clerk Maxwell published his famous paper on a dynamical theory of the electromagnetic field. See Maxwell 1864 5, page 499; also David J. Griffiths (1999), Introduction to electrodynamics, third Edition, ed. Prentice Hall, pp. 559-562"(as quoted in Gabriela, 2009).
Once this electromagnetic field has been produced from a given charge distribution, other charged objects in this field will experience a force (in a similar way that planets experience a force in the gravitational field of the Sun). If these other charges and currents are comparable in size to the sources producing the above electromagnetic field, then a new net electromagnetic field will be produced. Thus, the electromagnetic field may be viewed as a dynamic entity that causes other charges and currents to move, and which is also affected by them. These interactions are described by Maxwell's equations and the Lorentz force law. (This discussion ignores the radiation reaction force.)

Electromagnetic field as a feedback loop

The behavior of the electromagnetic field can be resolved into four different parts of a loop:
  • the electric and magnetic fields are generated by electric charges,
  • the electric and magnetic fields interact with each other,
  • the electric and magnetic fields produce forces on electric charges,
  • the electric charges move in space.
A common misunderstanding is that (a) the quanta of the fields act in the same manner as (b) the charged particles that generate the fields. In our everyday world, charged particles, such as electrons, move slowly through matter, typically on the order of a few inches (or centimeters) per second[citation needed], but fields propagate at the speed of light - approximately 300 thousand kilometers (or 186 thousand miles) a second. The mundane speed difference between charged particles and field quanta is on the order of one to a million, more or less. Maxwell's equations relate (a) the presence and movement of charged particles with (b) the generation of fields. Those fields can then affect the force on, and can then move other slowly moving charged particles. Charged particles can move at relativistic speeds nearing field propagation speeds, but, as Einstein showed[citation needed], this requires enormous field energies, which are not present in our everyday experiences with electricity, magnetism, matter, and time and space.
The feedback loop can be summarized in a list, including phenomena belonging to each part of the loop:
  • charged particles generate electric and magnetic fields
  • the fields interact with each other
    • changing electric field acts like a current, generating 'vortex' of magnetic field
    • Faraday induction: changing magnetic field induces (negative) vortex of electric field
    • Lenz's law: negative feedback loop between electric and magnetic fields
  • fields act upon particles
    • Lorentz force: force due to electromagnetic field
      • electric force: same direction as electric field
      • magnetic force: perpendicular both to magnetic field and to velocity of charge
  • particles move
    • current is movement of particles
  • particles generate more electric and magnetic fields; cycle repeats

Mathematical description

Main article: Mathematical descriptions of the electromagnetic field
There are different mathematical ways of representing the electromagnetic field. The first one views the electric and magnetic fields as three-dimensional vector fields. These vector fields each have a value defined at every point of space and time and are thus often regarded as functions of the space and time coordinates. As such, they are often written as E(x, y, z, t) (electric field) and B(x, y, z, t) (magnetic field).
If only the electric field (E) is non-zero, and is constant in time, the field is said to be an electrostatic field. Similarly, if only the magnetic field (B) is non-zero and is constant in time, the field is said to be a magnetostatic field. However, if either the electric or magnetic field has a time-dependence, then both fields must be considered together as a coupled electromagnetic field using Maxwell's equations.[2]
With the advent of special relativity, physical laws became susceptible to the formalism of tensors. Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws.
The behaviour of electric and magnetic fields, whether in cases of electrostatics, magnetostatics, or electrodynamics (electromagnetic fields), is governed by Maxwell's equations. In the vector field formalism, these are:
\nabla \cdot \mathbf{E} = \frac{\rho}{\varepsilon_0} (Gauss's law)
\nabla \cdot \mathbf{B} = 0 (Gauss's law for magnetism)
\nabla \times \mathbf{E} = -\frac {\partial \mathbf{B}}{\partial t} (Faraday's law)
\nabla \times \mathbf{B} = \mu_0 \mathbf{J} + \mu_0\varepsilon_0 \frac{\partial \mathbf{E}}{\partial t} (Ampère-Maxwell law)
where \rho is the charge density, which can (and often does) depend on time and position, \epsilon_0 is the permittivity of free space, \mu_0 is the permeability of free space, and J is the current density vector, also a function of time and position. The units used above are the standard SI units. Inside a linear material, Maxwell's equations change by switching the permeability and permittivity of free space with the permeability and permittivity of the linear material in question. Inside other materials which possess more complex responses to electromagnetic fields, these terms are often represented by complex numbers, or tensors.
The Lorentz force law governs the interaction of the electromagnetic field with charged matter.
When a field travels across to different media, the properties of the field change according to the various boundary conditions. These equations are derived from Maxwell's equations. The tangential components of the electric and magnetic fields as they relate on the boundary of two media are as follows:[3]
\mathbf{E}_{1} = \mathbf{E}_{2}
\mathbf{H}_{1} = \mathbf{H}_{2} (current-free)
\mathbf{D}_{1} = \mathbf{D}_{2} (charge-free)
\mathbf{B}_{1} = \mathbf{B}_{2}
The angle of refraction of an electric field between media is related to the permittivity (\varepsilon) of each medium:
\frac{{\tan\theta_1}}{{\tan\theta_2}} = \frac{{\varepsilon_{r2}}}{{\varepsilon_{r1}}}
The angle of refraction of a magnetic field between media is related to the permeability (\mu) of each medium:
\frac{{\tan\theta_1}}{{\tan\theta_2}} = \frac{{\mu_{r2}}}{{\mu_{r1}}}

Properties of the field

Reciprocal behavior of electric and magnetic fields

The two Maxwell equations, Faraday's Law and the Ampère-Maxwell Law, illustrate a very practical feature of the electromagnetic field. Faraday's Law may be stated roughly as 'a changing magnetic field creates an electric field'. This is the principle behind the electric generator.
Ampere's Law roughly states that 'a changing electric field creates a magnetic field'. Thus, this law can be applied to generate a magnetic field and run an electric motor.

Light as an electromagnetic disturbance

Maxwell's equations take the form of an electromagnetic wave in a volume of space not containing charges or currents (free space) – that is, where \rho and J are zero. Under these conditions, the electric and magnetic fields satisfy the electromagnetic wave equation:[4]
\left( \nabla^2 - { 1 \over {c}^2 } {\partial^2 \over \partial t^2} \right) \mathbf{E} \ \ = \ \ 0
\left( \nabla^2 - { 1 \over {c}^2 } {\partial^2 \over \partial t^2} \right) \mathbf{B} \ \ = \ \ 0
James Clerk Maxwell was the first to obtain this relationship by his completion of Maxwell's equations with the addition of a displacement current term to Ampere's Circuital law.

Relation to and comparison with other physical fields

Main article: Fundamental forces
Being one of the four fundamental forces of nature, it is useful to compare the electromagnetic field with the gravitational, strong and weak fields. The word 'force' is sometimes replaced by 'interaction' because modern particle physics models electromagnetism as an exchange of particles known as gauge bosons.

Electromagnetic and gravitational fields

Sources of electromagnetic fields consist of two types of charge – positive and negative. This contrasts with the sources of the gravitational field, which are masses. Masses are sometimes described as gravitational charges, the important feature of them being that there are only positive masses and no negative masses. Further, gravity differs from electromagnetism in that positive masses attract other positive masses whereas same charges in electromagnetism repel each other.
The relative strengths and ranges of the four interactions and other information are tabulated below:
Theory Interaction mediator Relative Magnitude Behavior Range
Chromodynamics Strong interaction gluon 1038 1 10−15 m
Electrodynamics Electromagnetic interaction photon 1036 1/r2 infinite
Flavordynamics Weak interaction W and Z bosons 1025 1/r5 to 1/r7 10−16 m
Geometrodynamics Gravitation graviton 100 1/r2 infinite

Applications

Static E and M fields and static EM fields

Main articles: electrostatics, magnetostatics and magnetism
When an EM field (see electromagnetic tensor) is not varying in time, it may be seen as a purely electrical field or a purely magnetic field, or a mixture of both. However the general case of a static EM field with both electric and magnetic components present, is the case that appears to most observers. Observers who see only an electric or magnetic field component of a static EM field, have the other (electric or magnetic) component suppressed, due to the special case of the immobile state of the charges that produce the EM field in that case. In such cases the other component becomes manifest in other observer frames.
A consequence of this, is that any case that seems to consist of a "pure" static electric or magnetic field, can be converted to an EM field, with both E and M components present, by simply moving the observer into a frame of reference which is moving with regard to the frame in which only the “pure” electric or magnetic field appears. That is, a pure static electric field will show the familiar magnetic field associated with a current, in any frame of reference where the charge moves. Likewise, any new motion of a charge in a region that seemed previously to contain only a magnetic field, will show that that the space now contains an electric field as well, which will be found to produces an additional Lorentz force upon the moving charge.
Thus, electrostatics, as well as magnetism and magnetostatics, are now seen as studies of the static EM field when a particular frame has been selected to suppress the other type of field, and since an EM field with both electric and magnetic will appear in any other frame, these "simpler" effects are merely the observer's. The "applications" of all such non-time varying (static) fields are discussed in the main articles linked in this section.

Time-varying EM fields in Maxwell’s equations

Main articles: near and far field, near field optics, virtual particle, dielectric heating and magnetic induction
An EM field that varies in time has two “causes” in Maxwell’s equations. One is charges and currents (so-called “sources”), and the other cause for an E or M field is a change in the other type of field (this last cause also appears in “free space” very far from currents and charges).
An electromagnetic field very far from currents and charges (sources) is called electromagnetic radiation (EMR) since it radiates from the charges and currents in the source, and has no "feedback" effect on them, and is also not affected directly by them in the present time (rather, it is indirectly produced by a sequences of changes in fields radiating out from them in the past). EMR consists of the radiations in the electromagnetic spectrum, including radio waves, microwave, infrared, visible light, ultraviolet light, X-rays, and gamma rays. The many commercial applications of these radiations are discussed in the named and linked articles.
A notable application of visible light is that this type of energy from the Sun powers all life on Earth that either makes or uses oxygen.
A changing electromagnetic field which is physically close to currents and charges (see near and far field for a definition of “close”) will have a dipole characteristic that is dominated by either a changing electric dipole, or a changing magnetic dipole. This type of dipole field near sources is called an electromagnetic near-field.
Changing electric dipole fields, as such, are used commercially as near-fields mainly as a source of dielectric heating. Otherwise, they appear parasitically around conductors which absorb EMR, and around antennas which have the purpose of generating EMR at greater distances.
Changing magnetic dipole fields (i.e., magnetic near-fields) are used commercially for many types of magnetic induction devices. These include motors and electrical transformers at low frequencies, and devices such as metal detectors and MRI scanner coils at higher frequencies. Sometimes these high-frequency magnetic fields change at radio frequencies without being far-field waves and thus radio waves; see RFID tags. See also near-field communication. Further uses of near-field EM effects commercially, may be found in the article on virtual photons, since at the quantum level, these fields are represented by these particles. Far-field effects (EMR) in the quantum picture of radiation, are represented by ordinary photons.

Health and safety

The potential health effects of the very low frequency EMFs surrounding power lines and electrical devices are the subject of on-going research and a significant amount of public debate. The US National Institute for Occupational Safety and Health (NIOSH) has issued some cautionary advisories but stresses that the data is currently too limited to draw good conclusions.[5]
The potential effects of electromagnetic fields on human health vary widely depending on the frequency and intensity of the fields. For more information on the health effects due to specific parts of the electromagnetic spectrum, see the following articles:

See also

References

  1. Spencer, James N. et al. (2010). Chemistry: Structure and Dynamics. John Wiley & Sons. p. 78. ISBN 9780470587119.
  2. Electromagnetic Fields (2nd Edition), Roald K. Wangsness, Wiley, 1986. ISBN 0-471-81186-6 (intermediate level textbook)
  3. Schaum's outline of theory and problems of electromagnetics(2nd Edition), Joseph A. Edminister, McGraw-Hill, 1995. ISBN 0070212341(Examples and Problem Practice)
  4. Field and Wave Electromagnetics (2nd Edition), David K. Cheng, Prentice Hall, 1989. ISBN 978-0-201-12819-2 (Intermediate level textbook)
  5. "NIOSH Fact Sheet: EMFs in the Workplace". United States National Institute for Occupational Safety and Health. Retrieved 2007-10-28.

Further reading

  • Maxwell, James Clerk (1865). "A dynamical theory of the electromagnetic field" (PDF). Philosophical Transactions of the Royal Society of London 155: p. 499. doi:10.1098/rstl.1865.0008.(This article accompanied a December 8, 1864 presentation by Maxwell to the Royal Society.)
  • Griffiths, David J. (1999). Introduction to electrodynamics, Third Edition, ed. Prentice Hall, pp. 559–562.
  • Gabriela, Davina (2009). Unpublished manuscript. Epistemology: Foundations for Clinical Theories, Endnote 5; p. 24.

External links

This page was last modified on 23 June 2014 at 00:52.
end quote from:

Electromagnetic field,

Most electrical events have an electromagnetic field including human beings. One of the ways to "see" this field around human beings is by being clairvoyant and seeing people's auras. However, then there is Kirlean photography which also takes a picture of this electromagnetic field around a human being.
Here is what wikipedia says about Kirlian photography:

  1. Kirlian photography - Wikipedia, the free encyclopedia

    en.wikipedia.org/wiki/Kirlian_photography
    Wikipedia
    Kirlian photography is a collection of photographic techniques used to capture the phenomenon of electrical coronal discharges. It is named after Semyon Kirlian ...
    ‎History - ‎Overview - ‎Research - ‎In popular culture
  2. Images for kirlian photography 
    • Image result for kirlian photography
    • Image result for kirlian photography
    • Image result for kirlian photography
    • Image result for kirlian photography
    • Image result for kirlian photography
    More images for kirlian photography
  3.  

  4. photography

    From Wikipedia, the free encyclopedia
    Kirlian photograph of two coins
    Kirlian photography is a collection of photographic techniques used to capture the phenomenon of electrical coronal discharges. It is named after Semyon Kirlian, who in 1939 accidentally discovered that if an object on a photographic plate is connected to a high-voltage source, an image is produced on the photographic plate.[1] The technique has been variously known as "electrography",[2] "electrophotography",[3] "corona discharge photography" (CDP),[4] "bioelectrography",[2] "gas discharge visualization (GDV)",[5] "electrophotonic imaging (EPI)",[6] and, in Russian literature, "Kirlianography".
    Kirlian photography has been the subject of mainstream scientific research, parapsychology research and art. To a large extent, It has been co-opted by promoters of pseudoscience, fringe science and paranormal health claims in books, magazines, workshops, and web sites.[7][8]

    History

    In 1889, Czech B. Navratil coined the word "electrography". Seven years later in 1896, a French experimenter, H. Baravuc, created electrographs of hands and leaves.
    In 1898, Russian engineer Yakov Narkevich-Iodko[9][note 1] demonstrated electrography at the fifth exhibition of the Russian Technical Society.
    In 1939, two Czechs, S. Pratt and J. Schlemmer published photographs showing a glow around leaves. The same year, Russian electrical engineer Semyon Kirlian and his wife Valentina developed Kirlian photography after observing a patient in Krasnodar hospital who was receiving medical treatment from a high-frequency electrical generator. They had noticed that when the electrodes were brought near the patient's skin, there was a glow similar to that of a Neon Discharge Tube.[10]
    The Kirlians conducted experiments in which photographic film was placed on top of a conducting plate, and another conductor was attached to the a hand, a leaf or other plant material. The conductors were energized by a high frequency high voltage power source, producing photographic images typically showing a silhouette of the object surrounded by an aura of light.
    In 1958, the Kirlians reported the results of their experiments for the first time. Their work was virtually unknown until 1970, when two Americans, Lynn Schroeder and Sheila Ostrander published a book, Psychic Discoveries Behind the Iron Curtain. Although little interest was generated among western scientists, Russians held a conference on the subject in 1972, at Kazakh State University.[11]
    Kirlian photography was used in the former Eastern Bloc in the 1970s. The corona discharge glow at the surface of an object subjected to a high voltage electrical field was referred to as a "Kirlian aura" in Russia and Eastern Europe.[12][13] In 1975 Belarusian scientist Victor Adamenko wrote a dissertation titled Research of the structure of High-frequency electric discharge (Kirlian effect) images.[14][15] Scientific study of what the researchers called the Kirlian effect was conducted by Victor Inyushin at Kazakh State University.[16][17]
    Early in the 1970s, Thelma Moss and Kendall Johnson at the Center for Health Sciences at the UCLA conducted extensive research[11] into Kirlian photography. Moss led an independent and unsupported parapsychology laboratory[18] that was shut down by the university in 1979.[19]
    Kirlian's research first became known in the United States after Shelia Ostrander and Lynn Schroeder's book "Psychic Discoveries Behind the Iron Curtain" was published in 1970. High voltage electrophotography soon became known to the general public as Kirlian Photography.

    Overview

    Typical Kirlian photography setup (cross section)
    Typical Kirlian photography setup (cross section)
    Kirlian photo of a fingertip
    Kirlian photo of two coins
    Kirlian photo of a Coleus leaf
    Kirlian photography is a technique for creating contact print photographs using high voltage. The process entails placing sheet photographic film on top of a metal discharge plate. The object to be photographed is then placed directly on top of the film. High voltage is momentarily applied to the metal plate, thus creating an exposure. The corona discharge between the object and the high voltage plate is captured by the film. The developed film results in a Kirlian photograph of the object.
    Color photographic film is calibrated to faithfully produce colors when exposed to normal light. Corona discharges can interact with minute variations in the different layers of dye used in the film, resulting in a wide variety of colors depending on the local intensity of the discharge.[20] Film and digital imaging techniques also record light produced by photons emitted during corona discharge (see Mechanism of corona discharge).
    Photographs of inanimate objects such as a coins, keys and leaves can be made more effectively by grounding the object to the earth, a cold water pipe or to the opposite (polarity) side of the high voltage source. Grounding the object creates a stronger corona discharge.[21]
    Kirlian photography does not require the use of a camera or a lens because it is a contact print process. It is possible to use a transparent electrode in place of the high voltage discharge plate, allowing one to capture the resulting corona discharge with a standard camera or a video camera.[22]
    Visual artists such as Robert Buelteman,[23] Ted Hiebert,[24] and Dick Lane[25] have used Kirlian photography to produce artistic images of a variety of subjects. Kirlian Photographer Mark D. Roberts, who has worked with Kirlian imagery for over 40 years, published a portfolio of plant images entitled "Vita occulta plantarum" or "The Secret Life of Plants", first exhibited in 2012 at the Bakken Museum in Minneapolis.

    Research

    Kirlian photography has been a subject of scientific research, parapsychology research and pseudoscientific claims.[7][8] There are no clear delineations between classic scientific research, fringe research, and claims made by promoters of alternative medicine and the like. Much of the research conducted around the middle of the 20th century occurred in the former Eastern Bloc before the cold war ended and has not held up to the scrutiny of stricter Western scientific standards[citation needed].

    Scientific research

    Results of scientific experiments published in 1976 involving Kirlian photography of living tissue (human finger tips) showed that most of the variations in corona discharge streamer length, density, curvature and color can be accounted for by the moisture content on the surface of and within the living tissue.[26] Scientists outside of the US have also conducted scientific research.
    Konstantin Korotkov developed a technique similar to Kirlian photography called Gas Discharge Visualization (GDV).[27][28][29] Korotkov's GDV camera system consists of hardware and software to directly record, process and interpret GDV images with a computer. The web site of Korotkov promotes his device and research in a medical context.[30][31] Izabela Ciesielska at the Institute of Architecture of Textiles in Poland used Korotov's GDV camera to evaluate the effects of human contact with various textiles on biological factors such as heart rate and blood pressure, as well as corona discharge images. The experiments captured corona discharge images of subjects fingertips while the subjects wore sleeves of various natural and synthetic materials on their forearms. The results failed to establish a relationship between human contact with the textiles and the corona discharge images, and were considered inconclusive.[9]

    Parapsychology research

    Around the 1970s, interest in paranormal research peaked. In 1968, Dr. Thelma Moss, a psychology professor headed UCLA's Neuropsychiatric Institute (NPI), which was later renamed the Semel Institute. The NPI had a laboratory dedicated to parapsychology research and staffed mostly with volunteers. The lab was unfunded, unsanctioned and eventually shut down by the university. Toward the end of her tenure at UCLA, Moss became interested in Kirlian photography, a technique that supposedly measured the "auras" of a living being. According to Kerry Gaynor, one of her former research assistants, "many felt Kirlian photography's effects were just a natural occurrence."[19]

    Claims

    Kirlian believed that images created by Kirlian photography might depict a conjectural energy field, or aura, thought, by some, to surround living things. Kirlian and his wife were convinced that their images showed a life force or energy field that reflected the physical and emotional states of their living subjects. They thought these images could be used to diagnose illnesses. In 1961, they published their first paper on the subject in the Russian Journal of Scientific and Applied Photography.[32] Kirlian's claims were embraced by energy treatments practitioners.[33]

    Torn leaf experiment

    A typical demonstration used as evidence for the existence of these energy fields involved taking Kirlian photographs of a picked leaf at set intervals. The gradual withering of the leaf was thought to correspond with a decline in the strength of the aura. In some experiments, if a section of a leaf was torn away after the first photograph, a faint image of the missing section would sometimes remain when a second photograph was taken. If the imaging surface is cleaned of contaminants and residual moisture before the second image is taken, then no image of the missing section will appear.[34]
    The living aura theory is at least partially repudiated by demonstrating that leaf moisture content has a pronounced effect on the electric discharge coronas; more moisture creates larger, more dynamic corona discharges. As the leaf dehydrates, the coronas will naturally decrease in variability and intensity. As a result, the changing water content of the leaf can affect the so-called Kirlian aura. Kirlian's experiments did not provide evidence for an energy field other than the electric fields produced by chemical processes, and the streaming process of coronal discharges.[4]
    The coronal discharges identified as Kirlian auras are the result of stochastic electric ionization processes, and are greatly affected by many factors, including the voltage and frequency of the stimulus, the pressure with which a person or object touches the imaging surface, the local humidity around the object being imaged, how well grounded the person or object is, and other local factors affecting the conductivity of the person or object being imaged. Oils, sweat, bacteria, and other ionizing contaminants found on living tissues can also affect the resulting images.[35][36][37]

    Qi

    Scientists such as Beverly Rubik have explored the idea of a human biofield using Kirlian photography research, attempting to explain the Chinese discipline of Qigong. Qigong teaches that there is a vitalistic energy called qi (or chi) that permeates all living things. The idea of qi as its own sort of field, not simply a creature's electromagnetic field, has been mostly disregarded by the scientific community.
    Rubik's experiments relied on Konstantin Korotkov's GDV device to produce images which were thought to visualize these qi biofields in chronically ill patients. Rubik acknowledges that the small sample size in her experiments "was too small to permit a meaningful statistical analysis."[38] Claims that these energies can be captured by special photographic equipment are criticized by skeptics.[33]

    In popular culture

    Kirlian photography has appeared as a fictional element in numerous books, films, television series, and media productions. Kirlian photographs have been used as visual components in various media, such as the sleeve of George Harrison's 1973 album Living in the Material World which features Kirlian photographs of his hand holding a Hindi medallion on the front sleeve and American coins on the back, shot at Thelma Moss's UCLA parapsychology laboratory.[39]

    See also

    Notes

    1. Alternatively transliterated Narkevich-Yodko. It is spelled Narkevich-Todko in some sources; In Russian: Наркевич-Йодко. Some sources state that he was Polish, rendering his name Jacob Jodko-Narkiewicz

    References

    1. Julie McCarron-Benson in Skeptical - a Handbook of Pseudoscience and the Paranormal, ed Donald Laycock, David Vernon, Colin Groves, Simon Brown, Imagecraft, Canberra, 1989, ISBN 0-7316-5794-2, p11
    2. Konikiewicz, Leonard W. (1978). Introduction to electrography: A handbook for prospective researchers of the Kirlian effect in biomedicine. Leonard's Associates.
    3. Lane, Earle (1975). Electrophotography. And/Or Press (San Francisco).
    4. Boyers, David G. and Tiller, William A. (1973). "Corona discharge photography". Journal of Applied Physics 44 (7): 3102–3112. doi:10.1063/1.1662715.
    5. Bankovskii, N. G.; Korotkov, K. G.; Petrov, N. N. (Apr 1986). "Physical processes of image formation during gas-discharge visualization (the Kirlian effect) (Review)". Radiotekhnika i Elektronika 31: 625–643.
    6. Wisneski, Leonard A. and Anderson, Lucy (2010). The Scientific Basis of Integrative Medicine. ISBN 978-1-4200-8290-6.
    7. Stenger, Victor J. (1999). "Bioenergetic Fields". The Scientific Review of Alternative Medicine 3 (1).
    8. Skrabanek, P. (1988). "Paranormal Health Claims". Cellular and Molecular Life Sciences 44 (4): 303–309. doi:10.1007/bf01961267.
    9. Ciesielska, Izabela L. (March 2009). "Images of Corona Discharges as a Source of Information About the Influence of Textiles on Humans". AUTEX Research Journal (Lodz, Poland) 9 (1). Retrieved 26 August 2012.
    10. Kirlian, S. D. (1949) Method for Receiving Photographic Pictures of Different Types of Objects, Patent, N106401 USSR.
    11. Richard Cavendish, ed. (1994). Man, Myth and Magic 11. New York, NY: Marshall Cavendish. p. 1481. ISBN 1-85435-731-X.
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    36. The Kirlian Technique: Controlling the Wild Cards. The Kirlian effect not only is explainable by natural processes; it also varies according to at least six physical parameters. Arleen J. Watkins and Williams S. Bickel, The Skeptical Inquirer 13:172-184, 1989.
    37. Carroll, Robert Todd (2003). The Skeptic's Dictionary: A Collection of Strange Beliefs, Amusing Deceptions, and Dangerous Delusions. Hoboken, NJ, USA: John Wiley & Sons. p. 446. ISBN 978-0-471-27242-7.
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    Further reading

    • Becker, Robert and Selden, Gary, The Body Electric:Electromagnetism and the Foundation of Life, (Quill/Williams Morrow, 1985)
    • Krippner, S. and Rubin, D., Galaxies of Life, (Gordon and Breach, 1973)
    • Ostrander, S. and Schroeder, L., Discoveries Behind the Iron Curtain, (Prentice-Hall 1970)
    • Iovine, John Kirlian Photography - A Hands on Guide , (McGraw-Hill 1993)

    External links

    Wikimedia Commons has media related to Kirlian photography.
     


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