HOME

TheInfoList



OR:

Joule heating (also known as resistive heating, resistance heating, or Ohmic heating) is the process by which the passage of an
electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
through a conductor produces
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
. Joule's first law (also just Joule's law), also known in countries of the former
USSR The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
as the Joule–Lenz law,Джоуля — Ленца закон
. ''Большая советская энциклопедия'', 3-е изд., гл. ред. А. М. Прохоров. Москва: Советская энциклопедия, 1972. Т. 8 ()
states that the power of heating generated by an
electrical conductor In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow of negatively c ...
equals the product of its resistance and the square of the current. Joule heating affects the whole electric conductor, unlike the Peltier effect which transfers heat from one electrical junction to another. Joule-heating or resistive-heating is used in many devices and industrial processes. The part that converts electricity into heat is called a
heating element A heating element is a device used for conversion of electric energy into heat, consisting of a heating resistor and accessories. Heat is generated by the passage of electric current through a resistor through a process known as Joule heating. He ...
. Practical applications of joule heating include but not limited to: * Buildings are often heated with electric heaters where
grid power Mains electricity, utility power, grid power, domestic power, wall power, household current, or, in some parts of Canada, hydro, is a general-purpose Alternating current, alternating-current (AC) electric power supply. It is the form of electri ...
is available. * Electric stoves and ovens use Joule heating to cook food. * Soldering irons generate heat to melt conductive solder and make electrical connections. * Cartridge heaters are used in various manufacturing processes. * Electric fuses are used as a safety device, breaking a circuit by melting if enough current flows to heat them to the melting point. *
Electronic cigarette An electronic cigarette (e-cigarette), or vape, is a device that simulates smoking. It consists of an atomizer, a power source such as a battery, and a container such as a cartridge or tank. Instead of smoke, the user inhales vapor. As such ...
s vaporize liquid by Joule heating. *
Food processing Food processing is the transformation of agricultural products into food, or of one form of food into other forms. Food processing takes many forms, from grinding grain into raw flour, home cooking, and complex industrial methods used in the mak ...
equipment may make use of Joule heating: running a current through food material (which behave as an electrical resistor) causes heat release inside the food. The alternating electrical current coupled with the resistance of the food causes the generation of heat. A higher resistance increases the heat generated. Joule heating allows for fast and uniform heating of food products, which maintains quality. Products with particulates heat up faster (compared to conventional heat processing) due to higher resistance.


History

James Prescott Joule James Prescott Joule (; 24 December 1818 11 October 1889) was an English physicist. Joule studied the nature of heat and discovered its relationship to mechanical work. This led to the law of conservation of energy, which in turn led to the ...
first published in December 1840, an abstract in the '' Proceedings of the Royal Society'', suggesting that heat could be generated by an electrical current. Joule immersed a length of wire in a fixed
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
of
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
and measured the
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
rise due to a known current flowing through the wire for a 30
minute A minute is a unit of time defined as equal to 60 seconds. It is not a unit in the International System of Units (SI), but is accepted for use with SI. The SI symbol for minutes is min (without a dot). The prime symbol is also sometimes used i ...
period. By varying the current and the length of the wire he deduced that the heat produced was proportional to the
square In geometry, a square is a regular polygon, regular quadrilateral. It has four straight sides of equal length and four equal angles. Squares are special cases of rectangles, which have four equal angles, and of rhombuses, which have four equal si ...
of the current multiplied by the
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...
of the immersed wire. In 1841 and 1842, subsequent experiments showed that the amount of heat generated was proportional to the
chemical energy Chemical energy is the energy of chemical substances that is released when the substances undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (20 ...
used in the
voltaic pile upright=1.2, Schematic diagram of a copper–zinc voltaic pile. Each copper–zinc pair had a spacer in the middle, made of cardboard or felt soaked in salt water (the electrolyte). Volta's original piles contained an additional zinc disk at the ...
that generated the template. This led Joule to reject the
caloric theory The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores ...
(at that time the dominant theory) in favor of the mechanical theory of heat (according to which heat is another form of
energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...
). Resistive heating was independently studied by
Heinrich Lenz Heinrich Friedrich Emil Lenz (; also Emil Khristianovich Lenz; ; 12 February 1804 – 10 February 1865), usually cited as Emil Lenz or Heinrich Lenz in some countries, was an Estonian physicist who is most noted for formulating Lenz's law in el ...
in 1842. The
SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...
of
energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...
was subsequently named the
joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
and given the symbol ''J''. The commonly known unit of power, the
watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
, is equivalent to one joule per second.


Microscopic description

Joule heating is caused by interactions between
charge carrier In solid state physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. ...
s (usually
electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
) and the body of the conductor. A
potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...
difference (
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
) between two points of a conductor creates an
electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...
that accelerates charge carriers in the direction of the electric field, giving them
kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
. When the charged particles collide with the quasi-particles in the conductor (i.e. the canonically quantized, ionic lattice oscillations in the harmonic approximation of a crystal), energy is being transferred from the electrons to the lattice (by the creation of further lattice oscillations). The oscillations of the ions are the origin of the radiation ("
thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
") that one measures in a typical experiment.


Power loss and noise

Joule heating is referred to as ''ohmic heating'' or ''resistive heating'' because of its relationship to
Ohm's Law Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...
. It forms the basis for the large number of practical applications involving
electric heating Electric heating is a process in which electrical energy is converted directly to heat energy. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electricity, electrical device t ...
. However, in applications where heating is an unwanted
by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...
of current use (e.g., load losses in electrical transformers) the diversion of energy is often referred to as ''resistive loss''. The use of
high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant sp ...
s in
electric power transmission Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is ...
systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents. The ring circuits, or ring mains, used in UK homes are another example, where power is delivered to outlets at lower currents (per wire, by using two paths in parallel), thus reducing Joule heating in the wires. Joule heating does not occur in
superconducting Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...
materials, as these materials have zero electrical resistance in the superconducting state. Resistors create electrical noise, called Johnson–Nyquist noise. There is an intimate relationship between Johnson–Nyquist noise and Joule heating, explained by the fluctuation-dissipation theorem.


Formulas


Direct current

The most fundamental formula for Joule heating is the generalized power equation: P = I (V_ - V_) where * P is the power (energy per unit time) converted from electrical energy to thermal energy, * I is the current travelling through the resistor or other element, * V_-V_ is the
voltage drop In electronics, voltage drop is the decrease of electric potential along the path of a current flowing in a circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are unde ...
across the element. The explanation of this formula (P = IV) is:''Electric power systems: a conceptual introduction'' by Alexandra von Meier, p67
Google books link
/ref> Assuming the element behaves as a perfect resistor and that the power is completely converted into heat, the formula can be re-written by substituting
Ohm's law Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...
, V = I R , into the generalized power equation: P = IV = I^2R = V^2/R where ''R'' is the resistance. Voltage can be increased in DC circuits by connecting batteries or solar panels in series.


Alternating current

When current varies, as it does in AC circuits, P(t) = U(t) I(t) where ''t'' is time and ''P'' is the instantaneous active power being converted from electrical energy to heat. Far more often, the ''average'' power is of more interest than the instantaneous power: P_ = U_\text I_\text = (I_\text)^2 R = (U_\text)^2 / R where "avg" denotes average (mean) over one or more cycles, and "rms" denotes
root mean square In mathematics, the root mean square (abbrev. RMS, or rms) of a set of values is the square root of the set's mean square. Given a set x_i, its RMS is denoted as either x_\mathrm or \mathrm_x. The RMS is also known as the quadratic mean (denote ...
. These formulas are valid for an ideal resistor, with zero reactance. If the reactance is nonzero, the formulas are modified: P_ = U_\textI_\text\cos\phi = (I_\text)^2 \operatorname(Z) = (U_\text)^2 \operatorname(Y^*) where \phi is phase difference between current and voltage, \operatorname means
real part In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the form ...
, ''Z'' is the complex impedance, and ''Y*'' is the
complex conjugate In mathematics, the complex conjugate of a complex number is the number with an equal real part and an imaginary part equal in magnitude but opposite in sign. That is, if a and b are real numbers, then the complex conjugate of a + bi is a - ...
of the
admittance In electrical engineering, admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the multiplicative inverse, reciprocal of Electrical impedance, impedance, analogous to how Electrical resistanc ...
(equal to 1/''Z*''). For more details in the reactive case, see
AC power In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the d ...
.


Differential form

Joule heating can also be calculated at a particular location in space. The differential form of the Joule heating equation gives the power per unit volume. \frac = \mathbf \cdot \mathbf Here, \mathbf is the current density, and \mathbf is the electric field. For a material with a conductivity \sigma, \mathbf=\sigma \mathbf and therefore \frac = \mathbf \cdot \mathbf = \mathbf \cdot \mathbf\frac = J^2\rho where \rho = 1/\sigma is the
resistivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity i ...
. This directly resembles the "I^2R" term of the macroscopic form. In the harmonic case, where all field quantities vary with the angular frequency \omega as e^, complex valued phasors \hat\mathbf and \hat\mathbf are usually introduced for the current density and the electric field intensity, respectively. The Joule heating then reads \frac = \frac\hat\mathbf \cdot \hat\mathbf^* = \frac\hat\mathbf \cdot \hat\mathbf^*/\sigma = \fracJ^2\rho, where \bullet^* denotes the
complex conjugate In mathematics, the complex conjugate of a complex number is the number with an equal real part and an imaginary part equal in magnitude but opposite in sign. That is, if a and b are real numbers, then the complex conjugate of a + bi is a - ...
.


Electricity transmission

Overhead power line An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors (commonly multiples of three) suspended by towers or poles. ...
s transfer electrical energy from electricity producers to consumers. Those power lines have a nonzero resistance and therefore are subject to Joule heating, which causes transmission losses. The split of power between transmission losses (Joule heating in transmission lines) and load (useful energy delivered to the consumer) can be approximated by a
voltage divider In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the i ...
. In order to minimize transmission losses, the resistance of the lines has to be as small as possible compared to the load (resistance of consumer appliances). Line resistance is minimized by the use of
copper conductor Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electri ...
s, but the resistance and
power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, electric current, current, and frequency to power ...
specifications of consumer appliances are fixed. Usually, a
transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
is placed between the lines and consumption. When a high-voltage, low-intensity current in the primary circuit (before the transformer) is converted into a low-voltage, high-intensity current in the secondary circuit (after the transformer), the equivalent resistance of the secondary circuit becomes higher and transmission losses are reduced in proportion. During the
war of currents The war of the currents was a series of events surrounding the introduction of competing electric power transmission systems in the late 1880s and early 1890s. It grew out of two lighting systems developed in the late 1870s and early 1880s: arc l ...
, AC installations could use transformers to reduce line losses by Joule heating, at the cost of higher voltage in the transmission lines, compared to DC installations.


Applications


Food processing

Joule heating is a
flash pasteurization Flash pasteurization, also called "high-temperature short-time" (HTST) processing, is a method of heat pasteurization of perishable beverages like juice, fruit and vegetable juices, beer, wine, and some dairy products such as milk. Compared with o ...
(also called "high-temperature short-time" (HTST)) aseptic process that runs an alternating current of 50–60 Hz through food. Heat is generated through the food's electrical resistance. As the product heats, electrical conductivity increases linearly. A higher electrical current frequency is best as it reduces oxidation and metallic contamination. This heating method is best for foods that contain particulates suspended in a weak salt-containing medium due to their high resistance properties. Heat is generated rapidly and uniformly in the liquid matrix as well as in
particulates Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspension (chemistry), suspended in the atmosphere of Earth, air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate ...
, producing a higher quality sterile product that is suitable for
aseptic processing Aseptic processing is a processing technique wherein commercially thermally sterilized liquid products (typically food or pharmaceutical) are packaged into previously sterilized containers under sterile conditions to produce shelf-stable products ...
. Electrical energy is linearly translated to thermal energy as
electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
increases, and this is the key process parameter that affects heating uniformity and heating rate. This heating method is best for foods that contain particulates suspended in a weak salt containing medium due to their high resistance properties. Ohmic heating is beneficial due to its ability to inactivate
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s through thermal and non-thermal cellular damage. This method can also inactivate antinutritional factors thereby maintaining nutritional and sensory properties. However, ohmic heating is limited by
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
, electrical conductivity, and
fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling, organic) or a non-living substance (inorganic). Fouling is usually distinguished from other surfac ...
deposits. Although ohmic heating has not yet been approved by the Food and Drug Administration (
FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
) for commercial use, this method has many potential applications, ranging from
cooking Cooking, also known as cookery or professionally as the culinary arts, is the art, science and craft of using heat to make food more palatable, digestible, nutritious, or Food safety, safe. Cooking techniques and ingredients vary widely, from ...
to
fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
. There are different configurations for continuous ohmic heating systems, but in the most basic process, a power supply or generator is needed to produce electrical current.
Electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
s, in direct contact with food, pass electric current through the matrix. The distance between the electrodes can be adjusted to achieve the optimum electrical field strength. The generator creates the electrical current which flows to the first electrode and passes through the food product placed in the electrode gap. The food product resists the flow of current causing internal heating. The current continues to flow to the second electrode and back to the power source to close the circuit. The insulator caps around the electrodes controls the environment within the system. The
electrical field An electric field (sometimes called E-field) is a physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) describes their capaci ...
strength and the
residence time The residence time of a fluid parcel is the total time that the parcel has spent inside a control volume (e.g.: a chemical reactor, a lake, a human body). The residence time of a set of parcels is quantified in terms of the frequency distribu ...
are the key process parameters which affect heat generation. The ideal foods for ohmic heating are viscous with particulates. * Thick soups * Sauces * Stews * Salsa * Fruit in a syrup medium * Milk * Ice cream mix * Egg * Whey * Heat sensitive liquids * Soymilk The efficiency by which electricity is converted to heat depends upon on salt, water, and fat content due to their
thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...
and resistance factors. In particulate foods, the particles heat up faster than the liquid matrix due to higher resistance to electricity and matching conductivity can contribute to uniform heating. This prevents overheating of the liquid matrix while particles receive sufficient heat processing. Table 1 shows the electrical conductivity values of certain foods to display the effect of composition and salt concentration. The high electrical conductivity values represent a larger number of
ionic compound In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions (Cation, cations) and negatively charged ions (Anion, anions), which results in a compound with no net electric charge (electrica ...
s suspended in the product, which is directly proportional to the rate of heating. This value is increased in the presence of polar compounds, like acids and salts, but decreased with nonpolar compounds, like fats. Electrical conductivity of food materials generally increases with temperature, and can change if there are structural changes caused during heating such as gelatinization of starch. Density, pH, and specific heat of various components in a food matrix can also influence heating rate. Benefits of Ohmic heating include: uniform and rapid heating (>1°Cs−1), less cooking time, better energy efficiency, lower capital cost, and heating simulataneously throughout food's volume as compared to
aseptic processing Aseptic processing is a processing technique wherein commercially thermally sterilized liquid products (typically food or pharmaceutical) are packaged into previously sterilized containers under sterile conditions to produce shelf-stable products ...
,
canning Canning is a method of food preservation in which food is processed and sealed in an airtight container (jars like Mason jars, and steel and tin cans). Canning provides a shelf life that typically ranges from one to five years, although under ...
, and PEF. Volumetric heating allows internal heating instead of transferring heat from a secondary medium. This results in the production of safe, high quality food with minimal changes to structural, nutritional, and
organoleptic Organoleptic properties are the aspects of food, water or other substances as apprehended via the senses—including taste, sight, smell, and touch. In traditional U.S. Department of Agriculture meat and poultry inspections, inspectors p ...
properties of food. Heat transfer is uniform to reach areas of food that are harder to heat. Less fouling accumulates on the electrodes as compared to other heating methods. Ohmic heating also requires less cleaning and maintenance, resulting in an environmentally cautious heating method. Microbial inactivation in ohmic heating is achieved by both thermal and non-thermal cellular damage from the electrical field. This method destroys
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s due to
electroporation Electroporation, also known as electropermeabilization, is a microbiological and biotechnological technique in which an electric field is applied to cells to briefly increase the permeability of the cell membrane. The application of a high-vo ...
of
cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
s, physical membrane rupture, and cell lysis. In electroporation, excessive leakage of
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s and intramolecular components results in cell death. In membrane rupture, cells swell due to an increase in moisture diffusion across the cell membrane. Pronounced disruption and decomposition of cell walls and cytoplasmic membranes causes cells to lyse. Decreased processing times in ohmic heating maintains nutritional and sensory properties of foods. Ohmic heating inactivates antinutritional factors like lipoxigenase (LOX), polyphenoloxidase (PPO), and
pectinase Pectinases are a group of enzymes that breaks down pectin, a polysaccharide found in plant cell walls, through hydrolysis, transelimination and deesterification reactions. Commonly referred to as pectic enzymes, they include pectolyase, pectozym ...
due to the removal of active metallic groups in enzymes by the electrical field. Similar to other heating methods, ohmic heating causes gelatinization of starches, melting of fats, and protein agglutination. Water-soluble nutrients are maintained in the suspension liquid allowing for no loss of nutritional value if the liquid is consumed. Ohmic heating is limited by viscosity, electrical conductivity, and fouling deposits. The density of particles within the suspension liquid can limit the degree of processing. A higher viscosity fluid will provide more resistance to heating, allowing the mixture to heat up quicker than low viscosity products. A food product's electrical conductivity is a function of temperature, frequency, and product composition. This may be increased by adding ionic compounds, or decreased by adding non-polar constituents. Changes in electrical conductivity limit ohmic heating as it is difficult to model the thermal process when temperature increases in multi-component foods. The potential applications of ohmic heating range from cooking, thawing, blanching, peeling, evaporation, extraction,
dehydration In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild deh ...
, and fermentation. These allow for ohmic heating to pasteurize particulate foods for hot filling, pre-heat products prior to canning, and aseptically process ready-to-eat meals and refrigerated foods. Prospective examples are outlined in Table 2 as this food processing method has not been commercially approved by the FDA. Since there is currently insufficient data on electrical conductivities for solid foods, it is difficult to prove the high quality and safe process design for ohmic heating. Additionally, a successful 12D reduction for '' C. botulinum'' prevention has yet to be validated.


Materials synthesis, recovery and processing

Flash joule heating (transient high-temperature electrothermal heating) has been used to synthesize
allotropes of carbon Carbon is capable of forming many allotropy, allotropes (structurally different forms of the same element) due to its Valence (chemistry), valency (Tetravalence, tetravalent). Well-known forms of carbon include diamond and graphite. In recent ...
, including graphene and diamond. Heating various solid carbon feedstocks (carbon black, coal, coffee grounds, etc.) to temperatures of ~3000 K for 10-150 milliseconds produces turbostratic graphene flakes. FJH has also been used to recover rare-earth elements used in modern electronics from industrial wastes. Beginning from a fluorinated carbon source, fluorinated activated carbon, fluorinated nanodiamond, concentric carbon (carbon shell around a nanodiamond core), and fluorinated flash graphene can be synthesized.


Gallery


Heating efficiency

Heat is not to be confused with
internal energy The internal energy of a thermodynamic system is the energy of the system as a state function, measured as the quantity of energy necessary to bring the system from its standard internal state to its present internal state of interest, accoun ...
or synonymously
thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
. While intimately connected to
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
, they are distinct physical quantities. As a heating technology, Joule heating has a
coefficient of performance The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...
of 1.0, meaning that every joule of electrical energy supplied produces one joule of heat. In contrast, a
heat pump A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm s ...
can have a coefficient of more than 1.0 since it moves additional thermal energy from the environment to the heated item. The definition of the efficiency of a heating process requires defining the boundaries of the system to be considered. When heating a building, the overall efficiency is different when considering heating effect per unit of electric energy delivered on the customer's side of the meter, compared to the overall efficiency when also considering the losses in the power plant and transmission of power.


Hydraulic equivalent

In the energy balance of groundwater flow a hydraulic equivalent of Joule's law is used: \frac = \frac where: * dE/dt = loss of hydraulic energy (E) due to friction of flow in x-direction per unit of time (m/day), comparable to P * v_x = flow velocity in x-direction (m/day), comparable to I * K =
hydraulic conductivity In science and engineering, hydraulic conductivity (, in SI units of meters per second), is a property of porous materials, soils and Rock (geology), rocks, that describes the ease with which a fluid (usually water) can move through the porosity, ...
of the soil (m/day), the hydraulic conductivity is inversely proportional to the hydraulic resistance which compares to R


See also

* * * * * * * * * * *


References

{{refs Electric heating Electricity Thermodynamics
Heating In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, atom ...
Food processing Cooking techniques