View source for Joule heating

{{Short description|Heat from a current in an electric conductor}}
[[File:Toaster-quartz element.JPG|thumb|A coiled [[heating element]] from an electric toaster, showing red to yellow [[incandescence]]]]
{{electromagnetism|Network}}

'''Joule heating''' (also known as resistive, resistance, or Ohmic heating) is the process by which the passage of an [[electric current]] through a [[conductor (material)|conductor]] produces [[heat]].

Joule's first law (also just ''Joule's law''), also known in countries of the former [[Soviet Union|USSR]] as the Joule–Lenz law,<ref name="BSE">[http://bse.chemport.ru/dzhoulya_-_lentsa_zakon.shtml Джоуля&nbsp;— Ленца закон] {{webarchive|url=https://web.archive.org/web/20141230003727/http://bse.chemport.ru/dzhoulya_-_lentsa_zakon.shtml |date=2014-12-30 }}. ''Большая советская энциклопедия'', 3-е изд., гл. ред. А. М. Прохоров. Москва: Советская энциклопедия, 1972. Т. 8 ({{cite book| section = Joule–Lenz law | title = Great Soviet Encyclopedia | editor = A. M. Prokhorov| language = ru | publisher = Soviet Encyclopedia | location = Moscow | year = 1972 | volume = 8|display-editors=etal| title-link = Great Soviet Encyclopedia }})</ref> states that the [[Power (physics)|power]] of heating generated by an [[electrical conductor]] equals the product of its [[Electrical resistance and conductance|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 multiple devices and industrial process. The part that converts electricity into heat is called a [[heating element]].

Among the many practical uses are:

* An [[incandescent light bulb]] glows when the filament is heated by Joule heating, due to [[thermal radiation]] (also called [[blackbody radiation]]).
* [[Fuse (electrical)|Electric fuses]] are used as a safety, breaking the circuit by melting if enough current flows to melt them.
* [[Electronic cigarette]]s vaporize propylene glycol and vegetable glycerine by Joule heating.
* Multiple heating devices use Joule heating, such as [[electric stove]]s, [[electric heating|electric heaters]], [[soldering iron]]s, [[cartridge heater]]s.
* Some [[food processing]] equipment may make use of Joule heating: running current through food material (which behave as an electrical resistor) causes heat release inside the food.<ref>{{cite web|last=Ramaswamy|first=Raghupathy|title=Ohmic Heating of Foods|url=http://ohioline.osu.edu/fse-fact/0004.html|publisher=Ohio State University|access-date=2013-04-22|url-status=dead|archive-url=https://web.archive.org/web/20130408043300/http://ohioline.osu.edu/fse-fact/0004.html|archive-date=2013-04-08}}</ref> The alternating electrical current coupled with the resistance of the food causes the generation of heat.<ref name=":0">{{Cite book|title=Food Processing Technology|last=Fellows|first=P.J|publisher=Elsevier|year=2009|isbn=978-0-08-101907-8|location=MA|pages=813–844}}</ref> A higher resistance increases the heat generated. Ohmic 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.<ref name=":1">{{Cite journal|last1=Varghese|first1=K. Shiby|last2=Pandey|first2=M. C.|last3=Radhakrishna|first3=K.|last4=Bawa|first4=A. S.|date=October 2014|title=Technology, applications and modelling of ohmic heating: a review|journal=Journal of Food Science and Technology|volume=51|issue=10|pages=2304–2317|doi=10.1007/s13197-012-0710-3|issn=0022-1155|pmc=4190208|pmid=25328171}}</ref>

== History ==
[[James Prescott Joule]] 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]] of [[water]] and measured the [[temperature]] rise due to a known current flowing through the wire for a 30 [[minute]] period. By varying the current and the length of the wire he deduced that the heat produced was [[proportionality (mathematics)|proportional]] to the [[square (algebra)|square]] of the current multiplied by the [[electrical resistance]] of the immersed wire.<ref name="APS">{{cite web|title=This Month Physics History: December 1840: Joule's abstract on converting mechanical power into heat|url=http://www.aps.org/publications/apsnews/200912/physicshistory.cfm| website=aps.org| publisher=American Physical society|access-date=16 September 2016}}</ref>

In 1841 and 1842, subsequent experiments showed that the amount of heat generated was proportional to the [[chemical energy]] used in the [[voltaic pile]] that generated the template. This led Joule to reject the [[caloric theory]] (at that time the dominant theory) in favor of the [[mechanical theory of heat]] (according to which heat is another form of [[energy]]).<ref name="APS" />

Resistive heating was independently studied by [[Heinrich Lenz]] in 1842.<ref name="BSE" />

The [[SI unit]] of [[energy]] was subsequently named the [[joule]] and given the symbol ''J''. The commonly known unit of power, the [[watt]], is equivalent to one joule per second.

== Microscopic description ==
{{see also|Electrical resistivity and conductivity|Drift velocity|Drude model}}

Joule heating is caused by interactions between [[charge carrier]]s (usually [[electrons]]) and the body of the conductor.

A [[Electric potential|potential]] difference ([[voltage]]) between two points of a conductor creates an [[electric field]] that accelerates charge carriers in the direction of the electric field, giving them [[kinetic energy]]. 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]]") 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]]. It forms the basis for the large number of practical applications involving [[electric heating]]. However, in applications where heating is an unwanted [[by-product]] of current use (e.g., [[load loss]]es in [[electrical transformer]]s) the diversion of energy is often referred to as ''resistive loss''. The use of [[high voltage]]s in [[electric power transmission]] systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents. The [[ring circuit]]s, 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 [[superconductivity|superconducting]] 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:
<math display="block">P = I (V_{A} - V_{B})</math>
where
* <math>P</math> is the [[electric power|power]] (energy per unit time) converted from electrical energy to thermal energy,
* <math>I</math> is the current travelling through the resistor or other element,
* <math>V_{A}-V_{B}</math> is the [[voltage drop]] across the element.

The explanation of this formula (<math>P = IV</math>) is:<ref name=meier>''Electric power systems: a conceptual introduction'' by Alexandra von Meier, p67, [https://books.google.com/books?id=qI2fNj2voe8C&pg=PA67 Google books link]</ref>

{{block indent | em = 1.5 | text = (''Energy dissipated per unit time'') = (''Charge passing through resistor per unit time'') × (''Energy dissipated per charge passing through resistor'')}}

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]], <math>V = I R </math>, into the generalized power equation:
<math display="block">P = IV = I^2R = V^2/R</math>
where ''R'' is the [[electrical resistance and conductance|resistance]].

Voltage can be increased in DC circuits by connecting batteries or solar panels in series.

=== Alternating current ===
{{Main|AC power}}

When current varies, as it does in AC circuits,

<math display="block">P(t) = U(t) I(t)</math>

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:

<math display="block">P_{\rm avg} = U_\text{rms} I_\text{rms} = (I_\text{rms})^2 R = (U_\text{rms})^2 / R</math>

where "avg" denotes [[Arithmetic mean|average (mean)]] over one or more cycles, and "rms" denotes [[root mean square]].

These formulas are valid for an ideal resistor, with zero [[electrical reactance|reactance]]. If the reactance is nonzero, the formulas are modified:

<math display="block">P_{\rm avg} = U_\text{rms}I_\text{rms}\cos\phi = (I_\text{rms})^2 \operatorname{Re}(Z) = (U_\text{rms})^2 \operatorname{Re}(Y^*)</math>

where <math>\phi</math> is phase difference between current and voltage, <math>\operatorname{Re}</math> means [[real part]], ''Z'' is the [[electrical impedance|complex impedance]], and ''Y*'' is the [[complex conjugate]] of the [[admittance]] (equal to 1/''Z*'').

For more details in the reactive case, see [[AC power]].

=== 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.

<math display="block">\frac{\mathrm{d}P}{\mathrm{d}V} = \mathbf{J} \cdot \mathbf{E}</math>

Here, <math>\mathbf{J}</math> is the current density, and <math>\mathbf{E}</math> is the electric field. For a material with a conductivity <math>\sigma</math>, <math>\mathbf{J}=\sigma \mathbf{E}</math> and therefore
<math display="block">\frac{\mathrm{d}P}{\mathrm{d}V} = \mathbf{J} \cdot \mathbf{E} = \mathbf{J} \cdot \mathbf{J}\frac{1}{\sigma} = J^2\rho</math>

where <math>\rho = 1/\sigma</math> is the [[resistivity]].  This directly resembles the "<math>I^2R</math>" term of the macroscopic form.

In the harmonic case, where all field quantities vary with the angular frequency <math>\omega</math> as <math>e^{-\mathrm{i} \omega t}</math>, complex valued [[phasors]] <math>\hat\mathbf{J}</math> and <math>\hat\mathbf{E}</math> are usually introduced for the current density and the electric field intensity, respectively. The Joule heating then reads
<math display="block">\frac{\mathrm{d}P}{\mathrm{d}V} = \frac{1}{2}\hat\mathbf{J} \cdot \hat\mathbf{E}^* = \frac{1}{2}\hat\mathbf{J} \cdot \hat\mathbf{J}^*/\sigma = \frac{1}{2}J^2\rho,</math>
where <math>\bullet^*</math> denotes the [[complex conjugate]].

== Electricity transmission ==
{{Main|Electric power transmission#Advantage of high-voltage power transmission}}
{{See also|Transformer|War of the currents}}
[[Overhead power line]]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 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]]s, but the resistance and [[power supply]] specifications of consumer appliances are fixed.

Usually, a [[transformer]] 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<ref>{{cite web|title=Transformer circuits|url=http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/tracir.html|access-date=26 July 2017}}</ref> and transmission losses are reduced in proportion.

During the [[war of currents]], [[alternating current|AC]] installations could use transformers to reduce line losses by Joule heating, at the cost of higher voltage in the transmission lines, compared to [[direct current|DC]] installations.

== Applications ==

=== Food processing ===
[[File:Ohmic Heating Process_for_Food_Processing.png|thumb|General process for joule heating in food]]

Joule heating is a [[flash pasteurization]] (also called "high-temperature short-time" (HTST)) aseptic process that runs an alternating current of 50–60&nbsp;Hz through food.<ref name=":2">{{Cite book|title=Food processing technology : principles and practice|last=Fellows|first=P.|orig-year=2016|date= 2017|publisher=Woodhead Publishing/Elsevier Science|isbn=9780081019078|edition=4th |location=Kent|oclc=960758611}}</ref> Heat is generated through the food's electrical resistance.<ref name=":2" /><ref name=":48">{{Cite book|title=Ohmic heating in food processing|date=2014|publisher=CRC Press|others=Ramaswamy, Hosahalli S.|isbn=9781420071092|location=Boca Raton, FL|oclc=872623115}}</ref><ref name=":010">{{Cite journal|last=Varghese|first=K. Shiby|last2=Pandey|first2=M. C.|last3=Radhakrishna|first3=K.|last4=Bawa|first4=A. S.|date=October 2014|title=Technology, applications and modelling of ohmic heating: a review|journal=Journal of Food Science and Technology|language=en|volume=51|issue=10|pages=2304–2317|doi=10.1007/s13197-012-0710-3|pmid=25328171|issn=0022-1155|pmc=4190208}}</ref><ref name=":113">{{Cite book|title=Food processing technology|last=Fellows|first=P.J.|publisher=Woodhead Publishing|year=2017|isbn=978-0-08-101907-8|pages=831–38}}</ref> As the product heats, electrical conductivity increases linearly.<ref name=":0" /> A higher electrical current frequency is best as it reduces oxidation and metallic contamination.<ref name=":2" /> This heating method is best for foods that contain particulates suspended in a weak salt-containing medium due to their high resistance properties.<ref name=":1" /><ref name=":2" />

Heat is generated rapidly and uniformly in the liquid matrix as well as in [[particulates]], producing a higher quality sterile product that is suitable for [[aseptic processing]].<ref name=":113" /><ref name=":24">{{Cite book|title=Handbook of food processing : food preservation|others=Varzakas, Theodoros,, Tzia, Constantina|isbn=9781498721769|location=Boca Raton, FL|oclc=924714287|last1 = Varzakas|first1 = Theodoros|last2=Tzia|first2=Constantina|date=2015-10-22}}</ref>

Electrical energy is linearly translated to thermal energy as [[electrical conductivity]] increases, and this is the key process parameter that affects heating uniformity and heating rate.<ref name=":113" /> This heating method is best for foods that contain particulates suspended in a weak salt containing medium due to their high [[Electrical resistance and conductance|resistance]] properties.<ref name=":010" /> Ohmic heating is beneficial due to its ability to inactivate [[microorganism]]s through thermal and non-thermal cellular damage.<ref name=":113" /><ref name=":36">{{Cite book|title=Ohmic Heating in food processing|publisher=CRC Press|year=2014|isbn=978-1-4200-7109-2|pages=93–102}}</ref><ref name=":52">{{Cite journal|last=Varghese|first=K. Shiby|last2=Pandey|first2=M. C.|last3=Radhakrishna|first3=K.|last4=Bawa|first4=A. S.|date=2014-10-01|title=Technology, applications and modelling of ohmic heating: a review|journal=Journal of Food Science and Technology|language=en|volume=51|issue=10|pages=2304–2317|doi=10.1007/s13197-012-0710-3|pmid=25328171|issn=0022-1155|pmc=4190208}}</ref>

This method can also inactivate [[antinutritional factor]]s thereby maintaining nutritional and [[Organoleptic|sensory properties]].<ref name=":36" /> However, ohmic heating is limited by [[viscosity]], electrical conductivity, and [[fouling]] deposits.<ref name=":48" /><ref name=":010" /><ref name=":113" /> Although ohmic heating has not yet been approved by the Food and Drug Administration ([[Food and Drug Administration|FDA]]) for commercial use, this method has many potential applications, ranging from [[cooking]] to [[Fermentation in food processing|fermentation]].<ref name=":113" />

There are different configurations for continuous ohmic heating systems, but in the most basic process,<ref name=":113" /> a power supply or generator is needed to produce electrical current.<ref name=":010" />  [[Electrode]]s, in direct contact with food, pass electric current through the matrix.<ref name=":010" /> The distance between the electrodes can be adjusted to achieve the optimum electrical field strength.<ref name=":010" />

The generator creates the electrical current which flows to the first electrode and passes through the food product placed in the electrode gap.<ref name=":010" /> The food product resists the flow of current causing internal heating.<ref name=":113" /> The current continues to flow to the second electrode and back to the power source to close the circuit.<ref name=":010" /> The [[Insulator (electricity)|insulator]] caps around the electrodes controls the environment within the system.<ref name=":010" />

The [[Electric field|electrical field]] strength and the [[residence time]] are the key process parameters which affect heat generation.<ref name=":113" />

The ideal foods for ohmic heating are viscous with particulates.<ref name=":113" />

* 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]] and resistance factors.<ref name=":36" /> 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.<ref name=":113" /> This prevents overheating of the liquid matrix while particles receive sufficient heat processing.<ref name=":48" /> Table 1 shows the electrical conductivity values of certain foods to display the effect of composition and salt concentration.<ref name=":113" /> The high electrical conductivity values represent a larger number of [[ionic compound]]s suspended in the product, which is directly proportional to the rate of heating.<ref name=":010" /> This value is increased in the presence of [[Chemical polarity|polar compounds]], like acids and salts, but decreased with [[Nonpolar|nonpolar compounds]], like fats.<ref name=":010" /> Electrical conductivity of food materials generally increases with temperature, and can change if there are structural changes caused during heating such as [[Starch gelatinization|gelatinization]] of starch.<ref name=":113" /> Density, pH, and specific heat of various components in a food matrix can also influence heating rate.<ref name=":36" />

{| class="wikitable" href="OCLC"
|+Table 1. Electrical conductivity of selected foods<ref name=":113" />
! scope="col" | Food
! scope="col" | Electrical Conductivity (S/m)
! scope="col" | Temperature (°C)
|-
| Apple Juice || 0.239 || 20
|-
| Beef || 0.42 || 19
|-
| Beer || 0.143 || 22
|-
| Carrot || 0.041 || 19
|-
| Carrot Juice || 1.147 || 22
|-
| Chicken meat || 0.19 || 20
|-
| Coffee (black) || 0.182 || 22
|-
| Coffee (black with sugar) || 0.185 || 22
|-
| Coffee (with milk) || 0.357 || 22
|-
| Starch solution (5.5%) || ||
|-
| (a) with 0.2% salt || 0.34 || 19
|-
| (b) with 0.55% salt || 1.3 || 19
|-
| (c) with 2% salt || 4.3 || 19
|}

Benefits of Ohmic heating include: uniform and rapid heating (>1°Cs<sup>−1</sup>), less cooking time, better [[Efficient energy use|energy efficiency]], lower capital cost, and heating simulataneously throughout food's volume as compared to [[aseptic processing]], [[canning]], and [[Pulsed electric field processing|PEF]].<ref name=":24" /> Volumetric heating allows internal heating instead of transferring heat from a secondary medium.<ref name=":48" /> This results in the production of safe, high quality food with minimal changes to structural, nutritional, and [[organoleptic]] properties of food.<ref name=":48" /> Heat transfer is uniform to reach areas of food that are harder to heat.<ref name=":113" /> Less fouling accumulates on the electrodes as compared to other heating methods.<ref name=":010" /> Ohmic heating also requires less cleaning and maintenance, resulting in an environmentally cautious heating method.<ref name=":48" /><ref name=":113" /><ref name=":24" />

Microbial inactivation in ohmic heating is achieved by both thermal and non-thermal cellular damage from the electrical field.<ref name=":52" /> This method destroys [[microorganism]]s due to [[electroporation]] of [[cell membrane]]s, physical membrane rupture, and [[cell lysis]].<ref name=":113" /><ref name=":36" /> In electroporation, excessive leakage of [[ion]]s and [[Organelle|intramolecular components]] results in cell death.<ref name=":36" /> In membrane rupture, cells swell due to an increase in moisture diffusion across the cell membrane.<ref name=":24" /> Pronounced disruption and decomposition of cell walls and cytoplasmic membranes causes cells to lyse.<ref name=":113" /><ref name=":36" /><ref name=":52" />

Decreased processing times in ohmic heating maintains nutritional and sensory properties of foods.<ref name=":48" /> Ohmic heating inactivates [[Antinutrient|antinutritional]] factors like [[Lipoxygenase|lipoxigenase]] (LOX), [[Polyphenol oxidase|polyphenoloxidase]] (PPO), and [[pectinase]] due to the removal of active metallic groups in enzymes by the electrical field.<ref name=":36" /> Similar to other heating methods, ohmic heating causes [[Starch gelatinization|gelatinization]] of starches, melting of fats, and [[Agglutination (biology)|protein agglutination]].<ref name=":113" /> Water-soluble nutrients are maintained in the suspension liquid allowing for no loss of nutritional value if the liquid is consumed.<ref>{{Cite journal|last=Kaur|first=Ranvir|last2=Gul|first2=Khalid|last3=Singh|first3=A.K.|title=Nutritional impact of ohmic heating on fruits and vegetables A review|journal=Cogent Food & Agriculture|volume=2|issue=1|doi=10.1080/23311932.2016.1159000|year=2016|doi-access=free}}</ref>

Ohmic heating is limited by viscosity, electrical conductivity, and fouling deposits.<ref name=":48" /><ref name=":010" /><ref name=":113" /> 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.<ref name=":113" /> A food product’s electrical conductivity is a function of temperature, frequency, and product composition.<ref name=":48" /><ref name=":010" /><ref name=":113" /> This may be increased by adding ionic compounds, or decreased by adding non-polar constituents.<ref name=":48" /> Changes in electrical conductivity limit ohmic heating as it is difficult to model the thermal process when temperature increases in multi-component foods.<ref name=":48" /><ref name=":010" />

The potential applications of ohmic heating range from cooking, thawing, [[Blanching (cooking)|blanching]], peeling, evaporation, extraction, [[Dehydration (food)|dehydration]], and fermentation.<ref name=":113" /> 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.<ref name=":010" /> Prospective examples are outlined in Table 2 as this food processing method has not been commercially approved by the FDA.<ref name=":010" /> 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.<ref name=":6">{{Cite web|url=https://www.fda.gov/downloads/food/foodborneillnesscontaminants/ucm545175.pdf|title=Kinetics of Microbial Inactivation for Alternative Food Processing Technologies|date=May 30, 2018|website=U.S. Food and Drug Administration}}</ref> Additionally, a successful [[D-value (microbiology)|12D reduction]] for ''[[Clostridium botulinum|C. botulinum]]'' prevention has yet to be validated.<ref name=":6" />

{| class="wikitable"
|+Table 2. Applications of Ohmic Heating in Food Processing <ref name=":010" />
! scope="col" |Applications
! scope="col" |Advantages
! scope="col" |Food Items
|-
|Sterilisation, heating liquid foods containing large particulates and heat sensitive liquids, aseptic processing
|Attractive appearance, firmness properties, pasteurization of milk without protein denaturation
|Cauliflower florets, soups, stews, fruit slices in syrups and sauces, ready to cook meals containing particulates, milk, juices, and fruit purees
|-
|Ohmic cooking of solid foods
|The cooking time could be reduced significantly. The centre temperature rises much faster than in conventional heating, improving the final sterility of the product, less power consumption and safer product
|Hamburger patties, meat patties, minced beef, vegetable pieces, chicken, pork cuts
|-
|Space food and military ration
|Food reheating and waste sterilization. Less energy consumption for heating food to serving temperature, products in reusable pouches with long shelf life. Additive free foods with good keeping quality of 3 years.
|Stew type foods
|-
|Ohmic thawing
|Thawing without increase in moisture content of the product
|Shrimp blocks
|-
|Inactivation of spores and enzymes
|To improve food safety and enhance shelf life, increased stability and energy efficiency, Reduced time for inactivation of lipoxygenase and polyphenol oxidase, inactivation of enzymes without affecting flavor
|Process fish cake, orange juice, juices
|-
|Blanching and extraction
|Enhanced moisture loss and increase in juice yield
|Potato slices, vegetable purees extraction of sucrose from sugar beets, extraction of soy milk from soy beans
|}

=== Materials synthesis, recovery and processing ===

Flash joule heating (transient high-temperature electrothermal heating) has been used to synthesize [[allotropes of carbon]], 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 [[Graphene#Turbostratic|turbostratic graphene flakes]].<ref>{{Cite journal|last1=Luong|first1=Duy X.|last2=Bets|first2=Ksenia V.|last3=Algozeeb|first3=Wala Ali|last4=Stanford|first4=Michael G.|last5=Kittrell|first5=Carter|last6=Chen|first6=Weiyin|last7=Salvatierra|first7=Rodrigo V.|last8=Ren|first8=Muqing|last9=McHugh|first9=Emily A.|last10=Advincula|first10=Paul A.|last11=Wang|first11=Zhe|date=January 2020|title=Gram-scale bottom-up flash graphene synthesis|url=https://www.nature.com/articles/s41586-020-1938-0|journal=Nature|language=en|volume=577|issue=7792|pages=647–651|doi=10.1038/s41586-020-1938-0|pmid=31988511|bibcode=2020Natur.577..647L|s2cid=210926149|issn=1476-4687}}</ref> FJH has also been used to [[rare-earth element#Recycling and reusing REEs|recover rare-earth elements]] used in modern electronics [[Circular economy|from industrial wastes]]<!-- with practical potential to reduce environmental/health impacts from mining, waste-generation and imports if it can be scaled up-->.<ref>{{cite news |title=Rare earth elements for smartphones can be extracted from coal waste |url=https://www.newscientist.com/article/2307608-rare-earth-elements-for-smartphones-can-be-extracted-from-coal-waste/ |work=New Scientist}}</ref><ref>{{cite journal |title=Rare earth elements from waste |journal=Science Advances|year=2022 |doi=10.1126/sciadv.abm3132 |last1=Deng |first1=Bing |last2=Wang |first2=Xin |last3=Luong |first3=Duy Xuan |last4=Carter |first4=Robert A. |last5=Wang |first5=Zhe |last6=Tomson |first6=Mason B. |last7=Tour |first7=James M. |volume=8 |issue=6 |pages=eabm3132 |pmid=35138886 |pmc=8827657 }}</ref> 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.<ref>{{Cite web|last=Michael|first=Irving|date=June 22, 2021|title=New method converts carbon into graphene or diamond in a flash|url=https://newatlas.com/materials/carbon-graphene-diamond-flash-joule-heating/|access-date=2021-06-22|website=New Atlas|language=en-US}}</ref><ref>{{Cite journal|last1=Chen|first1=Weiyin|last2=Li|first2=John Tianci|last3=Wang|first3=Zhe|last4=Algozeeb|first4=Wala A.|last5=Luong|first5=Duy Xuan|last6=Kittrell|first6=Carter|last7=McHugh|first7=Emily A.|last8=Advincula|first8=Paul A.|last9=Wyss|first9=Kevin M.|last10=Beckham|first10=Jacob L.|last11=Stanford|first11=Michael G.|date=2021-07-27|title=Ultrafast and Controllable Phase Evolution by Flash Joule Heating|url=https://doi.org/10.1021/acsnano.1c03536|journal=ACS Nano|volume=15|issue=7|pages=11158–11167|doi=10.1021/acsnano.1c03536|pmid=34138536|osti=1798515 |s2cid=235471710|issn=1936-0851}}</ref>

== Gallery ==
{{gallery
|title= Joule heating applications
|width=165
|Image:Livermore Centennial Light Bulb.jpg| An [[incandescent light bulb]]'s filament emitting light
|Image:Thermal image of an incandescent light.jpg|[[Infrared]]-[[thermal image]] of a light bulb
|Image:Carbon heater.jpg|Electric radiative space heater
|9=Image:Light Label Electric tabletop burner KCK-L103.jpg|10=Electric tabletop hotplate|11=Image:Morphy richards iron 1950.JPG|12=[[Clothes iron]] used to remove wrinkles from clothes|13=Image:Soldering gun.jpg|14=[[Soldering iron]], used to melt solder in electronic work|15=Image:Hairdryer 20101109.jpg|16=[[Hair dryer]], produces hot air flow}}

== Heating efficiency ==
{{Main|Electric heating}}

Heat is not to be confused with [[internal energy]] or synonymously [[thermal energy]]. While intimately connected to [[heat]], they are distinct physical quantities.

As a heating technology, Joule heating has a [[coefficient of performance]] of 1.0, meaning that every joule of electrical energy supplied produces one joule of heat. In contrast, a [[heat pump]] 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 ==
{{main|Darcy's law}}

In the [[Groundwater energy balance|energy balance of groundwater flow]] a hydraulic equivalent of Joule's law is used:<ref>{{cite book | title = The energy balance of groundwater flow | author = R.J.Oosterbaan, J.Boonstra and K.V.G.K.Rao | publisher = In: V.P.Singh and B.Kumar (eds.), Subsurface-Water Hydrology, Vol.2 of the Proceedings of the International Conference on Hydrology and Water Resources, New Delhi, India. Kluwer Academic Publishers, Dordrecht, The Netherlands | year = 1996 | isbn = 978-0-7923-3651-8 | pages = 153–160 | url = http://www.waterlog.info/pdf/enerbal.pdf }}</ref>

<math display="block"> \frac{dE}{dx} = \frac{(v_x)^2}{K} </math>

where:

* <math>dE/dt</math> = loss of hydraulic energy (<math>E</math>) due to friction of flow in <math>x</math>-direction per unit of time (m/day), comparable to <math>P</math>
* <math>v_x</math> = flow velocity in <math>x</math>-direction (m/day), comparable to <math>I</math>
* <math>K</math> = [[hydraulic conductivity]] of the soil (m/day), the hydraulic conductivity is inversely proportional to the hydraulic resistance which compares to <math>R</math>

== See also ==

* {{annotated link|Dielectric heating}}
* {{annotated link|Heating element}}
* {{annotated link|Induction heating}}
* {{annotated link|Joule's Second Law}}
* {{annotated link|Molybdenum disilicide}}
* {{annotated link|Nichrome}}
* {{annotated link|Overheating (electricity)}}
* {{annotated link|Resistance wire}}
* {{annotated link|Electric resistance welding}}
* {{annotated link|Thermal management (electronics)}}
* {{annotated link|Tungsten}}

== References ==

{{refs}}

[[Category:Electric heating]]
[[Category:Electricity]]
[[Category:Thermodynamics]]
[[Category:James Prescott Joule|Heating]]
[[Category:Food processing]]
[[Category:Cooking techniques]]