Joule heating

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

Joule's first law (also just Joule's law), also known in countries of the former USSR as the Joule–Lenz law,<ref name="BSE">Джоуля — Ленца закон Archived 2014-12-30 at the Wayback Machine. Большая советская энциклопедия, 3-е изд., гл. ред. А. М. Прохоров. Москва: Советская энциклопедия, 1972. Т. 8 (A. M. Prokhorov; et al., eds. (1972). "Joule–Lenz law". Great Soviet Encyclopedia (in русский). Vol. 8. Moscow: Soviet Encyclopedia.)</ref> states that the power of heating generated by an electrical conductor 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 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).
Electric fuses are used as a safety, breaking the circuit by melting if enough current flows to melt them.
Electronic cigarettes vaporize propylene glycol and vegetable glycerine by Joule heating.
Multiple heating devices use Joule heating, such as electric stoves, electric heaters, soldering irons, cartridge heaters.
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>Ramaswamy, Raghupathy. "Ohmic Heating of Foods". Ohio State University. Archived from the original on 2013-04-08. Retrieved 2013-04-22.</ref> The alternating electrical current coupled with the resistance of the food causes the generation of heat.<ref name=":0">Fellows, P.J (2009). Food Processing Technology. MA: Elsevier. pp. 813–844. ISBN 978-0-08-101907-8.</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">Varghese, K. Shiby; Pandey, M. C.; Radhakrishna, K.; Bawa, A. S. (October 2014). "Technology, applications and modelling of ohmic heating: a review". Journal of Food Science and Technology. 51 (10): 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 proportional to the square of the current multiplied by the electrical resistance of the immersed wire.<ref name="APS">"This Month Physics History: December 1840: Joule's abstract on converting mechanical power into heat". aps.org. American Physical society. Retrieved 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

Joule heating is caused by interactions between charge carriers (usually electrons) and the body of the conductor.

A 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. 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 losses in electrical transformers) the diversion of energy is often referred to as resistive loss. The use of high voltages in electric power transmission 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 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 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, Google books link</ref>

(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 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,

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:

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

These formulas are valid for an ideal resistor, with zero 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 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

Overhead power lines 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 conductors, 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>"Transformer circuits". Retrieved 26 July 2017.</ref> and transmission losses are reduced in proportion.

During the war of currents, 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 (also called "high-temperature short-time" (HTST)) aseptic process that runs an alternating current of 50–60 Hz through food.<ref name=":2">Fellows, P. (2017) [2016]. Food processing technology : principles and practice (4th ed.). Kent: Woodhead Publishing/Elsevier Science. ISBN 9780081019078. OCLC 960758611.</ref> Heat is generated through the food's electrical resistance.<ref name=":2" /><ref name=":48">Ohmic heating in food processing. Ramaswamy, Hosahalli S. Boca Raton, FL: CRC Press. 2014. ISBN 9781420071092. OCLC 872623115.{{cite book}}: CS1 maint: others (link)</ref><ref name=":010">Varghese, K. Shiby; Pandey, M. C.; Radhakrishna, K.; Bawa, A. S. (October 2014). "Technology, applications and modelling of ohmic heating: a review". Journal of Food Science and Technology. 51 (10): 2304–2317. doi:10.1007/s13197-012-0710-3. ISSN 0022-1155. PMC 4190208. PMID 25328171.</ref><ref name=":113">Fellows, P.J. (2017). Food processing technology. Woodhead Publishing. pp. 831–38. ISBN 978-0-08-101907-8.</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">Varzakas, Theodoros; Tzia, Constantina (2015-10-22). Handbook of food processing : food preservation. Varzakas, Theodoros,, Tzia, Constantina. Boca Raton, FL. ISBN 9781498721769. OCLC 924714287.{{cite book}}: CS1 maint: location missing publisher (link)</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 resistance properties.<ref name=":010" /> Ohmic heating is beneficial due to its ability to inactivate microorganisms through thermal and non-thermal cellular damage.<ref name=":113" /><ref name=":36">Ohmic Heating in food processing. CRC Press. 2014. pp. 93–102. ISBN 978-1-4200-7109-2.</ref><ref name=":52">Varghese, K. Shiby; Pandey, M. C.; Radhakrishna, K.; Bawa, A. S. (2014-10-01). "Technology, applications and modelling of ohmic heating: a review". Journal of Food Science and Technology. 51 (10): 2304–2317. doi:10.1007/s13197-012-0710-3. ISSN 0022-1155. PMC 4190208. PMID 25328171.</ref>

This method can also inactivate antinutritional factors thereby maintaining nutritional and 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 (FDA) for commercial use, this method has many potential applications, ranging from cooking to 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" /> Electrodes, 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 caps around the electrodes controls the environment within the system.<ref name=":010" />

The 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 compounds suspended in the product, which is directly proportional to the rate of heating.<ref name=":010" /> This value is increased in the presence of polar compounds, like acids and salts, but decreased with 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 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" />

Table 1. Electrical conductivity of selected foods<ref name=":113" />
Food	Electrical Conductivity (S/m)	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⁻¹), less cooking time, better energy efficiency, lower capital cost, and heating simulataneously throughout food's volume as compared to aseptic processing, canning, and 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 microorganisms due to electroporation of cell membranes, physical membrane rupture, and cell lysis.<ref name=":113" /><ref name=":36" /> In electroporation, excessive leakage of ions and 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 antinutritional factors like lipoxigenase (LOX), 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 gelatinization of starches, melting of fats, and 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>Kaur, Ranvir; Gul, Khalid; Singh, A.K. (2016). "Nutritional impact of ohmic heating on fruits and vegetables A review". Cogent Food & Agriculture. 2 (1). doi:10.1080/23311932.2016.1159000.</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, peeling, evaporation, extraction, 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">"Kinetics of Microbial Inactivation for Alternative Food Processing Technologies" (PDF). U.S. Food and Drug Administration. May 30, 2018.</ref> Additionally, a successful 12D reduction for C. botulinum prevention has yet to be validated.<ref name=":6" />

Table 2. Applications of Ohmic Heating in Food Processing <ref name=":010" />
Applications	Advantages	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 turbostratic graphene flakes.<ref>Luong, Duy X.; Bets, Ksenia V.; Algozeeb, Wala Ali; Stanford, Michael G.; Kittrell, Carter; Chen, Weiyin; Salvatierra, Rodrigo V.; Ren, Muqing; McHugh, Emily A.; Advincula, Paul A.; Wang, Zhe (January 2020). "Gram-scale bottom-up flash graphene synthesis". Nature. 577 (7792): 647–651. Bibcode:2020Natur.577..647L. doi:10.1038/s41586-020-1938-0. ISSN 1476-4687. PMID 31988511. S2CID 210926149.</ref> FJH has also been used to recover rare-earth elements used in modern electronics from industrial wastes.<ref>"Rare earth elements for smartphones can be extracted from coal waste". New Scientist.</ref><ref>Deng, Bing; Wang, Xin; Luong, Duy Xuan; Carter, Robert A.; Wang, Zhe; Tomson, Mason B.; Tour, James M. (2022). "Rare earth elements from waste". Science Advances. 8 (6): eabm3132. doi:10.1126/sciadv.abm3132. PMC 8827657. PMID 35138886.</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>Michael, Irving (June 22, 2021). "New method converts carbon into graphene or diamond in a flash". New Atlas. Retrieved 2021-06-22.</ref><ref>Chen, Weiyin; Li, John Tianci; Wang, Zhe; Algozeeb, Wala A.; Luong, Duy Xuan; Kittrell, Carter; McHugh, Emily A.; Advincula, Paul A.; Wyss, Kevin M.; Beckham, Jacob L.; Stanford, Michael G. (2021-07-27). "Ultrafast and Controllable Phase Evolution by Flash Joule Heating". ACS Nano. 15 (7): 11158–11167. doi:10.1021/acsnano.1c03536. ISSN 1936-0851. OSTI 1798515. PMID 34138536. S2CID 235471710.</ref>

Gallery

Joule heating applications

An incandescent light bulb's filament emitting light
Infrared-thermal image of a light bulb
Electric radiative space heater
Electric tabletop hotplate
Clothes iron used to remove wrinkles from clothes
Soldering iron, used to melt solder in electronic work
Hair dryer, produces hot air flow

Heating efficiency

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

In the energy balance of groundwater flow a hydraulic equivalent of Joule's law is used:<ref>R.J.Oosterbaan, J.Boonstra and K.V.G.K.Rao (1996). The energy balance of groundwater flow (PDF). 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. pp. 153–160. ISBN 978-0-7923-3651-8.</ref>

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>

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History

Microscopic description

Power loss and noise