Editing Microwave oven (section)

== Principles ==
{{further|Dielectric heating}}
[[File:Weibolu.jpg|thumb|250px|A microwave oven, c. 2005]]
[[File:Microwaveoventransient.webm|thumb|250px|Simulation of the electric field inside a microwave oven for the first 8 ns of operation]]

A microwave oven heats food by passing [[microwave radiation]] through it. Microwaves are a form of [[non-ionizing]] [[electromagnetic radiation]] with a [[frequency]] in the so-called [[Microwave|microwave region]] (300&nbsp;MHz to 300&nbsp;GHz). Microwave ovens use frequencies in one of the [[ISM band|ISM (industrial, scientific, medical) bands]], which are otherwise used for communication amongst devices that do not need a license to operate, so they do not interfere with other vital radio services.

It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food.{{Citation needed|date=February 2025}} Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times.{{Citation needed|date=February 2025}} Typically, consumer ovens work around a nominal 2.45&nbsp;[[gigahertz]] (GHz) – a [[wavelength]] of {{convert|12.2|cm|sigfig=3}} in the 2.4&nbsp;GHz to 2.5&nbsp;GHz ISM band – while large industrial / commercial ovens often use 915&nbsp;[[megahertz]] (MHz) – {{convert|32.8|cm|sigfig=3}}.<ref>{{cite web |title=For heat, tune to 915 or 2450&nbsp;Megacycles |year=2007 |publisher=[[Litton Industries]] |orig-date=1965 |via=Southwest Museum of Engineering, Communications, and Computation |url=http://www.smecc.org/litton_-_for_heat,_tune_to_915_or_2450_megacycles.htm |access-date=December 12, 2006}}</ref> Among other differences, the longer wavelength of a commercial microwave oven allows the initial heating effects to begin deeper within the food or liquid, and therefore become evenly spread within its bulk sooner, as well as raising the temperature deep within the food more quickly.<ref name=depth>{{cite web |title=Cooking with Microwave Ovens|website=Food Safety and Inspection Service |publisher=[[U.S. Department of Agriculture]] |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#3}}</ref>

A microwave oven takes advantage of the electric [[dipole]] structure of [[water]] [[molecule]]s, fats, and many other substances in the food, using a process known as [[dielectric heating]]. These molecules have a partial positive charge at one end and a partial negative charge at the other. In an alternating electric field, they will constantly spin around as they continually try to align themselves with the electric field. This can happen over a wide range of frequencies.<ref name="Soltysiak">{{cite conference |last1=Soltysiak |first1=Michal |last2=Celuch |first2=Malgorzata |last3=Erle |first3=Ulrich |title=Measured and simulated frequency spectra of the household microwave oven |conference=2011 IEEE MTT-S International Microwave Symposium |date=June 2011 |pages=1–4 |doi=10.1109/MWSYM.2011.5972844 |isbn=978-1-61284-754-2 |s2cid=41526758}}</ref><ref name=microwave>{{cite web |author=Bloomfield, Louis |title=Question 1456 |website=How Everything Works |url=http://www.howeverythingworks.org/page1.php?QNum=1456 |access-date=February 9, 2012 |archive-url=https://web.archive.org/web/20131017005928/http://www.howeverythingworks.org/page1.php?QNum=1456 |archive-date=October 17, 2013}}</ref><ref>{{cite web |first=Christopher S. |last=Baird |date=October 15, 2014 |title=Why are the microwaves in a microwave oven tuned to water |website=Science Questions with Surprising Answers |publisher=[[West Texas A&M University]] |place=Canyon, TX |url=http://wtamu.edu/~cbaird/sq/2014/10/15/why-are-the-microwaves-in-a-microwave-oven-tuned-to-water/}}</ref> The electric field's energy is absorbed by the dipole molecules as rotational energy.  Then they hit non-dipole molecules, making them move faster as well. This energy is shared deeper into the substance as molecular rotation and translational movement occurs, signifying an increase in the [[Temperature#Kinetic theory approach|temperature]] of the food. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other heat transfer by contact with a hotter body.<ref>{{cite book |last=Zitzewitz |first=Paul W. |date=February 2011 |title=The Handy Physics Answer Book |publisher=Visible Ink Press |isbn=9781578593576 |language=en |url=https://books.google.com/books?id=qGTkgFZBJZQC&q=microwave+oven+principle&pg=PA153 |via=Google Books}}</ref>

=== Defrosting ===
Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted.  Defrosting is done at a low power setting, allowing time for conduction to carry heat to still frozen parts of food. Dielectric heating of liquid water is also temperature-dependent: At 0&nbsp;°C, [[dielectric loss]] is greatest at a field frequency of about 10&nbsp;GHz, and for higher water temperatures at higher field frequencies.<ref name=Martin>{{cite web |last=Chaplin |first=Martin |title=Water and microwaves |url=http://www.lsbu.ac.uk/water/microwave.html |series=Water Structure and Science |publisher=[[London South Bank University]] |access-date=December 4, 2012 |date=May 28, 2012}}</ref>

=== Fats and sugar ===
[[Sugar]]s and [[triglyceride]]s (fats and oils) absorb microwaves due to the dipole moments of their [[hydroxyl groups]] or [[ester|ester groups]]. Due to their [[molecular dipole moment]], however, they are heated less efficiently.{{efn|Here "efficient" means that more energy is deposited and temperature rises faster, not necessarily that the temperature rises to a higher maximum. The maximum temperature is also a function of the material's [[specific heat capacity]], which for most substances is lower than water. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.{{Citation needed|date=January 2009}}}}

Although fats and sugar typically absorb energy less efficiently than water, paradoxically their temperatures rise faster and higher than water when cooking: Fats and oils require less energy delivered per gram of material to raise their temperature by 1&nbsp;°C than does water (they have lower [[specific heat capacity]]) and they begin cooling off by "boiling" only after reaching a higher temperature than water (the temperature they require to [[vaporize]] is higher), so inside microwave ovens they normally reach higher temperatures – sometimes ''much'' higher.<ref name=Martin/> This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional [[grilling|broiling (UK: grilling)]], braising, or deep fat frying.{{citation needed|date=July 2020}}

The effect is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures.{{Citation needed|date=February 2025}} Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic.{{Citation needed|date=February 2025}}

=== Cookware ===
[[Cookware]] must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents the microwaves from reaching the food.{{Citation needed|date=February 2025}} Cookware made of materials with high [[Relative permittivity|electrical permittivity]] will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of [[melamine resin]] is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware.{{Citation needed|date=February 2025}}

=== Thermal runaway ===
Microwave heating can cause localized [[thermal runaway]]s in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.<ref>{{cite conference |last1=Jerby |first1=Eli |last2=Meir |first2=Yehuda |last3=Faran |first3=Mubarak |title=Basalt melting by localized-microwave thermal-runaway instability |doi=10.13140/2.1.4346.1126 |url=https://www.eng.tau.ac.il/~jerby/Jerby_Basalt_Ampere-2013_Proc_PDF.pdf |conference=14th International Conference on Microwave and High Frequency Heating, AMPERE-2013 |location=Nottingham, UK |date=September 2013}}</ref>

=== Penetration ===
Another misconception is that microwave ovens cook food "from the inside out", meaning from the center of the entire mass of food outwards.{{Citation needed|date=February 2025}} This idea arises from heating behavior seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface.{{Citation needed|date=February 2025}} This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach a higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogeneous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers.{{Citation needed|date=February 2025}}

Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast with [[grilling|broiling / grilling]] (infrared) or convection heating methods which thinly deposit heat at the food surface. Penetration depth of microwaves depends on [[food composition]] and the frequency, with lower microwave frequencies (longer wavelengths) penetrating deeper.<ref name="depth"/>

=== Energy consumption ===
In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves,<ref>{{cite web |last1=Wirfs-Brock |first1=Jordan |last2=Jacobson |first2=Rebecca |date=February 23, 2016 |title=A watched pot: What is the most energy-efficient way to boil water? |website=Inside Energy |url=http://insideenergy.org/2016/02/23/boiling-water-ieq/}}</ref> but energy-efficient models can exceed 64% efficiency.<ref>{{cite web |title=Energy Label- Criteria |website=www.energylabel.org.tw |publisher=Bureau of Energy, Ministry of Economic Affairs, Taiwan |url=https://www.energylabel.org.tw/englishlabel/application_en/efficiency/upt.aspx?Cid=48 |access-date=March 7, 2022 |language=zh-Hant-TW}}</ref> Stovetop cooking is 40–90% efficient, depending on the type of appliance used.<ref>{{cite web |title=Level-up your cooking game with an energy-efficient stovetop |date=August 24, 2020 |url=https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ |access-date=December 23, 2022 |archive-date=December 23, 2022 |archive-url=https://web.archive.org/web/20221223161204/https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ |url-status=dead }}</ref>

Because they are used fairly infrequently, the average residential microwave oven consumes only 72&nbsp;kWh per year.<ref>{{cite journal |last1=Gallego-Schmid |first1=Alejandro |last2=Mendoza |first2=Joan Manuel F. |last3=Azapagic |first3=Adisa |title=Environmental assessment of microwaves and the effect of European energy efficiency and waste management legislation |journal=Science of the Total Environment |date=March 2018 |volume=618 |pages=487–499 |doi=10.1016/j.scitotenv.2017.11.064 |pmid=29145100 |bibcode=2018ScTEn.618..487G |url=https://www.research.manchester.ac.uk/portal/en/publications/environmental-assessment-of-microwaves-and-the-effect-of-european-energy-efficiency-and-waste-management-legislation(ac7e65c2-c53e-4d0f-9bc8-b03ceb65541f).html |access-date=}}</ref> Globally, microwave ovens used an estimated 77&nbsp;TWh per year in 2018, or 0.3% of global electricity generation.<ref>{{cite journal |last1=Detz |first1=Remko J. |last2=van der Zwaan |first2=Bob |title=Surfing the microwave oven learning curve |journal=Journal of Cleaner Production |date=October 20, 2020 |volume=271 |pages=122278 |doi=10.1016/j.jclepro.2020.122278 |s2cid=225872878 |access-date=|doi-access=free |bibcode=2020JCPro.27122278D }}</ref>

A 2000 study by [[Lawrence Berkeley National Laboratory]] found that the average microwave drew almost 3 watts of [[standby power]] when not being used,<ref>{{cite book |last1=Ross |first1=J.P. |last2=Meier |first2=Alan |title=Energy Efficiency in Household Appliances and Lighting |chapter=Whole-House Measurements of Standby Power Consumption |year=2001 |pages=278–285 |doi=10.1007/978-3-642-56531-1_33 |isbn=978-3-540-41482-7 |chapter-url=https://digital.library.unt.edu/ark:/67531/metadc743210/ }}</ref> which would total approximately 26&nbsp;kWh per year. New efficiency standards imposed in 2016 by the [[United States Department of Energy]] require less than 1&nbsp;watt, or approximately 9&nbsp;kWh per year, of standby power for most types of microwave ovens.<ref>{{cite web |last=de&nbsp;Laski |first=Andrew |date=June 3, 2013 |title=New standards cut "vampire" energy waste |website=Appliance Standards Awareness Project (ASAP) |url=https://appliance-standards.org/blog/new-standards-cut-vampire-energy-waste |access-date=October 4, 2021}}</ref>