FOOD ENGINEERING CLUB

Microwave ovens: How do they work?

 

Microwave ovens have been in our homes for more than 40 years. With our increasingly busy lifestyle, microwave ovens have made cooking faster and more convenient. In this webpage we’ll answer three common questions about domestic microwave ovens:

1. What are microwaves?

2. How do microwaves heat and cook my food? and

3. What is the difference between microwave and electric ovens?

 

1. What are Microwaves?

 

Microwaves are a type of radiation, that is, they are energy that travels in the form of high-speed particles and electromagnetic waves (Lee, 2014). We can also define microwaves, and other electromagnetic waves, as a flow of mass-less particles called photons traveling at light speed in wave patterns. The difference between the waves is in the amount of energy each photon has, the frequency (number of waves per second), and the length of the wave. This type of radiation is commonly found in everyday life. For example, the sun, light bulbs, fireflies, electric ovens, cell phones, and your favorite radio station all emit radiation at different energy levels. The waves used in a microwave oven have more energy than your radio station but less energy than a light bulb, the sun or the glow from fireflies (NASA, 2013). Is important to point out that radiation is something that occurs naturally and is all around us. Anything that has a temperature above absolute 0 (-273.15 °C or -459.67 °F) emits some kind of radiation, and that includes you! Yes! Humans emit radiation in the infrared part of the electromagnetic spectrum; and you have felt it too! Every time you stand close enough to someone or something and you feel their “body heat” without really touching them what you feel is the thermal radiation coming from their body. In physics we call this black-body radiation or a bio-energetic field, if the source is a live system, while people in metaphysics call it an aura. Regardless of the name, your “body heat” is a form of radiation and part of the electromagnetic spectrum (Stenger, 1999).

 

For more information on how the electromagnetic spectrum works please watch this NASA video or visit this NASA webpage. For more information on radiation please visit this NASA webpage.

Figure 1. Types of Electromagnetic Radiation (Source)

 

Radiation can be divided into two categories based on the amount of energy it possess: ionizing or non-ionizing.

Ionizing radiation refers to the electromagnetic waves with enough energy to remove electrons from the orbits of atoms, leaving a charged particle we call ion. This ionizing effect is different from the one obtained through chemical reactions. Chemical reactions remove electrons from the outer orbit of an atom, while ionizing radiation can remove electrons from other orbits if it has sufficient energy; the resulting ions are a lot more unstable and reactive. Examples of this type of radiation are X-rays and UV light (Lee, 2014), which is why too much exposure to the sun or tanning beds is not recommended by health experts. However, don’t forget that you do need sun light to make vitamin D, so it’s all in moderation. For more information on UV light exposure and cancer risk, please visit the American Cancer Society.

Non-ionizing radiation on the other hand, doesn’t have enough energy to remove electrons from atoms. Microwaves, light bulbs (which emit visible light) or radio waves (also called radio frequencies) belong to this type of radiation (Lee, 2014).

But how do we know microwaves don’t have enough energy to break bonds or make ions?

In order to break down molecules we need to use enough energy to separate the bond that holds them together. As we said before, microwaves are energy in the form of photons that travel in wave like patterns at the speed of light. However, this energy is not a continuous emission but a series of energy packages that German physicist Mark Planck called quanta. Planck postulated that the energy of the quanta can be related to its frequency using the following equation (Nguyen, 2015; Schombert, 2014): 

                            E= hf                                           (1)

Were E is the energy emitted in Joules (abbreviated as J), f is the frequency of the wave in Hertz (number of waves per second, abbreviated as Hz); and h is Planck’s constant h= 6.626 x 10-34 Joules per second (abbreviated as J*s) or 4.136 x 10-15 electron volt per second (abbreviated as eV*s).

Electromagnetic waves, like microwaves, have specific wavelength, frequency and energy. The microwaves used in your oven at home have a frequency of 2450 mega Hertz (MHz), and industrial microwave equipment typically uses 915 MHz waves. If we substitute the frequency in Planck’s equation, the quantum energy emitted by the waves in your microwave oven is 1.623x10-24 J (1.013*10-5 eV) for 2450 MHz and 6.063x10-25 J (or 3.784*10-6 eV) for 915 MHz.

But what does that mean?

Molecules are made of atoms which are kept together by different types of bonds. The most stable bond is called a covalent bond. Water for example is made out of an oxygen atom bound covalently to two atoms of hydrogen. If I wanted to break one of the bonds between oxygen and hydrogen (abbreviated as O-H), I would have to supply energy that we call “bond energy”. How much? 467 kJ/mol; there are approximately 6.022*1023 molecules of water in one mol of water; so if I wanted to break one O-H bond in one molecule of water, I would have to “hit” it with approximately 7.75*10-19 J. Fortunately the quanta in your microwave oven only have 1.623x10-24 J, so is not enough to break down water molecules into hydrogen and oxygen (Song and Le, n.d.). Even if you hit your water with more than one microwave at a time, as occurs in a microwave oven, you would only have enough energy to boil the water (separate molecules of water from one another), but not break it down. X-rays on the other hand emit 6.63x10-18-6.63x10-14 J, so with enough exposure you could eventually create some ions from water.

For more information on wave-particle duality and quantum mechanics, please refer to these webpages ChemWiki and UOregon .

 

2. How do microwaves heat and cook my food?

Your microwave oven has a magnetron that changes electric energy from the wall outlet into microwaves. The microwaves travel through a tube, called a waveguide, and then enter the box with your food, called a cavity. The inside of the cavity is made out of metal so it can reflect the waves and they can bounce around until they make it into your food.

Once the microwaves go into your food they create small electromagnetic fields that go from positive to negative 2.4 billion times every second. Food contains ions or charged particles and dipolar molecules, such as water, that have both positive and negative charges. As the electromagnetic field changes these charges and molecules will move to try to align themselves in the electromagnetic field. As the molecules quickly move around, they bump into each other, causing friction between them. The friction between the molecules creates heat and starts cooking your food. Friction heating is something we commonly use, such as when you quickly rub your hands together to warm them up when it’s cold outside.

To help you visualized how microwaves work, please take a look at this video (from the University of Illinois)

 

3. What is the difference between microwave and electric ovens?

The main difference between microwave ovens and traditional electric ovens used for baking is the type of energy that each oven uses to cook your food. Both ovens start with electricity from the wall outlet and then convert this electricity into different types of energy. Microwave ovens use electricity from the wall outlet to create electromagnetic waves that go directly into your food causing friction between molecules that generates heat. Using a microwave oven, your food is heated all at once as it shows in Figure 2. So you have cold (lowest temperature) and hot (highest temperature) spots everywhere depending on how the waves rebound around the oven cavity.

Figure 2. Diagram of microwave heating mechanisms for a pizza

 

Traditional electric ovens for baking use electricity from the wall outlet to heat a coil that heats the air inside the oven. The hot air heats the surface of your food by transferring energy from its molecules to the molecules in the food surface, we call this kind of heat transfer convection. Once the surface of your food starts to heat the energy will flow from the outside surface to the center of your food, we call this heat transfer conduction as it’s shown in Figure 3. So your hot spot is always the surface of your food, while your cold spot is always the center. Your food is also heated by radiating heat from the hot coil and metal sides of the oven since, as you already know from the answer in question 1, all objects emit some amount of thermal radiation.

 

Figure 3. Diagram of electric oven heating mechanisms for a pizza

 

Key points:

  • Microwaves have more energy than those emitted by your favorite radio station, but less energy than sun rays.
  • Microwaves heat food quickly because of friction from rotating water molecules and charged particles, such as salt dissolved in water.
  • Microwave ovens heat your food using the energy in the microwaves and the food heats all at once. This is different from electric ovens that heat your food using hot air and radiative heating, from the outside surface to the center.

                          

Credits:

  • Food Engineering Club at Washington State University
  • Sponsored in part by the USDA National Needs Program and USDA NIFA Grant AFRI 2011-68003-20096

 

References

  • Lee, K., 2014. What is space Radiation? [WWW Document]. Sp. Radiat. Anal. Group, Johnson Sp. Cent. URL http://srag-nt.jsc.nasa.gov/spaceradiation/what/what.cfm (accessed 12.4.15).
  • NASA, 2013. The Electromagnetic Spectrum [WWW Document]. URL http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html
  • Nguyen, D., 2015. Wave-Particle Duality [WWW Document]. Chemwiki.ucdavis.edu. URL http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/09._The_Hydrogen_Atom/Atomic_Theory/Electrons_in_Atoms/Wave-Particle_Duality
  • Schombert, J., 2014. Electromagnetic Radiation (a.k.a. Light) [WWW Document]. AST123 Galaxies Expand. Universe Cosmol. Coll. Semin. URL http://abyss.uoregon.edu/~js/ast123/lectures/lec06.html
  • Song, K., Le, D., n.d. Bond Energies [WWW Document]. Chemwiki.ucdavis.edu. URL http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/General_Principles/Bond_Energies
  • Stenger, V.J., 1999. Bioenergetic Fields [WWW Document]. URL http://www.colorado.edu/philosophy/vstenger/Medicine/Biofield.html
Contact Person:
Shuxiang Liu, President shuxiang.liu@wsu.edu

Questions or comments? Reach us: fec.wsucougs@gmail.com