First off, who am I and why should you trust me?…
I am a senior biochemistry student at DePauw University in beautiful, small, middle of no-where Greencastle, IN. I am originally from Knoxville, Tennessee, so if you’d like you can imagine this post was written in a Southurn accint. My background is mostly in biochemistry, but as an aspiring med student I’ve also taken two semesters of physics. I did pretty well in those two semesters, and I’m confident I learned enough to research the basics of microwave physics and explain them to you here. I have also have an extensive history with microwavable food…I currently eat Lean Cusines and Campbell’s Soup on a regular basis while I’m at school…so I’d like to think you can trust me when I say I am a microwave expert of sorts.
Everyday when my brother and I came home from school he would open a can of Spaghettios, pop them in the microwave, and wait the 2 or 3 minutes until they were warm and ready to eat. When I was younger I just inherently knew that if you put something in the microwave and turned it on for long enough the food would get warm, but how does a microwave oven actually heat up your food?
Microwave ovens actually use microwaves- a form of electromagnetic radio wave to heat food. Electromagnetic radiation is a form of energy that has both electric and magnetic field components and exhibits wave-like behavior as it travels through space (4). Radio waves have the longest wavelength and the lowest frequency in the electromagnetic spectrum (seen below), making them the lowest energy EM wave based on the Plank-Einstein relationship (E = hc/λ) which says the energy of an electromagnetic wave is directly proportional to the Planck Constant (h) and the speed of light (c) but indirectly proportional to wavelength (4).
In the case of microwave ovens, the frequency of radio wave usually used is about 2,500 megahertz (4). Interestingly, microwaves at this frequency are absorbed by water, fats and sugars and are not absorbed by most plastics, glass or ceramics. As the water, fats, and sugars in your Spaghetios or Pop-Tart absorb the microwaves they heat up by a process called “dielectric heating.” The molecules are dipoles, meaning they have a positive and negative charge on opposite ends. The dipoles begin to spin as they try to align themselves with the alternating electric field of the microwaves, causing them to rub together and create heat (3). The heat produced by the the water, fat, and sugar molecules in your food rubbing together begins to heat the molecules around them and, essentially, to cook your food! The length of time required to cook your food, therefore, depends on its water, fat, and sugar content in relation to its size.
…So, where do the microwaves come from? The microwaves are generated by a magnetron within the oven. Magnetrons were invented in 1921 and then vastly improved in the 1940s (2). The physics of a magnetron is a little beyond the grasp of this blog, but in layman’s terms it is essentially a tube that moves electrons through a magnetic field which causes the electron path to curve and create oscillating microwaves (2).
The microwaves are then corralled into the cooking box by the waveguide where they bounce around, reflected by the metal of the box, until they are absorbed by your food. Microwaves are a common household object around the world. We use them to heat up left-overs, cook microwave dinners, and to pop popcorn before we sit down to watch a movie. I hope you enjoyed learning about a little bit about the science behind these household marvels, but in case you’re craving a little bit more knowledge, here are some fun facts…
- Microwaves cook your food from the inside out, as opposed to a conventional oven which cooks food from the outside in by the process of convection. This is why Hot Pockets have a little metal casing around them which allows heat to be reflected back at its surface creating a crust! (1)
- Microwaves aren’t nearly as efficient at cooking frozen foods because the molecules are not free to rotate. (3)
- As of 1971 only about 1% of American homes had a microwave. That number rose to 25% by 1986, and as of 2009 90% of American households had a microwave.
- Microwaves convert Vitamin B12, an essential vitamin predominantly found in meat, to an inactive form. (3)
- However, spinach retains almost all of its folate when cooked in a microwave, but loses approximately 80% when it is cooked on a normal stove. (3)
- The first documented use of the term “microwave” was in 1931. It was used in a Telegraph and Telephone Journal, which said “When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon.” (3)
1. Brain, Marshall. “HowStuffWorks “Microwave Cooking”" HowStuffWorks “Home and Garden”2011. Web. 16 Nov. 2011. <http://home.howstuffworks.com/microwave2.htm>
2. Gallawa, Carlton. “The Magnetron Used in Microwave Ovens: Structure and Operation.” Gallawa Family Web Site. 2008. Web. 16 Nov. 2011. <http://www.gallawa.com/microtech/magnetron.html>.
3. “Why You Generally Shouldn’t Put Metals in the Microwave.” Today I Found Out: Why You Generally Shouldn’t Put Metals in the Microwave. Vacca Foeda, 2010. Web. 16 Nov. 2011. <http://www.todayifoundout.com/index.php/2010/08/why-you-generally-shouldnt-put-metals-in-the-microwave/>.
4. Vollmer, Michael. “Physics of the Microwave Oven.” Physics Education 39.1 (2004): 74-81. Print.