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How a Microwave Oven works
(This page has been updated as there were mistakes in the previous version. See at end of page)
Well you might say it's obvious - you just put the food in and the microwave heats it up. But why does it heat the food yet it doesn't heat the dish, and why is the inside of the oven always cold?
A microwave oven has in it a magnetron, which is a radio transmitter. If it was on a radio mast (don't try this) it would be able to send radio signals a long way. But it is inside a metal box which keeps the signal in. The frequency of the transmitter is 2450MHz (megahertz), which is a wavelength of 12cm (that's why it's micro waves, rather than short waves (several metres), medium waves (hundreds of metres) or long waves (thousands of metres). There's a good reason for the frequency being 2450 Megahertz, which I'll explain.
Food has a high percentage of water, and water is famously H2O. The molecule of water has the O (Oxygen) in the middle, and the two H's (Hydrogen) stuck on it like Mickey Mouse ears at a particular angle (105o). The H's are positive and the O is negative, so the molecule has a + and - end. It has "polarity".
Polarised molecules try to line themselves up with the electrical field, like compass needles trying to point at North.
But because the electrical field is changing 2,450 million times a second the molecules don't quite have time to line up one way before they have to try to line up the other way!
So, anything with water in it has all these molecules being moved this way and that by the electrical field, and heated up. The dishes, walls of the oven, etc, don't pick up radio, so don't get heated up.
Other things that pick up radio, anything approximately aerial-like such as the coily filament in a lightbulb, whether working or blown, will be energised if in a microwave oven. If you're silly enough to try any of this, I take no responsibility for what happens. You can take my word for it that the bulb lights up and then explodes. But in contrast, a glass full of petrol (at your own risk be it), which you might assume would explode, will go round and round and not get warmed up at all. That's because the molecules of petrol are long chains of carbon and not POLARISED like the H2O of water molecules. More about silly things to do with microwave ovens can be seen at the page of Microwave Tomfoolery! Plus, if you'd like to buy a microwave oven, see this page of Electrical Shops
Another explanation of How a Microwave oven works is at http://www.howstuffworks.com/microwave.htm
And there is a question-and-answer forum on this link: http://rabi.phys.virginia.edu/HTW//microwave_ovens.html
Plus, scientific explanations of water and microwaves: http://www1.lsbu.ac.uk/water/microwave.html - Welcome back!
Also see How a Fridge Works
The English language was short of a neat colloquial word for "microwave oven", so whereas telly, mobile, and sat-nav, all had a term to bandy about, the microwave oven was stuck with "microwave". However, in Cymru (Wales), a brilliant term was coined: POPTY PING. How about that?! Those good people who invented the word "penguin" have excelled again! Also, as well as "popty ping" meaning a microwave oven domestic appliance, it's also used in the connotation of "to microwave", ie to zap, nuke, frazzle, stonker, etc and it has been heard in sport commentary where a decisive victory resulted in the losing team ending up "popty ping", ie being subject to overwhelming power like a small item being zapped in a microwave oven!
Now about the faults in the previous version of the page:
Whilst it's true that water molecules can vibrate at resonant frequencies like a tuning fork, that's not the main mode of vibration in a microwave oven. Also, the molecules are MUCH smaller than a ¼ wave dipole aerial of 2450MHz.
There are textbooks which support the "tuning fork" or "resonance" theory of microwave ovens. Some people say they are right and some say they are wrong!
So, do water molecules RESONATE at 2450MHz? Let's get a graph of absorption versus frequency. I've seen microphones have graphs of frequency response, and Fraunhofer lines in star spectrum graphs, so, how about a microwave-water graph?!
Physics is not some kind of religious belief-based system, and there is no point in taking sides in such an argument. What's needed is to put these things to the test. It must be possible to test the frequency-response of water molecules by applying a varying frequency and see how much energy is absorbed.
How to measure the power of a microwave oven
Now here are a few helpful comments from contributors. Some of these explain the Microwave Oven issue in more technical detail, and some add other points...
The "vibrational" states of molecules like water is in the infrared spectrum. The "rotational" states are in the microwave spectrum. Only molecules with a "dipole moment" will have rotational states. A dipolar molecule has a net charge at each end. In the case of water it's negative at the oxygen end due to two pairs of unbonded electrons. At the other end it's slightly positive due to the single proton nucleus of each hydrogen atom being slightly exposed when its single electron each is covalently bonded (shared) with the oxygen.
Now along comes a microwave with its electric field. The partial positive and partial negative ends try to follow this electric field by rotating as the wave moves by. This creates friction in the food and thus heat.
Metals will get hot because of the law of conservation of energy. The metals absorb the microwaves and the energy is converted to heat.
Fat also has a dipole moment and will also rotate and get hot in a microwave oven. Just put a bowl of pure cooking oil in and watch it heat. Carbon dioxide has no dipole moment except for a tiny dipole during a bending vibrational state and an asymmetric stretching (but not symmetric stretching) vibrational state. Again, vibrational states are in the infrared spectrum and therefore very reduced at low temperatures. Therefore one cannot expect a microwave oven to heat dry ice.
For more information about dipole moments and the effects of microwaves, look up microwave spectroscopy. This would be either graduate level college chemistry of at least senior college chemistry.
I hope this helped.
What's going on is this:
=> in the vapor phase (when the molecules are isolated) they rotate and vibrate freely. Vibrational frequencies are mostly way above any microwave frequency (but see below). There are three fundamental rotational axes since water is not axisymmetric (unlike e.g. oxygen O2 which has an axis of symmetry from one O to the other, and another perpendicular axis, and so has only one rotational frequency). It's easy to calculate the frequencies corresponding to the smallest transition -- that is, from the ground state to one with a single "quantum" of rotation -- for each of these axes. For example, see Atkins' Physical Chemistry (Freeman), chapter 16 of the 5th edition. I discarded the scribbled pages with this calculation but I recall that one gets excellent agreement with the observed water vapor absorption peaks at around 180 GHz and (?) 320 GHz. In any case, these are WAY out of the range where a microwave oven would have an effect.
There is a microwave absorption of water at around 22 GHz. This is not a simple rotational transition. I haven't had time to look up the details, but it seems likely that it is of the same nature as the inversion line in ammonia (at 24 GHz), which is a transition between two rotational states with the same angular momentum but different symmetries with respect to the plane of the hydrogens ( http://poohbah.cem.msu.edu/courses/CEM495/cem495f1micro.pdf ).
=> in the liquid, individual water molecules are organized into transient ring structures, in which rotation is "hindered": that is, they can't rotate without banging into a neighboring molecule. Thus, there are no resonant transitions in the microwave region. Absorption takes place when the random motions of neighboring molecules allow a given molecule to follow the applied field for a short distance, after which it bangs into a neighbor, converting the motion into heat. This process involves all sorts of orientations of the molecules, and doesn't have any specific characteristic frequency but instead a wide range of frequencies: in particular, 2450 MHz plays no role, and 2300 or 2600 MHz would work just as well (but in the US the FCC would be after you). The use of 2450 MHz is a historical artifact having to do with frequencies licensed for industrial use, not related to any specific property of molecular or liquid water.
The absorption vs. frequency graph is in the web site I provided previously; it shows that as the water heats up, the peak in absorption increases in frequency, from about 15 GHz in cold water up to close to 100 GHz near the boiling point. The microwave oven frequency is chosen to be below the peak absorption point, so that as the surface of the cooked object heats up, absorption will decrease, allowing the microwave radiation to penetrate into the center of the object and heat it up and thus encouraging more uniform cooking.
Just measure the dielectric properties of water, and display the loss factor on an automatic network analyser (ANA). I have one at work that measures dielectric properties from 40MHz to 40GHz
Water heats because water molecules are not alone and choc with each other. That's what produces the thermal energy.
and Helmut talks of the "resonance MYTH"...
I know the problem with microwaves and how deep the belief in the resonance myth is. It will never disappear, never, as this makes it so easy to imagine. The myth of incompressibility of liquids, water in special is something likely hard to get rid of.
Thanks to everyone who has commented and contributed. The site is continually being improved!