Microwaves: why do they cook food?

Question

I have looked up several explanations of microwaves and how they work but I am unclear on a few key parts of how they actually make the food hotter.

How I understand it now:
In addition to the light and the rotating plate inside the microwave unit there is something called a magnetron that generates microwaves and focuses them at whatever is on the plate. The nature of microwave waves is that they do not affect glass, some plastics and a few other things like metals which reflect the microwaves. The microwave wave that is generated by the magnetron enters one of the molecules of the food and “is converted to kinetic energy” (why? How? By what? What effects does that process have? Why does that kinetic energy make the food hotter?)

That’s it. Can someone please explain to me how the microwave wave makes food hot?

For anyone reading this in the future, Bob’s answer AND Dipson’s answer were both very useful and explained different aspects of the question. There was even another answer here that was good and explained more. Honestly they are both solutions and both answers are good and now some of the comments even seem good but I do not know of a way to mark them both so I will just upvote them all. This thread is good. — brothman01, Dec 23 '20 at 21:06

Bob D · Accepted Answer · 2020-12-24T14:25:16.787

8

The primary mechanism involved with microwave cooking is the interaction between the alternating electric field component of the 2400 Mhz microwave electromagnetic field and the polar molecules of the water content of food. This gives the water molecules rotational kinetic energy. The rotational kinetic energy is then randomized by means of collisions between water molecules, increasing the translational kinetic energy of the molecules. That, in turn, results in an increase in the temperature of the food thus cooking the food.

Its a common misconception that microwave ovens "heat" food. Heat is energy transfer solely due to temperature difference. Microwave ovens don't cook like a conventional oven which uses heat. The energy transfer to the food involved with microwave ovens is work. Namely, the work that the alternating electric field performs on the water molecule dipole.

Are you saying that the microwaves increase the rotational energy of a molecule the wave enters? Why does the microwave increase the rotational kinetic energy? I am very curious how microwaves that enter a molecule increase that molecule’s rotational energy.

Yes.

There are two ways of looking at it, one using the wave (classical) model and one from the particle (quantum) model.

Wave Model:

The diagram below is perhaps a crude way of showing how the electric field component of the electromagnetic wave interacts with a water molecule inducing rotation. The water molecules is a an electric dipole owing to the spacial arrangement of the two hydrogen at one end and one oxygen atom at the other. It is overall electrically neutral, but is positive at one end and negative at the other forming a dipole, as shown.

At the electromagnetic wave propagates past the molecule, the electric field component alternates in polarity. When it does the molecule aligns itself (rotates) with the field due to the attraction and repulsion forces. That gives the molecules rotational kinetic energy.

Particle (Photon) Model:

In the photon (particle) model, the quantum energy of microwave photons is in the range 0.00001 to 0.001 eV which is in the range of energies separating the quantum states of molecular rotation and torsion. So in this model you can view the transfer of microwave electromagnetic energy in the form of photons absorbed by the molecule dipoles causing them to undergo rotation and torsion.

For a more detailed description of the interaction of electromagnetic radiation with matter, check out the following site: http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html

Hope this help.

edited Dec 24 '20 at 14:25

answered Dec 22 '20 at 17:21

Bob D

71,527

Are you saying that the microwaves increase the rotational energy of a molecule the wave enters? Why does the microwave increase the rotational kinetic energy? – brothman01 Dec 23 '20 at 21:16
@fall3n Yes. But I will update my answer to explain how this happens, both from an electromagnetic wave perspective and from the particle (photon) perspective. Please stand by. – Bob D Dec 23 '20 at 22:39
Thank you, please let me know when you do as I am very curious how microwaves that enter a molecule increase that molecule’s rotational energy. – brothman01 Dec 24 '20 at 00:22
@fall3n See update to my answer. – Bob D Dec 24 '20 at 14:25
Wow ok that diagram helps. Perhaps I am missing something obvious So please correct me if I am wrong in my understanding. The microwave wave is labeled in the diagram as “Alternating Electric Field”. The microwave wave has magnetic poles in the following order from left to right: +-+. The + poles In the microwave wave attract the - poles of the water molecules, so the molecules rotate so that those poles can be closer. The explanation in this comment is my understanding of the “warming” phenomenon so 1. please tell me if my understanding is correct or not. – brothman01 Dec 24 '20 at 14:51
Is it correct that the nature of a microwave is that it has alternating + and - poles throughout? (As shown in the diagram?)

brothman01

Dec 24 '20 at 14:52

@fall3n It is a sinusoidal wave. A sinusoidal wave continuously alternates between + and -. – Bob D Dec 24 '20 at 15:01

Yes the nature of a sinusoid always wave is that it alternates between + and -, however I have understand microwaves to only be sinusoidal sometimes. 1. Question 1 from two comments ago 2. Are the microwaves produced by microwave units in kitchens always sinusoidal? – brothman01 Dec 24 '20 at 16:29

@fall3n 1.Microwaves freely moving through space are electromagnetic waves having two orthogonal components, both sinusoidal, one the electric field the other the magnetic field. See the images in this link: https://www.google.com/search?q=electromagnetic+wave+images&oq=&aqs=chrome.0.69i59i450l8.514293917j0j15&sourceid=chrome&ie=UTF-8 – Bob D Dec 24 '20 at 17:35

In the microwave oven the electromagnetic waves do not propagate freely as they do in space since they reflect off of the walls of the oven. This can result in constructive and destructive interference, standing waves, etc. so that they may not be sinusoidal. But the food is on a rotating table so that there is relative motion between the electric field and the water molecules and even out the cooking process

– Bob D Dec 24 '20 at 17:36

Dipson Khadka · Answer 2 · 2020-12-23T08:07:57.040

4

When you start cooking, the magnetron takes electricity from the power outlet and converts it into high-powered, radio waves. The magnetron blasts these waves into the food compartment. The food sits on a turntable, spinning slowly round so the microwaves cook it evenly. The microwaves bounce back and forth off the reflective metal walls of the food compartment, just like light bounces off a mirror. When the microwaves reach the food itself, they don't simply bounce off. Just as radio waves can pass straight through the walls of your house, so microwaves penetrate inside the food. As they travel through it, they make the molecules inside it vibrate more quickly. Vibrating molecules makes it hot so, the faster the molecules vibrate, the hotter the food becomes. Thus the microwaves pass their energy onto the molecules in the food, rapidly heating it up.

In simple terms. Microwave cook food by generating kinetic energy between the molecules of the food.

edited Dec 23 '20 at 08:07

answered Dec 22 '20 at 15:07

Dipson Khadka

69

1

Re, "vibrating molecules" and "inter-molecular friction." There's no friction. Rather, the internal kinetic energy ("vibrating molecules") of a substance is what defines its temperature. https://en.wikipedia.org/wiki/Temperature#Microscopic_statistical_mechanical_scale – Solomon Slow Dec 22 '20 at 18:55
@SolomonSlow It sounds like you are saying that molecular vibration IS what makes things hot. Ok, that makes sense, but why does a microwave cause a molecule to vibrate? – brothman01 Dec 23 '20 at 21:14
@fall3n I'm not saying that molecular vibration makes things hot, I'm saying that it's the very definition of "hot." When you say that something is hot, you are saying that its molecules, on average, have relatively high kinetic energy. As for the mechanism at work in a microwave oven, I'm sorry but I don't think I am capable of giving you a better explanation than the ones that already are here in the answers. – Solomon Slow Dec 23 '20 at 21:47
Microwaves don't make molecules vibrate. They make them rotate. The vibration energy mode involves energy levels above that of microwaves. Infrared radiation makes molecules. – Bob D Dec 23 '20 at 23:28
@SolomonSlow ok thanks for your contribution, I feel that I understand this process a little more now. I am still unclear as to exactly how microwave waves “increase” rotational energy of molecules they enter but hopefully someone will explain that interesting answer. – brothman01 Dec 24 '20 at 00:25

Microwaves: why do they cook food?

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