# Microwave heating with a lid on

by ap_cycles
Tags: heating, microwave
 P: 36 HI there, I have a thought question. Normally when we wish to warm up food in a microwave oven, common sense tells us that the lid of the plastic containter (or container of whatever material) should be off, so that the microwave can reach the food. Just wondering....if the lid is on, ie the container is covered, can the food inside still be warmed? My guess is yes. My reasoning is that the microwave can pass through the material (ie. the material might be visually opqaue, but it allows microwaves through) and gain access to the food inside. My point is, a material might be opqaue to visible light, but at the same time, it is transparent to microwave. Of course, what determines whether the microwave can pass through the lid material is a matter of what the lid atoms do with the energies in the microwave (i.e. the electron energies level in the atoms). Is my reasoning correct? (Sorry i cant try it out as i do not have a microwave oven)
 PF Patron P: 691 Yes, food can heat with the lid on. What you have to be careful of is to not put it on too tight so that any steam can escape. If too much builds up, you can get scalded by it when you take the lid off.
 P: 36 Thanks Borg! The reply is quick! An additional question that just bugged me. I saw this question labelled as a "high difficulty" question in a student worksheet. I am a high school physics teacher btw. Basically, the question just boils down to asking why "when the frequency of the microwave is increased, its depth of penetration inside the food - say a big chunk of meat - drops". (There is no explanation for this phenomena inside the answer scheme.) This struck me as odd. I am thinking: for waves with high frequencies, say gamma rays, they can penetrate metals to a significant depth. So i apply this line of reasoning, a microwave with a higher frequency should also penetrate deeper into the food. But as it turns out, the answer is quite the contrary. I did a Net Seach and it yield answers in the form of ugly equations, with no worded explanations. I was just bearing some hope that this forum can yield me some satisfactory answers?
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P: 5,488

## Microwave heating with a lid on

Hello ap...a childhood friend of mine also teaches high school AP physics in NY..I used to tutor her in physics!!!

I take it you know microwaves are a type of electromagnetic wave governed by Maxwell's equations...a lid helps retain steam which has a secondary heating effect....just like a pot on a conventional stove....

Try this discussion on for size:
http://en.wikipedia.org/wiki/Microwave_ovens

Also note the numerous EXTERNAL LINKS at the bottom of the atrticle for additional
descriptions....
It sounds like the frequencies were chosen more for considerations other than heating....non communication and ability to generate them economically.....

Normally higher frequency waves carry more energy...say gamma vs visible light...so I am skeptical at the moment that higher frequency penetrate less...but you'll note a lot of the heating is due to the dipole moment....in the above reference...

so also check here:
http://en.wikipedia.org/wiki/Dielectric_heating

My guess without reading is that a higher frequency might be less effective in rotating a dipole???..it doesn't have time to shift??? but that's a pure guess...read POWER and PENETRATION....both seem to refute the idea that higher frequency penetrates less....I now think that assumption is wrong....

I used to do a bit of work with telcommunications waveguides....getting electromagnetic waves to and from big AT&T radio antennas....that copper ducting constrained and focused electromagnetic waves without heating...I'm guessing because the permitivity was low....yet in a microwave oven such conducting metals get hot...unsure why...
if you find out please let us know...
 PF Patron P: 5,488 More insights here: http://www.physicsforums.com/showthr...microwave+oven
P: 15,294
 Quote by ap_cycles ... the lid of the plastic containter ... should be off, so that the microwave can reach the food.
Not to put too fine a point on it but, to most people, the common sense reason is that leaving the lid on a container in the microwave will likely cause it to explode.
PF Patron
P: 5,488
 so that the microwave can reach the food.
just noticed that phrase....no,no....

A lid has virtually nothing to do with the microwave unduction heating of the food inside.....in fact most microwaves radiate into the food area from one side or the other....The klystron output is fed into the interior via a small waveguide type affair....you can find where you model feeds microwaves into the interior by the rectangular hole in the side of the metal envelope "box" inside the microwave door....the microwaves are feed thru the rectangular opening covered with a piece of thin plastic...usually removable for cleaning if you want to look in that output cavity....
 P: 36 Hi all, Thanks for the replies. But i still do not have a satisfactory answer on why "when the frequency of microwaves is increased, the depth of penetration of food warming decreases". I appreciate some of you posted links of old threads pertaining to microwave ovens, but still i could not find the answer i need. Or maybe i should post a new forum topic on this question?
 Mentor P: 15,563 They are talking about the skin depth, which is proportional to 1 over the square root of the frequency. So if you quadruple the frequency you will half the skin depth. However, you cannot take that relationship too far (e.g. comparing microwaves and gamma rays) because the skin depth is also directly proportional to the resistivity which is itself a function of the frequency.
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P: 5,488
Dalespam...
I still don't buy the posters premise....I am crudely familair with the skin effect in conductors....don't think it has anything to do with food in general....the dipoles don't go moving around to the surface in the manner of electrons, in a conductor right??? seems like a negligible effect in food.

 When an electromagnetic wave interacts with a conductive material, mobile charges within the material are made to oscillate back and forth with the same frequency as the impinging fields. The movement of these charges, usually electrons, constitutes an alternating electric current,
 PF Patron P: 5,488 The specific absorption of microwave energy in tissue is found here: http://en.wikipedia.org/wiki/Specific_absorption_rate and the energy absorption is clearly proportional to the strength of the electromagnetic field, E...... In turn, electromagnetic energy is proportional to frequency which means the poster's assumed premise is incorrect....higher frequency carries more energy and results in more absorption...hence more heating... does anybody think a 1 hertz microwave would do any heating???
 P: 36 Thank you all for the wonderful replies!
 P: 598 I'm not satisfied with these answers for some reason. I thought microwaves worked by exciting the rotational modes of water molecules? Which means microwaves are tuned to be around the resonant absorption frequency of water. The correct way to look at this question would be in terms of the molecular absorption spectrum of water I thought? Can anyone explain why what I'm missing? I thought that whether you increase or decrease the frequency of the radiation, you are going to move away from the resonant frequency? Edit: well apparently it is very difficult to excite the rotational modes of water because of very strong intermolecular forces.
P: 598
 Quote by ap_cycles HI there, Of course, what determines whether the microwave can pass through the lid material is a matter of what the lid atoms do with the energies in the microwave (i.e. the electron energies level in the atoms).

This does not make sense to me either, the energy to excite electrons lies in the ultraviolet spectrum I thought? Energies on the order of eV's? This question needs to be answered in terms of quantum mechanics.
 P: 598 From wikipedia: Microwave heating is more efficient on liquid water than on fats and sugars (which have a smaller molecular dipole moment), and also more efficient than on frozen water (where the molecules are not free to rotate).[6] Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz.[7] Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz—wavelength 328 millimetres (12.9 in)—also heat water and food perfectly well.[8] Verrrry interesting... Can somebody please set me straight here? What physical principle do microwave ovens work on? This is scaring the crap out of me, I am about to graduate with a degree in physics and all I can do is calculus, I don't even know how a microwave oven works!
PF Patron
P: 5,488
I would not worry; likely you don't know how a gyroscope works nor RADAR for that matter..neither do I. Each is a technological speciality...

And don't think some genius conceived that microwaves would heat food...I recall reading it was discovered accidentally during RADAR klystron testing....

I just got done investigating coatings for low e glass, solar heat gain conductance SHGC) and other heat loss and gain effects in modern windows...while decidinh what I wanted for my house.....talk about confusing!!!! The Department of Eenergy says one thing, then the IRS provides energy star tax credits for different parameters????? (Oh yes!!!)
Why mention that..in a weird way its related to this discussion!!!!! An ideal SHGC coating passes visible light and rejects infrared...to both keep warmth in winter and out in summer but let you see....I suspect analogous to the way dipoles in food react to one range of frequencies more than others....

Anyway, your last post from Wikipedia led me to conclude when I read Wiki that there is likely a fairly broad range of microwave frequencies that will heat foods/waters and so forth with reasonable efficiency. There is not precise "tuning" nor resonant frequencies involved...except to generate the microwaves,that is.

The post prior to that discusses the photoelectric effect....

 The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light
http://en.wikipedia.org/wiki/Photoelectric_effect

I would not assume there is a photoelectric effect taking place in my microwave oven...but would not rule it out...

Also did you notice my comment about a 1 hertz "microwave" above.....
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P: 5,488
While reading on another topic I came across the following which provides some additional insights on microwave oven heating:

 Quantum-mechanical interpretation In terms of quantum mechanics, permittivity is explained by atomic and molecular interactions. At low frequencies, molecules in polar dielectrics are polarized by an applied electric field, which induces periodic rotations. For example, at the microwave frequency, the microwave field causes the periodic rotation of water molecules, sufficient to break hydrogen bonds. The field does work against the bonds and the energy is absorbed by the material as heat. This is why microwave ovens work very well for materials containing water. There are two maxima of the imaginary component (the absorptive index) of water, one at the microwave frequency, and the other at far ultraviolet (UV) frequency. It should be noted that both of these resonances are at higher frequencies than the operating frequency of microwave ovens. At moderate frequencies, the energy is too high to cause rotation, yet too low to affect electrons directly, and is absorbed in the form of resonant molecular vibrations. In water, this is where the absorptive index starts to drop sharply, and the minimum of the imaginary permittivity is at the frequency of blue light (optical regime). This is why sunlight does not damage water-containing organs such as the eye.[12] At high frequencies (such as UV and above), molecules cannot relax, and the energy is purely absorbed by atoms, exciting electron energy levels. Thus, these frequencies are classified as ionizing radiation.
http://en.wikipedia.org/wiki/Permittivity

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