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I want to cook Turkish coffee on heated sand at school. I have difficulty accessing some easier method of heating, so I was going to try to heat sand in a microwave. It was then pointed out to me that sand, unlike water-dense foods, does not conduct electricity.

Will sand be heated by a microwave?

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  • $\begingroup$ If anyone has some good ideas about what to use as a container or what to use instead if sand cannot be heated this way, feel free to mention them. $\endgroup$ – technillogue Sep 19 '14 at 5:13
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    $\begingroup$ Did you do the experiment? Why not? :-) $\endgroup$ – CuriousOne Sep 19 '14 at 6:28
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    $\begingroup$ I have never attempted to microwave sand, however sand is mostly silicon dioxide and I have put glass (i.e. silicon dioxide) dishes in the microwave and they do not heat up. So I think it's safe to say sand will not heat up either. $\endgroup$ – John Rennie Sep 19 '14 at 6:59
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    $\begingroup$ In my opinion it should work when you put wet sand in the microwave oven. You then heat up the bound water and consequently the sand containing it. And I don't think it will work with dry sand... But just as CuriousOne said: try it out! $\endgroup$ – user42076 Sep 19 '14 at 7:48
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    $\begingroup$ Actually, no don't just try it out. Microwave ovens are often not well designed to handle operation without something to absorb the energy they produce. Continuous operation without load can damage the magnetron. Put a glass of water or something in there along with the dry sand, should be safe then. $\endgroup$ – user3823992 Sep 19 '14 at 8:43
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Well, I just put an empty glass plate in the microwave for 20 seconds and it did heat up ( hotter than my hand, maybe 40C), so people who think that silica does not absorb microwaves are wrong, at least for my microwave oven. I often heat up coffee in a glass, and the handle gets hot too, I was not sure it was not conduction from the coffee, so I tried the plate.

So I will agree with the other experimenter, Rob N, that sand which is mostly silica will heat up in the microwave.

Your problem then would be: to what temperature, as you will need excess of 100C to bring to a boil a turkish coffee. I think it will work, glass plates get really hot in the microwave.

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  • $\begingroup$ Βαρύ γλυκός στον microwave ? $\endgroup$ – user98038 Dec 5 '16 at 20:11
  • $\begingroup$ @aK1974 no, just reheating a cold cup of decaf $\endgroup$ – anna v Dec 6 '16 at 4:24
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Dielectric heating is the priciple of a microwave oven. Water $H_2O$ has a strong dipole moment.

Since the water molecule is not linear and the oxygen atom has a higher electronegativity than hydrogen atoms, the oxygen atom carries a slight negative charge, whereas the hydrogen atoms are slightly positive. As a result, water is a polar molecule with an electrical dipole moment.

The sand in your microwave absorbs the power density (per volume) $$\frac{P(f)}{V} = 2\pi\epsilon_0E^2\cdot\epsilon''_r(f)\cdot f$$ $$\approx const \cdot\epsilon''_r(f) \cdot f$$

Sand (partly consisting of silica $SiO_2$ and small rocks) has a smaller dipole moment. Water has a real part of relative permittivity of $\epsilon_r(20°C)=80.1$, while silica has $\epsilon_r=3.9$. I haven't found their imaginary part $\epsilon''_r$. The imaginary part of relative permittivity $\epsilon''_r$(f) essentially is the absorption and is dependant of the frequency. $$\epsilon''_{\text{water}}(2.45\,GHz) > \epsilon''_{\text{sand}}(2.45\,GHz)$$ Microwave ovens at home have a certain field strength of $E$ use a frequency of $f=2.45\,GHz$. Exact values depend on the type of sand and should be edited in here.

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    $\begingroup$ Interestingly enough, if you melt a little bit of silica (e.g. part of a glass bottle), it goes ionic and a microwave oven will then heat and melt the entire bottle. There's videos on YouTube of this happening.. $\endgroup$ – Carl Witthoft Sep 19 '14 at 11:35
  • $\begingroup$ All correct. I would add that the dielectric properties of water are due to orientational polarization and relaxation time (τ): $\varepsilon =5+75/(1-if/F)$ where $F={{(2\pi \tau )}^{-1}}=$ 10 GHz at 20°C and increases with temperature (inversely with viscosity). I cannot figure out the physics behind Im(ε) for silica. Do you have a clue? $\endgroup$ – Bert Barrois Jan 12 '20 at 12:42
  • $\begingroup$ @BertBarrois I appreciate the equation. As you stated, this relies on water relaxation time, which ist not known to me for sand/$SiO_2$, compared to $H_2O$. Absorption of this 2.45 GHz wave (tuned for water) would improve the answer. Feel free to add it to my answer. $\endgroup$ – Stefan Bischof Jan 13 '20 at 17:59
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    $\begingroup$ I have never seen any data on silica. At any rate, the mechanism is not molecular reorientation. It's lattice distortion -- oxygen getting pulled one way, silicon the other. I can only speculate that Im() is due to interactions with thermally excited acoustic phonons. $\endgroup$ – Bert Barrois Jan 14 '20 at 12:32
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I did heat sand up this morning in my little office microwave, I threw out my back and was looking into making a heat source to put on it, not have much out the office that I thought would work, I tried sand, ran some magnets through it to make sure there was no metal of any kind in it, it was all good, so this is what I did

1 - cut the strap off a junk back pack, then cleaned out the inside 2 - Taking sand I bought from Home Depot I filled up the strap with sand and zip tied it shout, leaving room inside for it tot move around 3 - place in the microwave and heated it for 30 second burst until I was happy with the heat it was producing

Seems to be working fine at the moment Sad to say I had the "hey what if" in me doing this before ANY research was done, but it seemed to work fine so I guess I got lucky, LOL :)

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There is evidence which shows that only certain kind of sand gets heated up. And if you microwave them they will acquire the strength of the rock . We can't say sand does not heat up.

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    $\begingroup$ Welcome on Physics SE and thank you for the contribution :) Could you provide sources or examples for that? $\endgroup$ – Sanya Nov 25 '16 at 8:57
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Clear glass doesn't heats up at all. Sand molecules are quartz and doesn't have dipoles so it shouldn't heat up. But molecules does absorb electromagnetic waves. So it may absorb a very small fraction of it.

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  • $\begingroup$ Welcome to stack exchange. Several people tried to heat "sand" in a microwave oven and they posted that it did heat up. However, as the question is about applied physics they did not characterise the composite/quality of their "sand". If you like to give a theoretical answer to this practical question, it would be helpful to quantify your answer by stating the amount of a "small fraction". $\endgroup$ – Semoi Jan 12 '20 at 12:51
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I would not experiment with a microwave oven without a decent load inside. Wet sand could work if you cook out the water. Note that the sand will reach 100 degrees at most and will cool off rapidly by evaporation of any remaining water once outside the oven. To make coffee you will probably need stable sand temperatures at or above 100 degrees and no rapid cooling. I am just guessing as I use the old fashioned filtration method myself.

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I assume that when you say 'microwave' you are talking about a microwave oven. As you say sand unlike water dense foods cannot conduct electricity. But in a microwave the foods are not heated up using electricity but by bombarding it with microwaves. The microwave use electricity to generate microwaves of frequency 2.4 GHz. In your question you have already said that sand is being heated up at your school. It is happening due to the light radiation being received by the sun. Microwaves are also a type of electromagnetic radiation like light. When light radiation is capable of heating up the sand then microwaves can also heat up the sand kept in the microwave oven.

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  • $\begingroup$ But there is no reason why the absorption of microwaves should be the same as the absorption of light, and indeed experiment shows that silicon dioxide (glass not sand) doesn't heat up in a microwave. Can you provide any links to support your claim that it does? $\endgroup$ – John Rennie Sep 19 '14 at 8:21
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    $\begingroup$ @JohnRennie, is on the right track here. Materials can exhibit vastly different absorption properties at different wavelengths. The fact that sand absorbs visible light is no indicator that it will absorb microwaves well. Consider that the opposite case exists for water. Visible light goes through, but microwaves are absorbed within a few centimeters. I've also dealt with ceramics and plastics (e.g. Teflon) that are quite opaque to visible light but transmit microwaves very well. $\endgroup$ – user3823992 Sep 19 '14 at 8:51
  • $\begingroup$ @JohnRennie please see my answer, I just tried with a glass plate, and it does heat up. Some of the microwaves can be absorbed evidently, maybe not as efficiently $\endgroup$ – anna v Dec 5 '16 at 19:31

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