Wikipedia:Reference desk/Archives/Science/2016 December 16

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December 16

Getting microwaves deeper into food (i.e. not cold in middle)

Would it be possible for a microwave to penetrate deeper into a container of food by tweaking the frequency up or down? --78.148.100.101 (talk) 15:36, 16 December 2016 (UTC)

There is a lot of physics wrapped up in the answer to this seemingly-simple question; the short answer is "maybe."

A good place to start is to read about dielectric heating - the physical mechanism by which microwave energy transfers heat into food.

You can also read about penetration depth, which explains the mathematical model that describes how microwaves get into the interior of the food (or other material).

Using the most simplistic equation, the answer is really simple: the lower the wavelength, the deeper the wave penetrates - so it would seem like we can emphatically answer "yes" to the original question - except for a zillion non-idealities about wave propagation in complex materials that you need to account for. To make things even more complicated, it's categorically true that if the wave is penetrating the material, it can't possibly be heating the material. The material is transparent to the wave! How can the energy get into a material, if the wave is able to pass right through? Again, you can introduce additional complications - like trying to apply geometric ray optics to microwave wavelengths to "focus" the beam at a particular depth...

...And of course, you care about heating the interior of the food, not getting microwave energy into it. So we can't neglect a complete model of thermal conductivity and heat flow in complex materials. It doesn't really matter where the microwaves end up, as long as heat flows throughout the food so it actually cooks!

So, to directly answer your question: lots of things are possible. That doesn't answer the relevant follow-up question: are these techniques simple enough to do in a consumer device? Nearly every consumer microwave runs at 2.4 GHz - which seems to imply that our captains of microwave industry all agree that this is the ideal operating frequency, accounting for efficacy, cost, safety, and commercially-available designs and parts.

Nimur (talk) 16:40, 16 December 2016 (UTC)

(ec) Adding to the list of "accounting for": Allowable RF interference. The ARRL notes that microwave ovens are covered by Title 47 CFR Part 18 and have certain allocated frequency bands, one of which is approximately 2.4 GHz. These frequency band assignments are generally international in nature. So while 2.4 GHz may not be the ideal operating frequency for cooking food, we can probably still conclude that the captains of industry have determined that it is the closest to ideal that they are legally permitted to use. — Lomn 17:06, 16 December 2016 (UTC)

You're getting close to the argument that "there must be a good reason for why we do X, so let's never consider any alternatives". While the way we "have always done things" often is the best way, it's not always, and we should remain open to new alternatives. Also, what's in the interest of the "captains of industry" isn't necessarily in the interests of the consumer, such as planned obsolescence. StuRat (talk) 18:33, 16 December 2016 (UTC)

Can you cite any evidence that cavity magnetron design with respect to frequency has ever been linked to planned obsolescence? Can you formulate (much less cite independent evidence of) a coherent argument whereby manufacturers gain an advantage by deliberately choosing the poorer of two operating frequencies, which arise strictly from the physical dimensions of the magnetron (as distinct from, say, choice of materials, manufacturing techniques or QA standards)? Of course, we could also go to our article on microwaves for a large part of the answer: 915 MHz microwaves are used in some places, but not worldwide due to discrepancies in frequency allocation between regions. Additionally, the magnetron is physically larger and the increased wavelength is less suitable for cooking small items; both factors are acceptable in the commercial space but have led to the discontinuance of 915 MHz microwaves in the small home appliance market. The next window down (425 MHz) is very narrow; the above linked magnetron article notes that they are not practical where tight control of the output frequency is required (the market concerns with the 915 MHz would also be amplified here). The windows above 2.4 GHz (5.8 GHz and 24 GHz) are not cost-effective. Which is all to say: frequency selection is not haphazard, unconsidered, or a grand conspiracy, even if it is not perhaps fully ideal. — Lomn 19:39, 16 December 2016 (UTC)

Settle down. Planned obsolescence is an example of how "captains of industry" don't always have the consumer's interest in mind, and is not about microwave ovens. If you want a microwave oven-specific example, cooking the food quickly is easier to market than cooking it evenly, as you can just compare how long it takes to boil a cup of water in the first case, but not in the 2nd. So, this may lead them to develop and market microwaves that way. I'm not saying this is the case, just that the argument that "whatever captains of industry do must be in the consumers' best interest" is invalid. StuRat (talk) 01:38, 17 December 2016 (UTC)

Yet there are plenty of examples of microwaves which claim to cook more evenly, and actually this is probably a more common point of difference nowadays than power or time at least at the higher end. Nil Einne (talk) 06:06, 17 December 2016 (UTC)

They can claim it, but can they prove it ? If not, then there's little reason for them to spend money to improve how evenly their microwaves heat food, when they can just lie about it instead. StuRat (talk) 18:00, 17 December 2016 (UTC)

Yes, the obvious problem is that a lower wavelength would pass right through small items without heating them. This could be addressed by using more than one frequency, but that would also require shielding designed for each frequency used. All that shielding might make the window unusable. StuRat (talk) 17:03, 16 December 2016 (UTC)

If you are interested in practical cooking information: most cooks skilled in the use of microwave ovens just use a working understanding of the heat equation to heat things through. If you microwave frozen chicken for 5 minutes on high, you get frozen interior with overcooked outside. If you cook it 10 minutes on half power, you get even cooking all the way through (N.B., numbers made up for illustrative purposes). The basic idea is that you pulse the radiation in a duty cycle, and let diffusion do its thing while the the power is off. Note that most microwave ovens do not actually modulate power output, but rather change the duty cycle when the "power" setting is changed.

More details on the power settings of microwaves, and how/why to use them can be found e.g. here http://cooking.stackexchange.com/questions/7394/how-does-the-power-setting-on-a-microwave-work] [1] [2]. SemanticMantis (talk) 16:51, 16 December 2016 (UTC)

When I make nachos in a large pot, I put my microwave on defrost to pulse the heat but even after 20 minutes the inside is not hot (can go straight into my sensitive mouth!) while the outside is too hot to touch the container. :( 78.148.100.101 (talk) 21:19, 16 December 2016 (UTC)

Yes, as discussed in our article, some newer microwave ovens have an inverter, which allows them to do constant "low-power cooking" instead of cycling the magnetron on and off. --47.138.163.230 (talk) 23:27, 16 December 2016 (UTC)

Agree with Nimur's summation of the issue. Here is a free access source to: > http://www.google.co.uk/url?q=http://www.emu.dk/sites/default/files/physics_of_microwave_oven.pdf&sa=U&ved=0ahUKEwiLubX6kfnQAhWqJ8AKHb0vCF44ChAWCB8wAg&usg=AFQjCNFpQnCmr5P_cLivqomqKM1A0RQ7bA< physics of the microwave oven. Thus, the current frequency has been chosen for very practical reasons. --Aspro (talk) 17:33, 16 December 2016 (UTC)

Assuming that you are only intending to move the frequency around by a small amount, even if that was permitted by EMI regulations, the main thing you are likely to achieve is to move the nulls around. Advantageous if the frequency is constantly moving because that would get you more even cooking and/or reduce the need for rotating the plate, but is not likely to get deeper penetration into the interior. SpinningSpark 12:16, 17 December 2016 (UTC)

Perhaps a combination of source would help? Matt Deres (talk) 15:08, 17 December 2016 (UTC)

LOL, but seriously, a microwave oven designed to penetrate deeply combined with a convection/infrared oven to brown the outside might be the best of both worlds. Quick, and also heats evenly. StuRat (talk) 18:49, 17 December 2016 (UTC)

What weapons could damage a plane made of flawless nanotubes/nanotube composite?

Depends on the size of the airplane, no? Sagittarian Milky Way (talk) 19:17, 16 December 2016 (UTC)

Pretty much any weapons that damage a normal airplane ? Unless I am missing something about nanotubes that makes them practically indestructible. StuRat (talk) 19:30, 16 December 2016 (UTC)

It's fairly clear SMW is talking about a General Products hull, which is made of a single molecule. Larry Niven has written about them extensively. μηδείς (talk) 19:55, 16 December 2016 (UTC)

Since that's a fictional thing, you can invent any other magical device you want to damage it. That's how fiction works.--Jayron32 21:26, 16 December 2016 (UTC)

Niven himself wrote a story about a GP hull mysteriously failing (and the owner claiming on the warrenty), the cause turning out to be a real-world thing that would indeed have the observed effect (which I won't specify further to avoid spoiling, but it's not the thing mentioned in the General Products article linked by μηδείς above.) {The poster formerly known as 87.81.230.195} 90.200.136.117 (talk) 11:51, 17 December 2016 (UTC)

Flatlander (short story) - I was going to say "it's a very dated story, I'd say practically unspoilable simply because people now know nothing like that actually happens"... but apparently NASA didn't, at least as of 2008, so I'd best not do that.^[3] Wnt (talk) 18:01, 17 December 2016 (UTC)

[4][5] Sagittarian Milky Way (talk) 21:51, 16 December 2016 (UTC)

Yes, nanotubes are very real and very strong. But Jayron's point still sort of stands. We currently have no way of mass producing them, let alone making an airframe out of them. But we might at some point in the future. So while your question is interesting, I think it is a bit too speculative for my abilities to help. However, someone may indeed find various references discussing the strength, toughness, or other aspects of how hard it is to cut/pierce/burn/whatever certain types of carbon nanotubes in gross structural forms, so the question is IMO in principle addressable with references. SemanticMantis (talk) 22:14, 16 December 2016 (UTC)

At times it seems like you could truly do anything with nanotubes, if you could do anything with nanotubes. But according to the carbon nanotube article, they are prone to buckling and have much less strength in compression. Mechanical properties of carbon nanotubes says a little more, not much. I'd love to see those articles get some tender loving care. But my untutored impression from what they say and similar things I've read is that your airplane would make a nice strong rope, but if you fire a shot into it those nanotubes are going to bend and buckle and collapse in on their hollow middles and generally not pose much of an obstacle. Wnt (talk) 23:44, 16 December 2016 (UTC)

There's got to be a nanotube equivalent of reinforced concrete, right? A piece of cake once you invent atom-by-atom 3D printing.. Sagittarian Milky Way (talk) 00:59, 17 December 2016 (UTC)

Reinforced concrete seems like kind of a bad example since it is a material with only compressive strength reinforced by steel with strength in all directions. And I doubt it's all that much more bulletproof than plain cement anyway. But I do imagine there are clever ways to get the tensile strength to add up; it certainly ought to be able to hold some armor plates together. Wnt (talk) 04:47, 17 December 2016 (UTC)

Of course even if you have an indestructible craft, if you want to put live humans or other things inside you still need to worry about keeping them alive. A nuclear bomb going off right on top of the craft probably wouldn't be too good for the passengers' health even if it didn't scratch the airframe. This is the central problem with manned spaceflight. The challenge is not getting somewhere per se; it's getting there with live passengers. --47.138.163.230 (talk) 01:31, 17 December 2016 (UTC)

I think you would find that they burn in oxygen (or fluorine). Also some molten metals that form alloys with carbon will quickly dissolve them. Molten iron would destroy them pretty fast. If you make a cloth out of it to make a composite, you will find that you can cut it with scissors. Graeme Bartlett (talk) 05:03, 17 December 2016 (UTC)

And of course a nearby nuclear explosion would vaporize it, no matter what its tensile strength might be. 2601:646:8E01:7E0B:AD2F:FE49:B9D3:9A64 (talk) 02:18, 18 December 2016 (UTC)