Why does the microwave heat up my mug before the tea?
April 11, 2013 3:53 AM   Subscribe

Why does the microwave heat up my mug before the tea?

In my understanding the magnetron [if that's what it's called] is situated above the rotating plate, and sends its energy downwards, so the tea's surface area is broader and in more direct line of fire. So why, when I zap my cold tea, does the mug burn my skin but the tea is sometimes still tepid?

I know the microwaves bounce around and give so-called full coverage inside the oven, and I understand that the energy excites hydrogen atoms [or is it water molecules?]. I would have thought there were more of both H & O atoms in the tea than in the ceramic.
posted by dash_slot- to Science & Nature (17 answers total) 1 user marked this as a favorite
 
I can't answer your physics question, but this varies with some property of the particular mug. Some work much better in microwaves than others do.
posted by jon1270 at 3:56 AM on April 11, 2013


This is a sign that your mug should not go in the microwave - microwave safe mugs will heat up less quickly than the water. Think - a metal mug would heat and spark like crazy.
posted by fermezporte at 4:10 AM on April 11, 2013 [4 favorites]


from the wiki article:

Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting.
posted by empath at 4:12 AM on April 11, 2013


To elaborate, the microwave works by creating a shifting electro-magnetic field in the oven. Any molecule which has electrical polarity (that is, one side is positive and the other is negative) will vibrate while it tries to line up with the field. That is what creates the heat. Water is one of those molecules, but there are lots of others, and they can be in ceramic mugs. If your mug heats up when you put it in the microwave by itself, don't use it in the microwave.
posted by empath at 4:17 AM on April 11, 2013 [6 favorites]


I am not a physicist, but I've made and used a lot of my own ceramic mugs.

If the mug is made from earthenware instead on stoneware (or has faults in the glaze) the clay body may still be absorbing a small amount of liquid. That small amount of water in the pores of the clay will get super-hot super fast.
posted by pantarei70 at 4:28 AM on April 11, 2013


I question your assumption that the waves are coming from the top. In every microwave I've ever seen, they come from the side. Here's Bill Hammack, the self-styled EngineeringGuy, showing the innards of one. He's got other cool videos; here's one of how a pop can tab works.
posted by at at 4:59 AM on April 11, 2013 [2 favorites]


Thank you guys - but the concept of 'microwave safe' is a new one on me! Is the danger that the mug may be damaged, either by heat or some other action of the microwaves?
posted by dash_slot- at 5:25 AM on April 11, 2013


I did search google images, the most common diagram seems to have the magnetron on top.
posted by dash_slot- at 5:33 AM on April 11, 2013


Your mug contains metallic particles (either in the body itself, or the glaze) which react to the microwaves, heating the mug. A microwave-safe mug would not do this.

I discovered this the hard way once, when I heated some dessert of some sort on a small plate. It was only in there a few seconds, but when I reached in to take the plate out, I found the plate to be painfully, searing hot. I actually blistered a finger on the thing.
posted by Thorzdad at 5:51 AM on April 11, 2013


Is the danger that the mug may be damaged, either by heat or some other action of the microwaves?

The mug could be damaged by uneven heating. The thermal stresses (combined with the colder liquid inside the mug) could eventually shatter the mug while cooking. You also, obviously, run the risk of a severe burn when trying to grab the mug out of the oven.

Some materials are designed to get super-hot inside a microwave, like the sputtered metal foil at the bottom of your microwave popcorn bag. But a mug shouldn't be one of those.
posted by JoeZydeco at 6:53 AM on April 11, 2013 [1 favorite]


Nthing to try a different mug. I have an assortment of handmade pottery mugs as well as standard ceramics, and there's one (and only one) of hte handmade ones that must have something odd in the clay, because it heats up like crazy in the microwave.

About "microwave safe" - when microwaves were becoming common in the 80's, people made a big fuss about things being (or not being) microwave-safe. (same thing with "dishwasher-safe") These days, the idea of making a not-okay product (something that can't stand the heat of the dishwasher or that has the wrong metallic compouds and heats up in the microwave) is just not acceptable for most manufacturers' concepts. The clay you get in pottery-making class is microwave-safe clay (and it didn't used to be). They've improved glass-paint so you can put decorated glasses in the dishwasher (and they used to be bleached out or flaking after just a few months of use). But in short, older things and artsier things (and sometimes foreign things) are much more likely to have applicance problems, and everything else can pretty much be assumed safe.
posted by aimedwander at 7:42 AM on April 11, 2013


You should use a different mug.

Your microwave doesn't really "send energy downwards" in the way that you're thinking, so the surface area of the tea and the mug and the tea don't matter. The microwave is a resonant cavity which the magnetron fills with oscillating electric fields. In a perfect cavity (evacuated, with superconducting walls and a particular special shape) the magnetron could fill the cavity with energy and the energy would just be stored there forever, like the potential energy in a tight rubber band. In a real cavity the energy gets dissipated somewhere --- mostly into whichever material in the oven couples most strongly to the oscillating field. Most ovens are designed to heat water and wet things, but heat metal too: oscillating electric fields push the free electrons in metals around, which produces heat.

The fact that the microwaves fill the whole cavity is the origin of the old statement that a microwave cooks the food "from the inside out" as compared to a conventional oven cooking food from the outside in. Though maybe it's more correct to say that the heat transfer into your food takes place in a layer which is kind of as thick as the wavelength of the microwaves.

But the short story is that you have a materials problem, not a geometry problem. Use a different mug.
posted by fantabulous timewaster at 10:27 AM on April 11, 2013 [3 favorites]


I have found that some kinds of colored glazes heat up CRAZY HOT in the microwave. I have a blue mug and a green mug that I can't even use. All my white mugs are fine.
posted by KathrynT at 10:37 AM on April 11, 2013


empath: from the wiki article:

Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting.
Fascinating - thanks for that note!
posted by IAmBroom at 7:03 PM on April 11, 2013


Thorzdad: Your mug contains metallic particles (either in the body itself, or the glaze) which react to the microwaves,
True, except the particles don't have to be metallic. In fact, many metals aren't particularly sensitive to microwave heating (although a fork in hot water will conduct the heat nicely, so it might still be hot even if it's not the microwaves that directly heated it).
posted by IAmBroom at 7:06 PM on April 11, 2013


The dangers aren't to the dish, but to the person!

I was taught, "Only use a dish that is labelled "microwave-save". If it doesn't say that on the bottom, don't use it." Plastic dishes and ceramics can leech toxins into food.

Tip #53: Glaze toxicity and Dinnerware safety
posted by at at 5:40 AM on April 13, 2013


at: The dangers aren't to the dish, but to the person!

I was taught, "Only use a dish that is labelled "microwave-save". If it doesn't say that on the bottom, don't use it." Plastic dishes and ceramics can leech toxins into food.

Tip #53: Glaze toxicity and Dinnerware safety
Your citation only really discusses ceramics leaching toxins.

Generally, if the plate doesn't heat faster than the food (and especially if it's hottest only where the food is, which would indicate it's a low absorber of microwaves), then it's unlikely to be reaching unsafe temperatures and leaching out anything.
posted by IAmBroom at 9:58 AM on April 15, 2013


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