13

So, this was originally a part of this question, but from the comments it seems like it might be beneficial to branch it.

I have been told that the microwave left "hot spots" — areas in a fluid which were substantially(?) warmer than the surroundings. It did not seem to me that they would exist in any non-trivial sense. So, my question:

  • Are hot-spots real?
  • If I were to chug a large amount of freshly microwaved <liquid>, assuming the average temperature is something which would not burn my tongue, would I need to worry about being burned?
  • 1
    You are missing a link to the source of the claim. This is important, because if the source is saying "If you remove the spinning tray, and cook a solid that melts, like chocolate, you will find a grid of hot-spots a few cms apart", we can confirm it with references. If they are saying (as you hint) "Be careful when reheating your mug of coffee, because parts of the liquid will be much hotter", we can debunk it. – Oddthinking Jan 14 '12 at 6:33
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    Why rely on someone else answering this for you? Science in action! Take a chocolate bar, remove the rotating plate and microwave on max for ~60s and you'll figure it out yourself! – Sklivvz Jan 14 '12 at 14:48
  • I hope we can reference the claim and keep the question going as the question has a great home science answer that I've given below (which is based on exactly the experiment @Sklivvz recommends). – matt_black Jan 14 '12 at 17:48
  • Use a microwave with a rotating glass plate and never place the object in the very center. You won't get hot spots: comsol.com/blogs/why-does-a-microwave-heat-food-unevenly – Communisty Feb 28 '18 at 12:49
10

Yes, they are very real.

There is a really great visualization of the microwave hot-spots from a bunch of different microwaves here, using extremely heat-sensitive raw Appalams.

enter image description here

As a side, note, EMSL is awesome. They did the experiment, and posted the picures. I just borrowed them (they're CC BY licensed).

13

Not only do they exist, but they make a really easy experiment that can illustrate the speed of light

Microwaves ovens really do create hotspots and it is an issue if it matters to have evenly cooked food. That's why many have turntables.

But the really cool thing about hotspots is that they can be used in a simple science experiment to get a good approximation of the speed of light. The experiment involves measuring the distance between the hotspots which are caused by standing waves in the microwave field that occur a half-wavelength apart. So knowing the frequency of the microwave (usually something like 2450MHz) allows a simple calculation of the speed of light since c=wavelength*frequency.

Links to the description of this experiment can be found here, here, here, and even illustrated on youtube (a google search for "speed of light microwave" with throw up a load more.)

  • +1 - I knew this from experience but did not understand exactly why thanks :) – Chad Jan 17 '12 at 14:31
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    +1 for the thought of measuring the distance between hotspots, but I think this is better viewed as estimating the microwave wave length and deducing its frequency given the speed of light than vice versa. – MZB Jan 17 '12 at 18:26

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