[John Chapman (OP)]

What would happen if I put my metal mug of cold coffee in the microwave? I’ve just done this and the result wasn’t what I expected. Can any of you predict what happened and I’ll tell you the result later. Then perhaps someone can tell me why.

In the interests of scientific accuracy I’ll lay out the conditions. The mug is stainless steel and double skinned with an insulating layer of air between the skins. It was half full (as opposed to half empty, because I’m an optimistic sort of guy) and I put it on the turntable midway between the outer edge and the centre with the handle pointing to the centre.

I was being cocky and assured my wife that the coffee would warm up just as if the mug was ceramic and that there would be no pyrotechnics. What do you think happened?
 

[techmind]

Was it a fairly tall mug? (It probably only makes a small difference).


With a glass/plastic/ceramic mug, the microwave energy can penetrate the mug and heat the contents from all directions.

Since conducting metal will tend to reflect and thus block microwaves (it nulls the E-field to zero), the first effect of an open-topped metal container is that the microwaves will only be able to enter the contained food from the top. This can have implications for the uniformity of heating. For a shallow metal tray containing food/water, this is not necessarily a big deal.


Just considering the geometry, this sheilding effect becomes more problematic for a half-full metal mug. A possible scenario is that you boil the top couple of centimetres of liquid while that at the bottom remains stone cold.

If you had an almost-full mug of typical mug-dimensions and made of copper 2mm thick, I'd have confidence in this result.


Now consider that the mug of which you speak is only half full, is probably about 6-7cm in diameter.
Consider also that the wavelength of the microwaves (2.45GHz) is about 12cm.

Given that the diameter of the mug is smaller than the microwave wavelength (and the top of the mug may be getting close to the top of the oven cavity) I suspect that the microwaves didn't very well penetrate down into the mug towards the liquid. With only a modicum of absorbtion therefore going on in the liquid, we can expect quite high field strengths in the oven cavity... and maybe "pyrotechnics" (you hint in your message).


Next consideration is that your mug is stainless steel - which has a reputation for being a fairly poor conductor of electricity, and it's probably a fairly thin sheet. So... with 700Watts of microwaves buzzing around and not much other than the lossy stainless to absorb them, I might guess something (i.e. the metal) got fairly hot somewhere and/or you had some arcing and a plasma fireball...?

You might well have had arcing across the rim or around the handle.

Go on, show us the video!  []


PS. Why bother? In my experience, microwave reheated (milky) coffee is not enjoyable to the taste anyway.

[John Chapman (OP)]

Hi techmind

That’s extremely interesting because your answer explains at least some of what happened which was, largely, nothing. And the coffee stayed cold. However the strange thing is this:

When I got the mug out it was also cold but the handle (only the handle) became hot over the 20 or 30 seconds following its removal from the microwave. How strange is that? I can only assume that maybe there was something at the base of the handle, maybe the spot-weld for instance, which DID get hot and this conducted up the length of the handle over a longer period of time. Is that possible, do you think?

Otherwise, what you said about the microwaves not being able to get deep enough into the cup to warm to coffee seems right on the money.
 

[techmind]

I'm so disappointed no fireworks!!!

Yes, it's reasonable that any local points of poorer conductivity relative to the surrounds (such as spot welds) would tend to be where the energy became dissipated. Stainless steel is a poor conductor of heat, so it wouldn't be surprising that it took a while for the heat to reach the rest of the handle.


(Actually, while stainless is a poor conductor for power-transfer, in this discussion the difference compared to copper may be negligible hence no major heating of the metal). Of course, if not very much is getting hot that you're aware of, then that 700Watts is still going somewhere - most likely being dissipated in the magnetron (microwave generator) of the oven and risk overheating killing/shortening its life.

[John Chapman (OP)]

What can I say? You have single handedly solved the mystery.

Give that man a nobel prize!

[traveler]

I love these type of threads.
 I accidentally made a similar experiment a few years ago and got a different result. I was reheating a holiday dinner on a fancy plate with a gold rim. Techmind is exactly correct. The food heated up properly and the gold rim created a little pyro show.

[Bored chemist]

Just a thought; would the handle act as a "shorted turn" and pick up more than its share of power that way?

[John Chapman (OP)]

Flippin' heck Bored Chemist. Just when I thought we had it all wrapped up you have to go and throw a spanner in the works. What is a 'shorted turn'?


Hello traveller

Yes, this is because microwaves are actually an electromagnetic wave and if you put something in the oven which is a conductor, like my metal mug, the waves will produce electricity. This can cause the metal to get hot but, if it is a good conductor that is all that will happen. If you put in something which conducts but does so poorly it tends to create spots of charge in adjacent positions which cannot easily discharge into each other and eventually the charge will ‘jump’ as a spark. Aunty Ethel’s tea set with the gold leaf trim will likely cause sparks because the metal leaf is so thin it will not conduct well enough to carry the charge across sites of different electrical potential.

So this is why your fancy holiday crockery caused sparks although, at other times, you can forget to remove the spoon from a dish and it won't. At least I think that's right. More or less. Sort of.
 

[traveler]

Let's take this to another level.
 Let's sayI took a ceramic mug full of pure H2O and put it into the microwave. Would it heat up the water? Pure water is an insulator.

[John Chapman (OP)]

I'm 95% sure this is correct. The conduction of electricity is what causes the sparking discussed before but isn't directly what heats up the food/water.

Microwave ovens work because water molecules are polarized and boomerang shaped and flip backwards and forwards as they try to align themselves with the alternating electric field of the microwaves. Vibrating atoms = heat.

I know this because I recently started a thread asking why a pair of underpants I once put in a microwave caught fire. The consensus of oppinion from the clever naked scientist bods is that cotton molecules are also polar.

[lyner]

Let's take this to another level.
 Let's sayI took a ceramic mug full of pure H2O and put it into the microwave. Would it heat up the water? Pure water is an insulator.

The water molecules can each absorb energy at their resonant frequency - and transfer the energy as internal energy ~(heat). Electrons do not need to move through the water.
In a metal, the absorption of energy is not due to resonance in the same way - it is due to the displacement of the conduction electrons, which are able to move throughout the metal - the energy transfer is due to the work done in moving the electrons through the lattice rather than their arrangement within a single molecule.

[Bored chemist]

Pure water is not an insulator.
It's  a lousy conductor but it does conduct. Also it gets more conductive as it gets hotter.
However, as has been pointed out it doesn't need to conduct.
It has a dipole moment and as it is exposed to the electric field of the microwaves it trys to line up with that field. In doing so the molecules move. Since they are moving in a viscous liquid they dissipate energy. That's why it gets hot. I'm not even sure they use a resonant wavelength.