Most microwave ovens won't start up again just by closing the door.

If yours does, best press STOP/CLEAR before you close it.

Peter,

you  say: "...we agree that the microwaves vibrate water molecules, so, if you're right (and I do believe you), where is the water in the electrical components that suffer as a result of being bombarded?"

Well actually our agreement is only qualified. Certainly microwaves of a suitable frequency (such as those in the typical microwave oven) do vibrate the water molecules, and various other suitably polar molecules, such as common fats and oils. However, it does not follow that nothing else can absorb microwave energy. In particular any kind of microwave aerial also can absorb microwaves; that is precisely how they work!

In particular a transmission aerial such as a magnetron can absorb microwaves that suitably match the very microwaves that they transmit in the first place. It is very much like the way that if you twang the string of a musical instrument, creating a sound in the air, then the echo of that sound striking the string again, will amplify its vibrations again. And of course, if the energy cannot escape it is prone to convert into other forms, in the end typically heat. Poor magnetron!

You also asked: "... are we really saying that there is so much leakage (billions of waves a second), that having nothing in the microwave doesn't make much difference?"

Actually, what we said was billions of bounces (reflections if you like) a second. And some wastage at every bounce. We never said anything about having nothing in the oven not making much difference. Suppose the oven is empty; then the photons bouncing around have a half life, say, 100 nanoseconds. After a few half lives, say 10 or so, if you cut the power, the microwave glow will have died away for most practical purposes. However, if we put a microwave-absorbent plate of food or jug of milk in the oven, it will absorb microwaves strongly, and the half life of a bouncing microwave might drop to say, 10 ns. These figures are the wildest thumb sucks, but you can see that your magnetron in the second example would have only about one 10th as much energy to absorb, which I am sure you can see could make quite a lot of difference. It might not sound like much, put that way, but think of it like this: you might feel very comfortable carrying a 50 kg load a few hundred metres, but that does not mean that you could carry a 500 kg load anywhere, right?

You protest: "When I think of the energy required to warm a cup of tea in a microwave, I find it difficult to believe that when nothing is there, the whole energy furnished by the electrical supply has just .....gone......" and very right too!

Of course it has not just... gone... Energy is conserved remember? For all everyday practical purposes anyway. Some of it leaks out of the oven and warms a grateful universe, including you if you are nearby. A lot more of it goes into heating up your much-abused, long-suffering magnetron.

No mystery there, or am I misunderstanding your point?

You presume that: "...there is leakage when something is in there too, so something, to my simplistic mind, does not add up."

You are right about the leakage of course, but on the contrary, everything does add up, quite mercilessly in fact. Some leaks out, some heats up the magnetron (not a great deal we hope!) and most (we devoutly hope!) heats up our food. And between that wastage and that usage of power we consume precisely as much energy as we had fed into the oven.

Simple, right? Yes, but I sense your (hopefully residual) discomfort. What happened to the energy absorbed by the food? Am I right?

On the assumption that I am (forgive me if I am not) let me point out three major fates for any energy absorbed by the food. (There are umpteen others, but generally minor.)

Firstly, some of the absorbed energy spins or distorts polar molecules (prominently fats, proteins and water) and generally sets them bouncing around in a state we call being hot. That energy represents the equivalent of microwaves that have been absorbed and converted. As microwaves they are gone. As energy they still are there in the form of heat.

Secondly, part of the microwaves and the heat they generated causes chemical reactions, such as hydrolysis of some foods or charring of others. This might cause more heat to be given off, but also absorbs those microwaves responsible for the effect. Some of the new molecules formed in the process store such energy as is not given off in the form of heat.

Thirdly, the hot food re-radiates heat in the form of photons are far higher energy and far higher frequency (meaning far smaller wavelength). The wavelengths in question are in the infrared. Now, your microwave oven is partly metal, which is opaque both to microwaves and to infrared. However, the front panel, usually set into the door, is partly of glass and partly a metal grid. The grid does not permit long wavelengths such as those of microwaves (a few centimetres long) to pass through. The wavelengths of the infrared are in comparison minute, typically more like a few micrometres. They pass through the grid as comfortably as any torch beam. Now, there is leakage!

The glass of the door is not very infrared-opaque, though it is not especially transparent either, and it is a major route for escape of energy.But as I said, the whole lot adds up pretty comfortably!

Cheers

Jon

The energy from the magnetron is transmitted into the oven cavity. If something is in there the energy is absorbed. If nothing is in there some fairly high voltages can build up thet will cause arcing from sharp points. Modern ovens tend not to have sharp points in the oven cavity and so this is not an issue. There are obviously some losses in the oven cavity walls and through the door , nothing is perfect. The bulk of the enrgy is reflected back to the magnetron which generated it. The magnetron heats up. Magnetrons are not too good at taking this and eventually go bang.

The whole process of a microwave oven is technically similar to transmission and reception of any other electromagnetic wave. You need a transmitter, say the source of microwave inside the oven, and a receiver, typically some food with moisture or fat content. The receiver acts as a matching impedance to the ambient microwave. If you remove the destination (of matching impedance), then the microwave transmission does not flow into a target. Most of the actual radiation does not even leave the electronic circuits. That is why you could eventually fry a transmitter of any frequency if antenna connection is removed for a long period.

I understand all this too. However, I hope (and think) we agree that the microwaves vibrate water molecules, so, if you're right (and I do believe you), where is the water in the electrical components that suffer as a result of being bombarded?

One other thing I can't get straight here - are we really saying that there is so much leakage (billions of waves a second), that having nothing in the microwave doesn't make much difference?

When I think of the energy required to warm a cup of tea in a microwave, I find it difficult to believe that when nothing is there, the whole energy furnished by the electrical supply has just .....gone......

Presumably there is leakage when something is in there too, so something, to my simplistic mind, does not add up.

Yes, yes, and yes.

The energy changes partly because it got absorbed by the target. The loss of energy is partly by red-shifting (look up Raman scattering and spectroscopy, and read the book "Why the Sky is Blue" by Hoeppe.) partly by absorption, and partly by loss of photons, leading to a loss of amplitude.

And of course, if you change the frequency, such as by Raman effects, then of course the wavelength changes too, right?

There are other secondary and emergent effects too of course, such as the emission of infra-red as the food gets much hotter. One chap on the internet explains how to melt glass in your kitchen microwave. He carefully warns that you should only do this if you don't expect to need the oven again, because you won't be able to.

So don't.

not really an answer, but another related question.

what does the electro magnetic wave look like once it has been through the food.

i know it will have less energy but does the frequency, amplitude or wave length change

after answering your question i realised i didn't completely answer the question. i started to asking similar questions to my self. i know that energy can not be created or destroyed, so the energy must go some were.

so to be honest im not sure exactly were the energy goes

in my analogy with the swing kinetic energy not used to increase the potential energy of the swing(pushing when the swing is moving towards you). the energy from your push and some of the energy from the swing is converted into sound(and maybe some broken ribs).

in the microwave when the food is heated up the particles are vibrating more. when we heat air up  half the microwave energy is energy is used to "hit" the particles to slow them down. is this the same as the swing? does your microwave make moor noise when there is no food in it? 1000w of noise?. now, at a smaller level sound is just pressure difference. so i suspect that the energy causes a pressure increase in the surrounding air, even though it is probably a very small one.

just another thing to think about. temperature at a particle level is just vibrations and pressure at a particle level is also just vibrations(as i understand it) so why do you get a presure increase instead of a temperature increase.

my science really falls down at a microscopic level instead of macroscopic

Not only are you indeed using the same energy, and not only are you wasting it, but if you do not stop in time, something has to give. If you read any instructions that you get with a microwave oven, you will see that they strongly warn you against running the oven without a reasonable mass of microwave-absorbing material inside (such as a meal for example.)

You rightly assume that the energy must go somewhere. It reflects to and fro inside the chamber, something like 1 billion times a second, with a small percentage of leakage and absorption  at each bounce. The amount that leaks out won't harm you much, but the part that is absorbed is not good for the oven. The main parts to suffer are the electronic components that produce the microwaves.

It might clarify the picture for you to imagine what happens to your car if you put your foot on the clutch and on the accelerator. All that happens is that the engine turns with very little resistance, which does little harm for a little while, but if you keep it up and build up very high speeds of rotation, you can be sure of serious damage, even of effectively total destruction of the engine.

The energy that should have gone into driving the car instead goes into turning the engine over at excessive speed. It is much the same with any device that works by exerting energy on some object other than its own structure. For example if you draw a bow and shoot without an arrow on the string, you stand a good chance of ruining string and bow, and possibly injuring yourself if you loose as though you were shooting an arrow.

Ok, I think I understand your answer John, but this begs the question - am I really using the same amount of energy when the microwave is on and 'buzzing', whether there is something in there to cook or nothing at all?

It seems a little strange that there is a scenario where I switch on a microwave with nothing in it, and monitor the electricity used. Can it really be true that when I put a ready meal in there to cook, the meal cooks, but the monitor is reading the same usage?

Which goes back to the original point - where is the excess energy going, when there is nothing in the microwave?