Two Questions about my Microwave Oven

1. May 5, 2005

The Rev

These are probably the newbest of newb questions, possessing of a level of ignorance that moves right past funny into sad....however, being the curious type that I am, I'm gonna tempt the cat's proverbial fate, and ask anyway.

1) My understanding is that microwave ovens work on the principle that EM waves of a certain frequency (microwaves) agitate the water molecules in the food, and as a result of the agitation, heat is produced which then cooks your food. Now, what I don't know about quantum mechanics is alot, but doesn't this basically come down to photon-electron interactions, where the photons are of just the right frequency to cause the electrons to change state within the hydrogen and oxygen atoms in water?

If this is so, then nuking food is basically a quantum interaction, right?

Well, if this is so, then one might assume that the food is in a superposition of states until observed, at which time a state of "cooked" or "uncooked" is determined by the collapse of the microwave energy's wave function.

What happens if you watch the food the entire time it's "cooking"? How can the food change states if it's being observed the entire time?

2) How do they keep the microwave frequency photons from escaping the oven if visible light can get out?

Thank you for indulging me.

$$\hbar$$

The Rev

2. May 5, 2005

vincentchan

1. Why don't you just regard the microwave as a very quick changing E field... The water molecule is just a electric dipole flipping in the changing E field... Does that solve your problem?

2. How do they keep the microwave frequency photons from escaping the oven if visible light can get out?
If you look at the microwave (oven) glass door carefully, you will see ______. That is the reason microwave cannot pass through..... (please go to your microwave oven and tell me what you see)

3. May 5, 2005

The Rev

Like I said, I don't know THAT much about QM, so my answer is, "Maybe." If I understood your answer, I might be able to collapse that Maybe Function into a Yes or No.

Alright, smarty, yes, I have seen the little screen. What makes it keep in the microwave frequency photons, but pass the shorter wavelength visible light?

$$:)$$

The Rev

4. May 5, 2005

Claude Bile

1. Microwave photons interacting with a water molecule are not electronic interactions per se, as they do not excite electrons into higher orbitals etc. Microwaves are only able to excite electrons into higher rotational energy states. Microwave energy absorbed by the water molecules will manifest as additional rotational and translational energy, which we identify on a macroscopic level as heat.

As soon as you involve large numbers of photons and molecules (around 10^20 to 10^24 for both in this case), classical physics is quite sufficient to describe the processes that occur in a microwave. Trying to apply quantum principles to macroscopic fields and objects is inappropriate.

2. Microwaves establish a standing wave between two paralled metal plates. The field in this case is actually very well confined for this reason, and does not rely on the absorptive properties of the microwave's outer casing to prevent the microwave radiation from escaping. Visible light is able to enter/exit the microwave due to the plastic window being transparent throughout most of the optical spectrum.

Claude.

5. May 5, 2005

DaveC426913

Waves will effectively not pass through a grating that has a smaller resolution than the wavelength of the wave (This works in water as well). Microwaves are in the centimeter band, and will not pass through a grill smaller than that.

Radio telescopes are even more coarse: little more than a wire mesh - you can stick your fingers through them! (But they are opaque to radio.)

6. May 6, 2005

The Rev

This is where I was mistaken. I thought the electrons were being excited into higher orbitals. I've only read Gribbin's two books, and while they stimulate ideas, they aren't exactly comprehensive texts. I need to get Quantum Mechanics in Exhaustive Detail for Dummies. :rofl:

That's really cool.

$$\phi$$

7. May 6, 2005

Locrian

Firstly, the microwaves may establish standing waves within the cavity, but if you do not have a good screen, the microwaves will leak out easily. If the screen is damaged you can get microwave leaks. If the door is damaged you can get leaks. There is a potential produced on the screen of your microwave door and it needs to be grounded. I hope no posts in this thread will be taken to suggest that the microwave seal of the enclosure is not of great importance.

Secondly, there is a general misunderstanding with how microwaves heat food. Water vapor has a rotational energy mode right around the frequency microwave ovens operate at, and many think this is the operation that generates heat in the water. It isn't. Water molecules in liquid form do not have quantised rotational modes, and suggesting it is the alternating rotation of the molecules isn't the full story. Most of the heat has to do with electrical currents created in the material and the heat is generated by the material's resistance. This heating operation has been studied by chemists interested in using it.

8. May 6, 2005

Burnsys

A time ago i found in the net, an article that explained how to build a weapon out of a microwave oven.....

it said that a 2000w magnetron pointed at a human for 5sec could kill..

9. May 6, 2005

Locrian

It would certainly blind them and cause permanent damage to parts of their brain.

But uh... it's going to have to be plugged in. It's hard to focus radiation of that wavelength, so it will be very heavy. It's going to have a very short range.

Just hit em with a baseball bat instead.

10. May 9, 2005

vanesch

Staff Emeritus
I don't know if this is urban legend or not, but I heard the story of a technician repairing an F-16 combat radar while someone was inadvertedly standing in front of the airplane, and the person was reported to feel dizzy without a reason. I think these radars are also in the KW range (but in pulsed mode).

cheers,
Patrick.