Pulsed direct current?

Question

How are the microwaves coming from the sun generated by pulsed direct current, as opposed to microwave generated by magnetron?

What exact is pulsed direct current?

In order to provide a context for those two questions, they arised from the issue of microwave heating in the reference below under the section Heating Food:
http://www.mercola.com/article/microwave/hazards2.htm

Thanks very much in advance.

Sorry, typo: exact should be exactly.

Could you also post any links you have on this particular topic; I don't have much success googling on this one.

Answer 1

The information in the referenced link is junk. There is no difference in microwaves of the same frequency or wavelength regardless of where they originate. Actually the magnetron in microwave ovens and radars receives pulsed direct current.

Answer 2

I guess I'm a little suspicious of their being any different "kinds" of microwaves. They could be generated in different ways, but that wouldn't affect how they behave, they'd all travel at the speed of light and have comparable wavelengths (by definition).

The article seems to be saying that there is some difference between natural (from the sun) and artificial (from a microwave oven) microwaves. I don't really think this is the case.

As for pulsed direct current, it would be just what it says, a big pulse of current going in one direction could generate microwaves the same way that alternating current generates them. By accelerating electrons, and then coupling that into an electromagnetic wave via some kind of antenna, you'd have a traveling microwave. It would just be a pulse of microwaves and not many continuous wavefronts like comes out of a microwave's magnetron.

I would think the principle that heats food would work in both cases. Microwaves excite a vibrational mode (I forget which one) in water molecules, and it is this motion that becomes heat.

Answer 3

All the author is trying to say is that light from the sun has its power spread across (roughly) the entire (visible and some invisible) spectrum and thus has very little power concentrated in the single small section of the spectrum that microwave ovens use.

"Direct current" has all its energy concentrated at DC. "Pulsed direct current" spreads its energy out in the frequency domain under an envelope of a sinc function (a sinc(f)/f is an unnormalized sinc function). In other words, you can choose a handful of pure tones of light to add together such that the sum is "pulsed DC" (lots of little pulses turning on and off in time). You actually need an infinite number of these pure tones, but together they form a "nowhere dense set" of frequencies, and most of their energy comes from contributions closest to DC. Thus, energy is spread out everywhere along the spectrum, but not much of it is concentrated in one particular region (like the microwave region). "DC" technically isn't the best term to use here.

A microwave oven only operates at a single frequency (2450 MHz) and thus concentrates a great deal of power at that frequency. This single frequency is used because it most efficiently causes dielectric heating inside water (and fat and sugar) molecules (which are polar, which is what makes the microwave work).

I think the author is trying to deal with the fact that many people can't tell the difference between "microwaves" from a microwave oven and the more general "microwaves" from other sources that emit low-frequency invisible light, like the sun.

(note that the sun's light is only "white" in the visible range; it tapers off at higher and lower frequencies, like a black body. Also, you should not think of the sun as being a "pulsed DC" source, but the author is trying to make his point using that analogy)