And the OP was likely hoping to split water "for free" (which can't ever be done) by zapping it at the right frequency, which is well into the terahertz and so can't be done with current technology even if it was free, which it is not.

I don't like to think of posters as free lunchers, but sadly I think you are right.

That is why I was offering to convert the thread to something useful, namely a proper discusion about resonance for those who may be interested.

NSAspook has the right idea in that the water molecule (when free), being polarized, tries to align itself (rotate) with the changing electric field of the microwave. This works if the frequency is compatible ie resonance takes place.

 

Sounds good. But I'm not sure what the OP's "resonance energy" would refer to in that scenario. I took it to refer to a molecular bond strength that must be exceeded in order to break the bond, by some sort of resonant application of energy. It wouldn't be the first time I misunderstood an OP!

 

...
That is why I was offering to convert the thread to something useful, namely a proper discusion about resonance for those who may be interested.

NSAspook has the right idea in that the water molecule (when free), being polarized, tries to align itself (rotate) with the changing electric field of the microwave. This works if the frequency is compatible ie resonance takes place.

I'd be appreciative if you wanted to explain in more depth.

From my (limited!) understanding of mechanical resonance and heat, if somethind resonates mechanically, applying energy at resonant freq increases its mechanical movement at that freq, and energy is stored within the thing AS movement. The process is efficient and little heat is achieved. Like if you apply a freq to a tuning fork it doesn't heat, it vibrates. And then if you stop the source energy, it continues to vibrate at resonance for a period while it gives off that same freq as energy back to the environment.

But if you apply that same frequency to a non-resonant material (like a clay tuning fork) it can't resonate efficiently and the energy must be converted to internal heat. Similar effect if the energy is applied to a resonant material, at non-resonant frequency; low efficiency, internal heating, no ringing when energy is removed.

I think a MWO acts much more like the second model. And it doesn't matter too much what the frequency is, because it's nowhere near the resonance of water molecules and the result must always be heat.

The magnetron in a MWO contains resonant cavities at 2.4GHz, and they are MUCH larger than a water molecule.

 

Think of it more as an impedance match. If the frequency of the radiation is too high, the polar molecules do not have time to rotate to align with the fluctuating field. It is this induced rotation that becomes molecular motion, or heat. The water is only resonant at the molecular level, not the atomic bond level.

Maybe resonant isn't the right word, since there is no ringing after the power is removed. But it is resonant in the sense that just the right frequency can cause the water molecule to rotate continuously.

At lower frequencies, I suppose the water molecules still align with the field, but at a lower rate and therefore with less rotations per time and thus less heat production.

 

Hello,

NMR (nuclear magnetic resonance) is making use of the spins of an atom in a molucule and a strong magnetic field.
You could look at a molucule as a construction of atoms with elastic band between them.
With NMR you are putting the molecule into a strong magnetic field.
In this field the atoms will allign.
Using an Radio Frequent puls, you let the atoms swing.
The frequency of the swinging is dependend on the position of the atoms in the molecule.
The frequency used to let the atoms swing is dependend on the fieldstrenght and the nucleus you are looking at.

Here is a link to a PDF from the vanderbilt institute:
http://www.vanderbilt.edu/AnS/Chemistry/Rizzo/chem220a/Ch13slides.pdf

Bertus

 

I think a MWO acts much more like the second model. And it doesn't matter too much what the frequency is, because it's nowhere near the resonance of water molecules and the result must always be heat.

The magnetron in a MWO contains resonant cavities at 2.4GHz, and they are MUCH larger than a water molecule.

Right, so they just spin and that spin couples the energy in to the water by simple dielectric loss at microwave oven frequencies.

 

So it's simply energy causing movement in the water (or other liquid) and subsequent heat, and not "resonance"?

 

NMR uses a quantum effect and relies on the spin quantum number, which is not a mechanical spin at all.

Microwave oven heating is quite different, but still a resonance phenomenon, despite the claims of some websites to the contrary.

I have been experimenting with some sketches to descibe the processes and will post when I am happy with them as it is complicated to bring out the proper sequence of events.

Remember also that I noted in my first post on the subject that water has other resonance modes besides what is happening here.

 

So it's simply energy causing movement in the water (or other liquid) and subsequent heat, and not "resonance"?

It's not resonance of individual molecules but because it's a dielectric form of current flow the molecules as a whole move in sync with the shifting phase and polarity with the waveform much like a dielectric antenna that form nodes.

 

It's not resonance of individual molecules but because it's a dielectric form of current flow the molecules as a whole move in sync with the shifting phase and polarity with the waveform much like a dielectric antenna that form nodes.

A bunch of compass needles following a changing magnetic field might be more visual example.

The clue as to whether it is resonance or not is that it is frequency selective. Not all EM frequencies cause absorbtion of the EM energy and subsequent heating.

 

So it's simply energy causing movement in the water (or other liquid) and subsequent heat, and not "resonance"?

I want to say yes, but I think we're all hung up on different meanings of "resonance".

If a transfer of energy is frequency-dependent, I think that qualifies as resonance. Like going over a rumble strip in your car at just the right speed. Microwave energy makes water molecules (and other dipoles) rotate in synch to the field. This becomes kinetic energy transferred to nearby molecules - heat.

A plucked guitar string continues to oscillate after the plucking energy is removed. Isn't it resonating also? But that's a different phenomenon. I guess a spinning water molecule keeps spinning until the energy dissipates. Maybe it's not so different.

 

A bunch of compass needles following a changing magnetic field might be more visual example.

The clue as to whether it is resonance or not is that it is frequency selective. Not all EM frequencies cause absorbtion of the EM energy and subsequent heating.

At the frequencies of a MWO the variation in overall absorbtion is minor because so much energy is directly coupled into the overall mass as heat (increases the random movements of atoms) it dampens resonance modes.

 

Gosh why does everyone keep talking about spinning?

Anyone would think these molecules are like ice skaters.

Take a lump of sugar, or better grow a large single crystal, and place it in a microwave oven.

What happens?

Does it spin round like an ice skater?

The rotations are small, variable and keep reversing in direction. It is this reversal that leads to a resonance phenomenon. Otherwise it would not be resonance.

 

Gosh why does everyone keep talking about spinning?

Because that's what is happening - the dipoles align to the field and reverse when the field does. It's true that the molecule may not continue rotating in the same direction and instead just reverse itself, but it does fully rotate to opposite polarities on each cycle.

 

Because that's what is happening

So you are telling me that when I switch on my microwave my solid sugar crystal will completely rotate 360 degrees or more in 0.0000000004 seconds?

 

Certainly not. The sugar molecule is not a dipole, for one thing, and the crystal as a whole isn't either. A fraction of the entrained water molecules will rotate.

Ice is much slower to heat than liquid water, since the dipoles in an ice crystal are constrained. But as soon as a few knock loose, heating speeds up.

 

Hello,

Perhaps this is a simple idea how the magnetron works:
http://ffden-2.phys.uaf.edu/104_spring2004.web.dir/arts_mcnulty/howmicrowaveovenswork.htm

Bertus

 

I didn't ask about ice or entrained water.
As a matter of interest I have never had any trouble cooking from frozen in my oven, there is no time difference whatsoever to a bowl of frozen peas with visible ice.

But I did note what will happen to a mince pie in an earlier post.

Sugar is about the one molecule that interacts more strongly than water in the microwave oven.

The whole point about the crystal is that the molecules in a crystal cannot rotate far.

And yes sugars (there are many) are polar.

Wikipedia, since you like it so much
Polar molecules[edit]
The dipole moment of the water molecule.A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (H2O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The dipoles do not cancel out resulting in a net dipole. Due to the polar nature of the water molecule itself, polar molecules are generally able to dissolve in water. Another example includes sugars (like sucrose), which have many polar oxygen–hydrogen (-OH) groups and are overall highly polar.

If you seriously believe that a single cycle of microwave energy can rotate a water molecule 360 degrees twice, try this calculation,
How much energy will this take and how much is available in a single cycle?

 

As a matter of interest I have never had any trouble cooking from frozen in my oven, there is no time difference whatsoever to a bowl of frozen peas with visible ice.

The effect of temperature on microwave heating is well established. Less heating of ice and cold water than warm.

Sugar is about the one molecule that interacts more strongly than water in the microwave oven.

Sugar increases the dielectric properties of liquid water. I'm no expert, but I believe the thinking is that this leads to better energy absorption by the water, as opposed to the sucrose molecules themselves absorbing the radiation and converting it to heat. Sucrose is indeed polar - I misspoke - but I'm not sure if it behaves as a rotating dipole in a field.

The whole point about the crystal is that the molecules in a crystal cannot rotate far.

Right, and crystals absorb far less microwave energy as a result.

If you seriously believe that a single cycle of microwave energy can rotate a water molecule 360 degrees twice, try this calculation,
How much energy will this take and how much is available in a single cycle?

A microwave oven transfers energy to water by rotating some of the molecules in the field. If they don't rotate, they don't absorb energy from the field.

There are many references I can provide on these topics if anyone is interested. Here's one I have open at the moment. I've seen better and I can dig them up if needed.

 

they don't absorb energy from the field.

The whole point is that they don't 'spin', they rock back and forth a small amount.

Right, and crystals absorb far less microwave energy as a result.

It is nonsense to suggest that the absorbtion is less in the solid state, the dipole moment is identical.
What actually happens is that in order to rotate the cryustal bonds must stretch (or even break). This is where the energy goes.

Sugar increases the dielectric properties of liquid water.

There is no liquid water in a mince pie, but lots of sugar.

We use 'wheat bags' in place of old fashioned hot water bottles. The starch in the wheat is another molecule that absorbs the microwave energy and results in a safer, more user friendly 'heat bag'.

I seriously suggest you wait until I have sorted out these sketches as they explain it all. Please remember there are many websites promoting nonsense about microwaves ovens.