Power inside a microwave oven due to reflections

[genekuli]

That is how much more power would it consume if one was to try and produce the same amount of heating power with just the waveguide holes and no walls, so no reflections?

 

[Drakkith]

You might have a non-equilibrium situation shortly after the microwave starts up, but you very quickly will reach equilibrium where the amount of power being pumped into the cavity equals the amount of power being absorbed by the food and the microwave components.

That is how much more power would it consume if one was to try and produce the same amount of heating power with just the waveguide holes and no walls, so no reflections?

I suppose that depends on where you place the food in relation to the waveguide along with the size, shape, and makeup of the food you're heating. There's no simple answer here. The point of the cavity is to contain the microwave radiation so that it doesn't escape and make it so that as much as possible is absorbed into the food.

 

[genekuli]

The point of the cavity is to contain the microwave radiation so that it doesn't escape and make it so that as much as possible is absorbed into the food.

yes, and what % is that of the total. how much is reflected and how much is from the source?

 

[Drakkith]

yes, and what % is that of the total. how much is reflected and how much is from the source?

I suppose you'd have to compare a sample with and without a cavity. And as I said already, that's going to change with the specifics of the food you're trying to cook. So there's no single answer. Stick a big container of soup at the end of the waveguide and you'll probably absorbed nearly all the incoming radiation since water strongly absorbs microwaves. However, if you put a bagel in front of the waveguide you'll likely lose most of the microwave radiation.

 

[genekuli]

if there was a drop of water in the oven, how much more energy does it receive as opposed to only the waveguide hole at the same distance in the open

 

[genekuli]

I suppose you'd have to compare a sample with and without a cavity. And as I said already, that's going to change with the specifics of the food you're trying to cook. So there's no single answer. Stick a big container of soup at the end of the waveguide and you'll probably absorbed nearly all the incoming radiation since water strongly absorbs microwaves. However, if you put a bagel in front of the waveguide you'll likely lose most of the microwave radiation.

yes of course, but if it was a small water droplet say so as to not significantly block the majority of the RF

 

[Drakkith]

yes of course, but if it was a small water droplet say so as to not significantly block the majority of the RF

Then it would only absorb a small amount and you'd lose most of the microwaves.

 

[genekuli]

yes, but approximately how much of the energy would be from reflections?

 

[genekuli]

is the oven's metal box a safety and convenience solution or does it significantly add to the cooking?

 

[Drakkith]

yes, but approximately how much of the energy would be from reflections?

Hard to say. A drop of water is so small that even inside a cavity most of the radiation is going to bounce around and get absorbed by the microwave components instead of the droplet.

is the oven's metal box a safety and convenience solution or does it significantly add to the cooking?

Both. By containing the microwaves inside the cavity, you make it so that the food absorbs most of the microwaves and virtually none are lost to the environment where they can harm someone.

 

[genekuli]

By containing the microwaves inside the cavity, you make it so that the food absorbs most of the microwaves and virtually none are lost to the environment where they can harm someone.

yes, but what would be the approximate % of the "most"

 

[genekuli]

in the same scenario in a convection oven, the containment of the energy by the container is almost 100%. without the container almost no energy would be absorbed by the food. and in a microwave oven?

 

[A.T.]

yes, and what % is that of the total. how much is reflected and how much is from the source?

To estimate the wave energy E_0 that has not been reflected yet, multiply the power output of the waveguide by the average time before first reflection (distance / speed of light).

To estimate the wave energy that has been reflected at least once, you would have an infinite sum: E_r = Σ E_0 * rⁱ, where r is the average fraction of reflected energy for one reflection and i goes from 1 to infinity.

 

[genekuli]

for example, in an engineering magazine it might say; most of the power of the microwave oven is due to the resonant cavity reflections of the metal box [my statement].
i'm just asking to quantify the "most" a little into a %
(not referring to the magnetron here)

 

[Drakkith]

yes, but what would be the approximate % of the "most"

I honestly can't give you a concrete number. This paper gives a 'utilization efficiency' between 0.38 and 0.62 for a sample of water. This efficiency depended on both the output power and the time irradiated. This would almost certainly change drastically as you change your sample shape, size, and makeup too.

 

[sophiecentaur]

is the oven's metal box a safety and convenience solution or does it significantly add to the cooking?

I would say Totally!
If you put a lump of wet food in a waveguide and pass 2GHz waves through, the food may absorb very little energy (depending on its size and shape) and the power can mostly go up the spout. Modify the waveguide so it becomes a resonant cavity and the resistance of a few parts of the wet food will match with the standing wave, dissipating Power. That's why you get hot spots in a simple oven. You can either move the food about or use a 'mode stirrer' to move the waves relative to the food and get more even heating. You could say the wave gets several go's at the target. Without the food present, you can get high standing wave levels which can damage the Magnetron. At least that used to be true - it's not hard to back off the power these days and I haven't actually heard of "exploding microwaves" recently.