# Power inside a microwave oven due to reflections

• genekuli
In summary, the oven's metal box absorbs a small amount of energy while the oven is cooking. This box significantly increases cooking time.
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?

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.

genekuli said:
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
Drakkith said:
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?

genekuli said:
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

Drakkith said:
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

genekuli said:
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.

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

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

genekuli said:
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.

genekuli said:
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
Drakkith said:
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"

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?

genekuli said:
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)

genekuli said:
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.

genekuli
genekuli said:
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.

genekuli

## 1. How does the power inside a microwave oven change due to reflections?

When microwaves are emitted from the magnetron in a microwave oven, they bounce around the metal walls of the oven and can reflect off of them. These reflections can either add to or subtract from the total power inside the oven, depending on the location and angle of the reflection. This can result in hot and cold spots within the oven and affect the overall cooking time.

## 2. Can reflections inside a microwave oven be harmful?

No, the reflections inside a microwave oven are not harmful. The microwaves used in cooking are non-ionizing, meaning they do not have enough energy to cause damage to DNA or other molecules in the body. However, it is important to never operate a microwave oven with the door open, as this can result in direct exposure to the microwaves emitted from the magnetron.

## 3. How do reflections affect the cooking time in a microwave oven?

Reflections inside a microwave oven can affect the cooking time by creating hot and cold spots within the oven. This means that some areas of the food may be cooking faster than others, resulting in unevenly cooked food. To minimize this effect, it is recommended to stir or rotate the food during the cooking process.

## 4. Can the design of a microwave oven reduce the impact of reflections?

Yes, the design of a microwave oven can help reduce the impact of reflections. Many modern microwave ovens are designed with rotating turntables and multiple magnetrons to help distribute the microwaves more evenly and reduce the impact of reflections. Additionally, the interior walls of the oven are often coated with a material that helps to absorb and diffuse the microwaves.

## 5. Are there any safety precautions to take when using a microwave oven?

Yes, there are some safety precautions to take when using a microwave oven. It is important to never operate the oven with the door open, as this can result in direct exposure to microwaves. It is also recommended to use microwave-safe containers and to avoid using metal or aluminum foil, as these materials can cause sparks and potentially damage the oven. Additionally, it is important to follow the manufacturer's instructions and never attempt to repair a microwave oven yourself.

• Optics
Replies
4
Views
1K
• Optics
Replies
9
Views
2K
• Optics
Replies
5
Views
2K
• Optics
Replies
5
Views
6K
• Optics
Replies
5
Views
3K
• Optics
Replies
2
Views
1K
• Electrical Engineering
Replies
72
Views
4K
• Electrical Engineering
Replies
18
Views
2K
• Electrical Engineering
Replies
12
Views
2K
• Other Physics Topics
Replies
33
Views
2K