It seems to have lost its "soul". Why doesn't this microwave oven rotate now?

In the traditional impression, a microwave oven is an appliance closely associated with "rotation". Turn the knob, and the glass turntable drives the food to rotate in the warm yellow light. Then, accompanied by a "ding", the steaming hot meal is ready to be served on the table.

Even microwave heating is called "giving it a spin". Rotation can be said to be the soul of a microwave oven | BuzzFeed/giphy

However, many modern microwave ovens have changed their appearance. They not only come with various complex new functions, but some even don't have a turntable, leaving only a flat bottom.

Why is the turntable gone? | Photo by the author

A microwave oven without a turntable has fewer restrictions on the size of the heating container and is easier to clean. The user experience is indeed good. But can a microwave oven really heat food well without a turntable? Which is better, with or without a turntable?

First, why is there a turntable?

A microwave oven heats food through electromagnetic waves in the microwave frequency band. These electromagnetic waves are generated from the magnetron and form a changing electromagnetic field inside the microwave oven. Polar molecules in food (such as water molecules) will react to the electromagnetic field. They rotate, vibrate, and collide with each other, generating heat in the process.

A microwave oven can make polar molecules (such as water) collide with each other to generate heat | Wikipedia

Compared with traditional stoves, a microwave oven allows energy to reach the food more directly, so it heats up quickly. It can reheat leftover food in just two or three minutes. However, the unique heating principle also means that a microwave oven is bound to face the problem of uneven heating.

Inside the microwave oven, electromagnetic waves are emitted from a fixed position in a fixed form. These electromagnetic waves are reflected between the inner walls of the microwave oven and interact with each other. As a result, there is always a fixed pattern of standing waves inside the microwave oven.

A simplified schematic diagram of a standing wave. The blue and green waves are superimposed to form a red standing wave. There are some points on the standing wave where the amplitude is always 0, which are called nodes; while other points always maintain the maximum amplitude, and these positions are called antinodes. | Wikipedia

Standing waves make the heating effects at different positions inside the microwave oven very different. At the position of the antinode, the electromagnetic field oscillates greatly, and the molecules in the food also generate a large amount of heat. But at the node, the electromagnetic field is calm, and the food naturally won't heat up at all.

Standing waves make the heating effect inside the microwave oven uneven. The experimenter heated water in a shallow glass dish without using a turntable and used thermal imaging to capture the uneven temperature distribution. | Michael Vollmer

The turntable is designed to solve this problem. It allows the food to constantly change positions during heating, making the electromagnetic wave environment it is exposed to more even.

Without a turntable, there are other ways

Some microwave ovens don't have a turntable because the turntable is not the only means to improve uneven heating.

Another commonly used method is to install special-shaped metal "blades" inside the microwave oven. Although the position of the food is not changed, the rotating blades can continuously change the way electromagnetic waves are reflected inside the microwave oven. In this way, the original fixed standing waves will be disrupted, and the heating intensity at different positions will become more uniform.

The removed metal blade, which is responsible for stirring electromagnetic waves inside the microwave oven | RepairClinic.com

These metal blades are called "mode stirrers". They are usually located below the glass or ceramic bottom plate or hidden behind a plastic cover and are not usually noticed. Although these covers look opaque, they can transmit microwaves and will not affect the function of the stirrer. Some microwave ovens have both a stirrer and a turntable.

The microwave stirrer hidden in the plastic cover | RepairClinic.com

How effective are they?

From the experimental data, both the stirrer and the turntable can significantly improve the heating uniformity, and there is no obvious difference in their advantages and disadvantages. However, they also have different limitations.

Comparison of heating uniformity without a stirrer (left) and with a mode stirrer (right). The experimenter heated a layer of moist color-changing silica gel in the microwave oven. The silica gel that was not fully heated remained light in color, while the fully heated and dried silica gel darkened in color. | H. S. Hauck

The limitation of the stirrer is that even if the microwaves are constantly disrupted, the microwave intensity inside the oven cannot be completely uniform. Even if the blade is reasonably designed, its "averaging" effect may still be slightly worse than directly changing the position of the food.

The limitation of the turntable is that there is always an inevitable temperature difference between the center of the rotating shaft and the outer periphery. This is because the food at the center of the rotating shaft cannot effectively change its position during heating.

The researcher heated water in a plastic tray divided into grids and used thermal imaging to show the temperatures at different positions of the microwave oven turntable. The temperature around the turntable was uniform, but there was a significant temperature difference at the center. | Krishnamoorthy Pitchai et al.

Apart from these factors, uneven microwave heating is also related to the composition and geometric shape of the food, which cannot be avoided even by improving the design of the microwave oven.

For example, under the same microwave intensity, the heating effects of water and fat in food are very different. When heating meat in the microwave, we often find that the fatty part gets hot first - this is because the specific heat capacity of fat is much smaller than that of water. The efficiency of fat in absorbing microwaves to generate heat is not as high as that of water, but it only needs less energy to increase the temperature more.

Therefore, to really heat the food evenly, it's best to pause in the middle and take the food out to turn it over.

References

[1] https://iopscience.iop.org/article/10.1088/0031-9120/39/1/006

[2] https://www.sciencedirect.com/science/article/abs/pii/S0260877407001434

[3] https://ieeexplore.ieee.org/document/4181134

[4] https://pubs.aip.org/aapt/pte/article-abstract/28/7/474/270637/Standing-waves-in-a-microwave-oven

[5] https://www.researchgate.net/publication/282231090_Heating_Performance_Assessment_of_Domestic_Microwave_Ovens

This article is from the WeChat public account "Guokr" (ID: Guokr42). Author: Chuang Qiaoyu. Republished by 36Kr with permission.