This article is from WeChat official account:Bring science home (ID: steamforkids), author: Mirror, the original title: “The fruits do not easily heated in a microwave, or else ……” head Figure from: vision China
Every day at noon, the migrant workers with meals begin to gather in front of the microwave oven to heat up their meals.
During the waiting time, the editor thought of a series of questions about microwave ovens:
How did the microwave oven be invented?
Why does the food inside the microwave heat up first when heated?
Why can’t microwave oven heat metal?
Why do two grapes sparkle in the microwave?
Why hasn’t it been my turn yet?
So there is this article.
From accidental discovery
One day in the early part of the last century, the American engineer Percy Spencer (Percy Spencer) was studying his military radar device and found it in his pocket The chocolate melted somehow. This aroused Spencer’s interest. After excluding other factors, he thought that this might be a “trick or treat” by radar, so he and a few colleagues started to get military equipment and started cooking experiments.
Caricatures of microwave heating food in the last century | Source: Wikipedia
The food that first devoted himself to the microwave heating experiment was corn kernels, which became the world’s first microwave popcorn.
Probably thinking that this effect is not cool enough, Spencer and others put the eggs in a kettle again, and then stuffed the magnetron that emits electromagnetic waves in it. As a result, the egg gave them a “surprise”, transformed into a “bomb” and splashed the faces of the participants who were staring at it.
Eggs: I can’t be the only one dedicated to science | Source: programmer sought
Later, Spencer specially designed a metal box,Install the magnetron inside to make the electromagnetic field more concentrated and controllable without leaking out. This is the earliest prototype of a microwave oven.
The heat effect near the radar was not the first thing Spencer noticed, but he was the first to explore it and discover its application value.
Early microwave oven | Source: Wikipedia
However, like computers, the earliest commercial microwave ovens were very bulky, about 1.8 meters high and 340 kilograms in weight, and the price was as high as several thousand dollars in that era. It was not until 1967 that the size and price of microwave ovens became relatively close to the people, and they began to enter the kitchens of thousands of households.
Heating from the inside out?
The difference from the oven is that the food heated in a microwave is often cold outside, but hot inside. Even if the microwave can penetrate the food, it should be heated inside and outside together. Why is the inside priority?
To understand this, we must first understand the nature of the microwave itself.
Although it is called “microwave”, its wavelength in the electromagnetic spectrum is longer than visible light and infrared light(1mm~1m)< /span>, shorter than radio waves. The electromagnetic wave used in microwave ovens has a wavelength of about 12 cm and a frequency of 2.45 GHz.
Electromagnetic spectrum | Source: Wikipedia
Whether it is a stove or an oven, it essentially uses electromagnetic waves (infrared rays) to heat food, and the food surface closer to the heat source absorbs first The heat is then transferred inward. The microwaves emitted by the microwave oven can directly act on certain molecules in the food.
The basic structure of modern microwave ovens | Source: BusinessInsider
Of course, food will be affected by microwave inside and outside, but not all molecules will respond positively, mainly relying on the cooperation of “active elements” such as water molecules that can be polarized. The water molecule as a whole is not charged, but its positive and negative charge centers do not overlap, which is a polar molecule.
In the periodically changing electromagnetic field of the microwave oven, water molecules begin to “dance” like a small magnet, changing the direction of rotation in accordance with the pace of the electromagnetic field, and the vibration frequency approaches the microwave frequency, reaching 2.45 billion times per second. Other surrounding molecules also start to vibrate under the drive of water molecules. The more violent the movement of molecules in a substance, the higher the temperature it exhibits.
Schematic diagram of periodic vibration of water molecules with electromagnetic field changes| Source: engineerguy
The water molecules are so active that the parts of food with high water content naturally heat up faster. Many foods are dry on the outside and wet on the inside, or when the surface layer of water loses too much, it will heat up inside and then conduct outward. If the ingredients are uniform, the inside and outside will be almost hot.
Because microwave heating relies on molecular vibration, in frozen food, the vibration of water molecules is limited, and the heating efficiency is not so high.
So, why choose microwave at a specific frequency of 2.45GHz?
Actually, home WiFi is also in this frequency band. It is a frequency band regulated by the International Telecommunication Union that is open to industrial, scientific, and medical fields, avoiding the frequency bands used in communications and other fields. In addition, the use of this frequency in microwave ovens also takes into account heating efficiency and cost.
Why can’t the microwave heat metal?
Open the microwave oven manual, there must be a warning that metal cannot be heated. However, it is puzzling that there is a metal structure in the microwave oven, the most obvious is the metal mesh on the microwave door.
The outer shell of the microwave oven is essentially a metal cage, equivalent to a Faraday cage, which can trap electromagnetic waves inside.
Faraday cage can shield electromagnetic field | Source: Wikipedia
The microwaves are reflected everywhere in the furnace, but there is no escape. Such microwave energyThen they can be gathered and absorbed by the food.
Although a microwave oven is not a perfect Faraday cage, the microwave intensity leaked by a qualified microwave oven is far less than the radiation intensity of the sun, so you don’t have to worry about being “cooked” by the microwave.
However, under certain circumstances, metal can turn your microwave oven into a disaster site.
There are many freely moving electrons inside the metal, and they will redistribute in the changing electromagnetic field. When the potential difference accumulates to a certain level, lightning may occur like cumulonimbus clouds in summer.
The aluminum foil paper (top) and fork (bottom) in the microwave produce electric sparks, please do not imitate! | Source: programmersought
Thin and wrinkled aluminum foil and sharp, bifurcated forks are high-risk items. Their horns and corners are easy to accumulate electric charges, form potential differences, generate arcs, and even damage metal materials and microwave ovens.
Two grapes “bring out” intense sparks
It’s not an accident that metal sparks in the microwave. It’s incredible that twoGrapes will also “wipe” sparks in the microwave, even more intense than metal (dangerous, do not imitate). Just use one grape or replace it with other berries.
Two grapes in the microwave set sparks | Source: Khattak, H. K., Bianucci, P. and Slepkov, A. D. (2019)
This phenomenon has been hotly debated a few years ago, but no one has given a reasonable explanation. It was not until last year that a scientific research team solved the mystery.
What makes grapes sparkle is a phenomenon called Mie resonance(Mie resonance). Mie resonance, also known as structural resonance(morphology-dependentresonance), refers to when the size of an object with a certain shape and material is similar to the electromagnetic wave wavelength , The resulting interaction.
We mentioned earlier that the microwave wavelength of a microwave oven is about 12 cm, which refers to its wavelength in the air.
In different media, the wavelength and refractive index of the microwave are different. The wavelength in the grape pulp (mainly water) is only the original wavelength 1/10 of that, or about 1.2 cm, this length is exactly close to the diameter of a grape, which meets the conditions of Mie resonance. Microwaves are trapped inside the grape and reflect continuously, forming an oscillating electromagnetic field.
The heat distribution of two grapes in a microwave oven| Source: Khattak, HK, Bianucci, P. and Slepkov, AD (2019)
At this time, if the distance between the two grapes is less than one wavelength, the electromagnetic field inside them will interact, the electromagnetic field strength at the contact point will increase greatly, and finally the ions in the grapes will be reduced.(mainly sodium ions and potassium ions) ionize, excite plasma, the air will be broken down, forming electric sparks.
In the experiment, the researchers replaced the two grapes with hydrogel beads of similar size and achieved the same effect. They also thought of applying this principle to finer lithography techniques.
Just behind the successful experiment, I am afraid that many microwave ovens have sacrificed heroically…
Don’t try! Do not try! Do not try!
Even if you don’t care about sacrificing the microwave oven, violent electric sparks and explosions are likely to hurt people or cause fires. So whether it’s metal or grapes, if you don’t have the professional protective measures of experimenters, don’t try it rashly.
In addition, ordinary plastic packaging and ordinary cling film should not be heated in a microwave, because these plastics will be heated and contaminate food.
Among plastics, only high-temperature resistant materials (such as PP plastic) that can be put into microwave ovens can be heated by microwave ovens.
There are also sealing systems such as eggs and closed-packaged foods that are not suitable for microwave ovens. If you don’t believe it, please look back at the complaining eyes of colleagues Spencer.
As long as you don’t die, microwave ovens are actually quite safe and efficient kitchenwareAnd experimental props.
Source: “The Gate of Destiny Stones”
Reference: https://shimo.im/docs/3gTxRPryJKgVwG33
This article is from WeChat official account:Bring science home (ID: steamforkids), author: Mirror