Introduction:
Recently, a woman in Guangzhou bought a box of frozen dumplings near the company, the results of cooking dumplings to play a new height, the girls in the company microwave heating frozen dumplings, the results of the final dumplings are cooked, but also really with the same boiled. The following is based on the process of microwave heating dumplings to understand the working principle of microwave ovens and some of the precautions to prevent microwave leakage, and from learning some knowledge of the impact of microwave ovens on the human body.
Basic principles of microwave heating:
A microwave oven is a microwave heating device, which you can simply understand to work on the same principle as the sun heating your face through radiation. And fromPrinciples of EMC TestingMicrowave radiation can have an effect on the human body, especially people with pacemakers, so it is especially important to pay attention to the principles of microwave heating in microwave ovens!
Operating Frequency:
Before we get into the structure of a microwave oven, let's understand what frequencies the microwaves heated by a microwave oven operate at.
Radio frequency bandingWhen the microwave is designated as 300MHz (wavelength 0.9 meters) ~ 300GHz (wavelength 0.0009 meters) operating frequency, while microwave ovens generally use 2.24GHz (wavelength 12.2 centimeters) operating frequency, which determines whether or not it passes through the metal grid on the glass window! This you can understand more in the basic structure below.
Basic structure of a microwave oven:
Basic structure of a microwave oven
- Metal cavities:That is, the enclosure of the microwave oven is made entirely of metal and is a shield, which is the same as the one we make for theelectromagnetic shielding roomThe principle is the same.
- Metal mesh windows:We can generally see a glass panel on the front of the microwave oven, in fact, there is a metal mesh on the inside of the panel, which is also a small waveguide, you see the smaller the aperture represents that it only allows fixed-frequency microwave signals to pass through, we introduced above the microwave oven's wavelength of the operating frequency is generally 12.2cm, so as long as the aperture of the metal mesh is much smaller than the wavelength of the microwave, then the microwave will be locked in the metal mesh window.
- Control Panel:Setting basic functions such as working hours.
- High voltage transformers:There is a high voltage transformer on the right side of the microwave oven, which functions to convert 220V to several thousand V high voltage (depending on the power of your product).
- Magnetron:The core device of a microwave oven, a high-power vacuum tube, converts electrical energy into long-distance microwave radiation.
- Antenna/Microwave Stirrer:The microwaves from the magnetron will be emitted by the antenna, while the metal microwave stirrer constantly changes the direction of polarization of the microwaves, preventing continuous directional emission of the microwaves.
- Waveguide:Waveguide is a hollow metal tube that we use in theAnechoic chamberWaveguides are also commonly used to transmit non-metallic signal lines such as fiber optics. The microwave signal from the antenna will be transmitted through the waveguide to the
- Microwave stirrer:The microwave stirrer is made of a metal sheet that constantly changes the direction of polarization of the microwaves.
As can be seen from the structure of the microwave oven, as long as the metal cavity is intact and the metal mesh window is closed properly, the microwave signals inside the microwave oven will not be transmitted to the outside and will not affect the human body. However, we cannot rule out the possibility that the microwave oven does not detect normally whether the metal mesh window is closed normally, at which time the microwave signal will overflow and even heat the human body, or produce aEMSInterferes with the operation of other electrical equipment.
Heating principle:
Microwave heating is mainly through the wavelength of 12.2 centimeters of microwave radiation to the food, which is the natural resonance frequency of the water molecule, we all know that the chemical formula of the water molecule is H2O, H hydrogen molecules are positively charged, O oxygen atoms are negatively charged, so it is a polar molecule.
According to the principle of physics, when the microwave to the water molecule to maintain a fixed direction of radiation, the electric field under the action of the charge movement direction to follow the following law: positive charge along the direction of the electric field (high potential to low potential) to move, negative charge against the direction of the electric field (low potential to high potential) to move. So the water molecules will be parallel to the direction of the electric field lines.
Direction of positive and negative charge movement under the action of an electric field when the direction of the microwave electric field is fixed
And microwave ovens use a stirrer to constantly change the direction of polarization of the microwave signal, which means that an alternating electric field is created, constantly switching between the high and low potential directions shown above.
Water molecules oscillate alternately under the force of an alternating electric field
At this time, the positive and negative charges in the water molecules will drive the water molecules to swing from side to side, and the neighboring water molecules will rub violently against each other because of the swing.
According to the period equation T = 1/f:
T= 1/2.45GHz =4*10^(-10) s
This means that it takes only 4*10^(-10) (seconds/time) for a water molecule to oscillate once, which translates to a friction frequency (times/second) = 2.45*10^9, or 2.45 billion times!
With such intense friction, the food containing water molecules will gain heat quickly and eventually the food becomes cooked.
Notes on heating ingredients:
The lady used a mixture of microwaveable glass bowl + frozen dumplings + hot water to heat up the food directly in the microwave oven, which is in line with the basic requirements of microwave oven usage. From the working principle of microwave oven introduced before, microwave oven mainly heats food by heating water molecules, so using non-metallic containers such as glass or ceramic is the best choice.
Points of Attention:
- However, be cautious of using "paper and plastic" containers to heat food, although their composition does not contain water molecules that can be heated and will not be heated by microwave ovens, they will still be hot food temperature transfer to the vessel, especially plastic fast food lunch boxes may melt after heating and even produce carcinogens, and paper containers will become soft and flatten, losing their function as containers. flatten out and lose the role of the container.
- In addition, we can not use metal containers, because the microwave transmission to the metal medium will be converted to surface-induced current, once the metal containers on the spikes or irregular edges, metal objects in the microwave electric field under the action of the arc sparks or even fire!
Use a glass bowl and hot water to mix the dumplings to heat
Precautions for heating with a microwave oven:
From the above introduction of the microwave oven working principle, we know that after the microwave oven is started, the microwave signal begins to continuously irradiate the water molecules inside the food inside the microwave oven, so that the water molecules of positive and negative charges in the alternating electric field of the force of the oscillation and heat, the temperature of the food will be increased until cooked. And the microwave will penetrate your food and heat the food as a whole. At this point we just need to make sure that the metal grid window of the microwave oven is properly closed and shielded from microwave signals, and we'll have a steaming plate of food.
Frequently Asked Questions:
Do you heat the inside or the outside of the food in the microwave first?
According to the principle of microwave heating in a microwave oven, theoretically the food should be heated together as a whole. But the actual microwave signal will be absorbed by the food, when it reaches the inside of the food microwave capacity will be weaker, so most of the cases are the outside of the food part is too hot, of course, if you heat the food is very small, it can achieve the theoretical effect of the whole heated together, if the food is very large in size, or the food density is very large, then most of the microwave energy will be absorbed by the outside of the food to become warmer, and then through the heat conduction If the food is very large or the food is very dense, then most of the microwave energy will be absorbed by the outside of the food and then through heat conduction, the inside of the food will be heated.
If there are foods with high water content, most of the microwave energy will penetrate to the inside of the food, this is the inside of the food because the heat is blocked by the outside, the inside of the food will become hot preferentially, while the outside of the food will be later.
This is where the microwave differs from the oven, which must heat from the outside of the food first and then conduct it to the inside, whereas the way the microwave heats up depends on a number of factors such as the material, volume and density of the ingredients.
For example, this girl heated the frozen dumplings in the microwave oven at work. There is very little water inside the dumplings and it is unwise to heat the dumplings directly, so the dumplings will be heated more easily after she poured water in the bowl. First, the water is heated, and then the heat transfer to the inside of the dumplings heats up the dumplings.
Why do glass containers get hotter than food when heated?
The possible reason for this is that the material of the glass container contains debris within it, such as metal molecules or other metal oxides, and the glass bowl is exposed to microwave radiation from the microwave oven earlier than the food, so it will get hotter.
Is a microwave oven harmful if it leaks?
Before we introduced the working principle of the microwave oven explains the microwave oven using a metal shell shielding microwave signal leakage, but also use the metal mesh window from the front to isolate the microwave leakage, but when the microwave oven because of quality problems that is, the shielding effect is ineffective, the microwave radiation will be emitted to the outside world, but because of the following factors, we do not need to be gripped by the microwave oven's radiation.
The power is not high:
The power of microwave oven is only about 1000W, according to the principle of antenna emission, the microwave radiation generated at the end of the antenna will be logarithmically reduced with the increase in distance, which is also called the exponential reduction, so a certain distance away, even if the microwave oven after all the leakage of the human body to reach the remaining power will be very small, may be only with the cell phones to emit the same power. And each country for the microwave radiation emission limits, such as the U.S. Food and Drug Administration regulations in the 5 cm when the distance does not exceed 5 milliwatts per square centimeter, if you need to measure, we can also use theMicrowave Radiation TesterThe residual power of the microwave radiation is measured at different locationss.
Even if you're standing next to a microwave oven, you'd have to be exposed to high levels of radiation for a long time to have an effect on your body.
However, the risk of microwave use by some people carrying pacemakers cannot be ruled out because pacemakers are electronics, which are more sensitive to electromagnetic interference.
Is radiation from microwave ovens harmful?
Microwave ovens emit radiation, does it make the irradiated food radioactive? Is it harmful if it is directly received by the human body? I think this should be what the public wants to know most. We only need to make it clear that the microwave emitted by a microwave oven is a non-ionizing signal, which means that it does not have sufficient ability to change the atomic structure of the irradiated object or make the food radioactive! Its function is simply to make the water molecules in the food oscillate, not change into other elements. Of course, if your body is exposed to large doses of microwaves for an extended period of time, you may get hot, much like you would if you were sunbathing.
So microwave radiation can heat your body as much as it heats your food. Prolonged exposure to high-intensity microwaves can cause burns. The eyes in particular are more susceptible to microwave heating because there is relatively little blood flow in the eyes to carry away excess heat.
Does a microwave oven consume electricity?
The work of the microwave oven needs to be boosted, the magnetron produces microwave signals, but also be stirred through the waveguide to the food, the process will consume a lot of electricity; microwave oven shell is metal, it will also absorb the heat of the entire chamber; when we heat the food and then open the metal mesh window, you can feel that the chamber is very hot, which is also the energy loss. That is to say that the microwave oven in addition to providing energy to the food, but also to lose a lot of energy in the structure, these energy losses will lead to the use of microwave ovens when the power consumption increases.
Is microwave heating efficient?
A microwave oven heats up very efficiently, and if you don't take into account power loss, it will heat up more efficiently and faster than an oven!
But if you are just heating a cup of water, microwave oven may not be the best choice, according to the study found that the efficiency of microwave oven to heat water is only 40% that of electric water heater.And if you are heating rice, retaining some moisture in the rice will heat up faster. So the heating efficiency of a microwave oven depends on what kind of food you are heating.
Moreover, microwave ovens have a limited volume and cannot be used to heat large volumes of food, so the average family will also purchase an oven as a supplement.