Physics is a natural discipline with a long history. As an important branch of natural science, physical science not only plays an important role in promoting the progress of material civilization and deepening human understanding of the natural world, but also plays an important role in promoting human development. The development of thinking also had an indispensable influence. From the natural philosophy of Aristotle's era, to the classical mechanics of Newton's era, to the theory of relativity and quantum mechanics in modern physics, they are all tangible manifestations of the scientific quality, scientific spirit and scientific thinking of physicists. With the development of science and technology and the progress of society, physics has penetrated into all areas of human life. For example, just looking for knowledge about optics in cars has the following points:
1. The rearview mirror outside the car cab is a convex mirror
Using the divergent effect of the convex mirror on light and the characteristics of upright, reduced, and virtual images, the actual object seen is small and the observation range is larger. Ensure driving safety.
2. The reflector in the car headlight is a concave mirror
It is made by utilizing the property of a concave mirror to reflect the light emitted by the light source placed at its focus into parallel light.
3. Car headlights are always equipped with horizontal and vertical striped glass shades
Car headlights are composed of a light bulb, a reflector and a front glass cover. According to the knowledge of lenses and prisms, the car headlight glass cover is equivalent to a combination of lenses and prisms. When driving at night, drivers must not only see clearly the road conditions ahead, but also clearly see people on the roadside, road signs, forks, etc. Lenses and prisms refract light, so the lampshade disperses the light in the required direction according to actual needs through refraction, so that the light can evenly and softly illuminate the road ahead of the car and the scenery on the roadside. At the same time, this astigmatism lampshade can also Make part of the light slightly refracted upward to illuminate road signs and milestones, thereby ensuring driving safety.
4. After the car is equipped with tinted glass, it is difficult for pedestrians to see the faces of the people in the car.
The tinted glass can reflect part of the light and absorb part of the light, so the light penetrating into the car is weak. To see a passenger's face clearly, strong enough light must be reflected from the face and transmitted outside the glass. Because the light inside the car is weak and not enough light is transmitted through, it is difficult to see the faces of the passengers clearly.
5. Except for large passenger cars, the front windows of most cars are tilted
When the front window glass of the car is tilted, the images of the passengers in the car reflected by the glass are in the front and top of the country, while the images on the road are It is impossible for pedestrians to appear in the air above, so the images of passengers in the car are separated from the pedestrians on the road, so the driver will not have any illusions. Large buses are larger, and the front windows are much higher from the ground than cars. Even if the front windows are installed vertically, as if they are at the same height as the windows, pedestrians on the road cannot appear at this height, so the driver will not move the passengers The image outside the window is confused with the pedestrians on the road.
Another example:
People love to eat spiced tea eggs, especially when eaten hot, they taste better. Careful people will find that when the eggs are just taken out of the boiling marinade, if you are eager to peel the eggs and eat them, you will inevitably peel off the shells and "meat" together. To solve this problem, a trick is to soak the freshly cooked eggs in cold water for a while before peeling them. The eggshells will be easier to peel off.
General substances (with a few exceptions) have the characteristics of thermal expansion and contraction. However, different materials expand and contract at different speeds and amplitudes when heated or cooled. Generally speaking, substances with low density expand and contract more easily than substances with higher density, and the extent of expansion and contraction is also larger. Substances that conduct heat quickly expand and contract more easily than substances that conduct heat slowly. Eggs are composed of hard eggshells and soft egg whites and yolks, and their expansion and contraction conditions are different. When the temperature does not change much, or when the change is relatively slow and even, it will not show anything; once the temperature changes drastically, the expansion and contraction pace of the eggshell and albumen will be inconsistent. Immediately immerse a boiled egg in cold water. The temperature of the eggshell drops and it shrinks quickly. However, the albumen is still at its original temperature and has not yet shrunk. At this time, a small part of the albumen is squeezed by the eggshell into the empty end of the egg. Then the egg white gradually shrinks due to the lowering of temperature, and the egg shell shrinks very slowly at this time, so that the egg white and egg shell are separated. Therefore, when peeled, the shell and "meat" will not come off together.
Understanding this truth is very useful to us.
For things that need to withstand large temperature changes, if they are made of two different materials, then when selecting the materials, you must consider their thermal expansion properties. The closer the two are, the better. Engineers widely use reinforced concrete when designing houses and bridges. This is because the expansion degrees of steel and concrete are almost exactly the same. Although the temperatures in spring, summer, autumn and winter are different, no harmful forces will be generated, so reinforced concrete buildings are very strong. .
In addition, some electrical components are made of two metals with very different thermal expansion properties. For example, the thermal expansion of a copper sheet is greater than that of an iron sheet. The bimetallic sheet that nails the copper sheet and the iron sheet together will bend due to different degrees of expansion when heated under the same conditions. Many automatic control devices and instruments have been made using this property. There is a small bimetallic piece in the "starter" of the fluorescent lamp. It can automatically flex and extend as the temperature changes, thus automatically turning on the fluorescent lamp.
There are countless such examples. Physics is a very practical science and is extremely closely related to industrial and agricultural production and daily life. The laws of physics themselves are the summary and abstraction of natural phenomena.
When it comes to physics, some students find it difficult; when it comes to physical inquiry, some students find it unfathomable; when it comes to physicists, some students feel that they are not mortals. It is true that there are only a handful of people who become physicists, but as long as you are diligent in observation, good at thinking, courageous in practice, dare to innovate, and move from life to physics, you will find that: in fact, physics is around you. As Marx said: "Science is the science of experiment, and science consists in using rational methods to organize perceptual materials." Physics is not only one of our subjects, but more importantly, it is also a science.
Physics exists among physicists. Galileo, an Italian physicist who was diligent in observing, was worshiping in the Cathedral of Pisa. The swing of the copper chandelier suspended in the church aroused his great interest. Later, he repeatedly observed and studied it and invented the isochronism of the pendulum. Franklin, an American physicist who has the courage to practice, in order to understand the nature of "the anger of God", risked his life on a day of lightning, thunder, wind and rain, and used a common kite to bring the "fire of God" down to earth. The lightning rod was invented; Henry Atchar, an innovative British scientist, went to the post office to do business. At that time, a foreigner next to me took out a large edition of new stamps and was going to cut one out and paste it on an envelope, but he didn't have a knife. I asked Achar to borrow it, but Achar didn’t have it either. The foreigner had an idea, took off the pin from his suit tie, stabbed a circle of small holes neatly around the stamp, and then tore off the stamp neatly. The outsider left, but left Achar with a series of deep thoughts, which led to the invention of the stamp punching machine, and the toothed stamps were also born; in ancient Greece, Archimedes discovered that Achar Meade's principle; German physicist R?ntgen's discovery of X-rays;... There are countless examples of physicists who have studied trivial matters around them and made great achievements.
Physics also exists around the students. After learning the preliminary knowledge of measurement, the students started making soft rulers one after another. One classmate was creative and wrapped the prepared kraft paper ruler with transparent glue to make it stronger. Then, a large bubble gum packaging box was used as the outer shell of the tape measure. In the center of the box, an iron wire was used to make a central axis of the crank handle. The end of the tape measure was fixed on the axis. In this way, a tape measure that could be packed away and used repeatedly was born. At the same time, this classmate was inspired by the soft ruler and solved an exercise through experiments: When using a soft ruler to measure the length of an object, if the soft ruler is stretched longer, will the measured value be too large or too small? He did such a simulation experiment: draw a straight line on white paper, mark the scale, then paste it with transparent glue, and then pull it off to make a "soft ruler". Using the "soft ruler" not only found the above question The answer is, and they can clearly see that the graduation value has become larger. They know this and why; after learning the relevant knowledge of electricity, the students explored the maximum voltage that the earthworm can withstand: when adding voltage to it When a voltage of 1.5V is applied, the earthworm quickly secretes mucus, struggles hard, and jumps out of the bottle.
When a voltage of 3V is applied to it, the earthworm is electrocuted into two pieces; when some students were measuring the power of a small light bulb with "2.4V, 0.5A" and studying its luminescence, they were not satisfied with adding 2.4V to the light bulb. Instead, he used the small light bulb he had prepared for a destructive experiment and continued to increase the voltage at both ends of the light bulb. He did not stop exploring until the voltage reached 9V and the light bulb filament burned out. Some students took the trouble to learn the knowledge of evaporation. Ji Zuo observed the same two drops of water at the table (one drop of water spread out), observed them attentively, and then analyzed and compared them to find out the factors that affect evaporation;... It is not uncommon for students to capture trivial matters around them for investigation.
The things around us are inexhaustible. For physics subjects that are closely related to real life, they are often used. Students use examples around them to explain and summarize physical laws. If it sounds familiar, it will be easier to accept. As long as we always pay attention and summarize frequently, we will continue to discover things that are beneficial to physics teaching, enrich our classrooms, activate the teaching atmosphere, and simplify concepts and rules. The new curriculum standards tell us that “physics courses at the compulsory education stage should be close to students’ lives, conform to students’ cognitive characteristics, stimulate and maintain students’ interest in learning, reveal hidden physical laws by exploring physical phenomena, and apply them to production and life. Practical, cultivate students' lifelong interest in exploration, good thinking habits and preliminary scientific practical abilities."
Today, all mankind's amazing scientific and technological achievements, such as sheep cloning, the Internet, nuclear power plants, etc. Aviation technology, etc., are all based on early scientists' observation and research of trivial matters around them. During study, students should establish scientific awareness, think big and start small, go through activities such as observation, thinking, practice, innovation, etc., gradually master scientific learning methods, and train scientific thinking. Soon you will have the skills of a scientist. Mind, lay a solid foundation for your amazing future development and a better life in the future.
Main references:
Ministry of Education: New Curriculum Standards for Junior High School Physics (Experimental Draft)
Xing Hongjun: "On the Development of Science and Technology and the Reform of Middle School Physics Curriculum ""Middle School Physics Teaching Examination" Issue 4, 1998
Li Rongming and Zhang Yunsheng: "Using Prototype Inspiration to Cultivate Students' Innovative Thinking" "Discussion on Physics Teaching" Issue 8, 2000
[ U.S.] L. Epstein[U.S.] P. Harwitt's "Answers to Interesting Physics"
Article source: China Foundation Network
Interesting physical phenomena around you
1. When the quartz clock hung on the wall stops moving because the battery is exhausted, its second hand often stops at the "9" position on the dial. This is because the second hand is most hindered by the gravity moment at the "9" position.
2. Sometimes when the water pipe is discharging water from a nearby faucet, bursts of sound occasionally occur. This is because the water pipes vibrate when water rushes out of the faucet.
3. When taking pictures on the TV screen, you should turn off the camera flash and indoor lighting, so that the picture will be clearer. . Because the reflected light from the flash and lighting on the TV screen will interfere with the transmitted light of the TV screen.
4. Frozen pork thaws faster in water than in air of the same temperature. A hot iron nail cools faster when placed in water than in air of the same temperature. A cup filled with scalding boiling water cools faster when immersed in water than when immersed in air of the same temperature. These phenomena all show that water has better heat transfer properties than air.
5. When there is cold water in the pot, the water droplets attached to the outer surface of the bottom of the pot will take a long time to be dried on the flame, and will not boil until it is dried. This is because the water droplets, the pot and the pot The three water in the pot maintain heat conduction and have roughly the same temperature. As long as the water in the pot does not boil, the water droplets will not boil either. The water droplets will gradually be burned dry by evaporation on the flame.
6. The mirror is out of shape. The further away the person is from the mirror, the more out of shape it becomes. Because the image in the mirror is formed by the reflection of the silver-plated surface behind the mirror, uneven silver-plated surface or uneven glass thickness will cause aliasing.
For a mirror that is out of shape, the further away a person is from the mirror, the greater the deviation of the reflected light from the silver-plated surface from the normal position due to the principle of light amplification, and the more out of shape the mirror will be.
7. There are several small holes on the side of the gas nozzle of the natural gas furnace that are connected to the outside world, but the natural gas will not eject from the small holes on the side, but only from the nozzle. This is due to the flow of natural gas at the nozzle. The speed is high. According to the principle of fluid mechanics, the flow speed is high and the pressure is low. The surface pressure of the air flow is less than the atmospheric pressure outside the side holes, so the natural gas will not be ejected through the small holes on the side of the nozzle.
8. After blowing up the balloon, hold the mouthpiece with your hands, and then suddenly let go. The air in the balloon will spurt out, and the balloon will move due to recoil. You can see that the route of the balloon movement is winding and varied. There are two reasons for this: First, the thickness and tension of the inflated balloon are uneven everywhere, which causes the balloon to shrink unevenly and swing when it is deflated, so that the direction of movement constantly changes; second, the shape of the balloon keeps changing during the contraction process. Changes, so the airflow speed on the surface of the balloon is also constantly changing during the movement. According to the principle of fluid mechanics, the flow speed is large and the pressure is small, so the pressure of the air on the surface of the balloon is also constantly changing, and the balloon swings accordingly, thereby changing the direction of movement. Just keep changing.
9. When the ceiling fan rotates normally, the pulling force on the suspension point is smaller than when it is not rotating. The greater the rotation speed, the more the pulling force is reduced. This is because when the ceiling fan rotates, the air has an upward reaction force on the ceiling fan blades. The greater the speed, the greater the reaction force.
10. The "combustion" of electric furnaces is the conversion of electrical energy into internal energy, which does not require oxygen. Oxygen can only oxidize the electric furnace wires and shorten their service life.
11. Even if there is no wind, the path of a thin piece of paper falling from a height will be twists and turns. This is because each part of the paper has different convex and concave shapes and different shapes. Therefore, during the falling process, the air flow speed on the surface is different. According to the principle of fluid mechanics, the flow speed is large and the pressure is small, resulting in the air force acting on various parts of the paper. It is uniform and changes with the movement of the paper, so the paper keeps rolling and twisting and falling.
Why is a bottle filled with water not easy to break?
There are two identical glass bottles, one empty and one filled with water. They fall to the ground from the same height at the same time. Which bottle is more likely to break?
It is generally said that heavy bottles are easy to break. However, when the bottle is filled with water, the water in the bottle has another effect, which can reduce the deformation of the bottle, making it less likely to break.
Glass bottles break, mostly due to deformation. The empty bottle falls to the ground, and the ground exerts pressure on the bottle. The bottle deforms from the outside inward, and finally breaks. The bottle is filled with water. Since water is incompressible, it reduces deformation and makes the bottle less likely to break. Fill the bottle with water and tighten the cap, making it less likely to break.
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Interesting oscillation phenomenon
During the Tang Dynasty, a strange thing happened in a temple in Luoyang. There is a copper chime in the temple room. No one knocks it, but it often buzzes by itself. What is the reason?
It turns out that the frequency of this chime and a big bell in the dining hall are exactly the same as the number of vibrations per second when they sound. Whenever the young monk rings the big bell, the vibration of the big bell makes the surrounding air vibrate. When the sound wave reaches the chime in the old monk's room, because the frequency of the chime is the same as the frequency of the sound wave, the chime also vibrates. stand up. There was a "buzzing" sound. This is the oscillation phenomenon of vibration, also called oscillation.
Have you ever noticed that the lower end of the Huqin has a large "belly" - a bamboo tube covered with snake skin. When you play the huqin with great interest, the vibration of the strings will cause the air in the "belly" to rumble through the snake skin, making the sound not only loud, but also rich, melodious and melodious. People call this "belly" a ringing box. You see, dulcimer, pipa, violin, piano and other musical instruments, don’t they all have sound boxes of various shapes and sizes?
In addition to the buzzing box, people have done a lot of good things by using the phenomenon of buzzing.
When construction workers build a house, whether they are pouring concrete walls or floors, in order to improve the quality, they always use an oscillator to vibrate the concrete while pouring it, so that the concrete will be tighter and stronger due to the vibration.
Pedestrians on the street, the noise of vehicles, the rumble of machines - these continuous noises not only affect people's normal life, but also damage people's hearing. There is a kind of anti-vibration muffler, which is composed of an orifice plate and a cavity with many small holes. When the frequency of the transmitted noise is the same as the natural frequency of the oscillator, it will vibrate violently with the air column in the small hole. In this way, sound can be converted into heat energy when it vibrates, so that a considerable part of the noise is "swallowed" away.
In addition, pulverizers, vibration meters, electric vibration pumps, etc. also use the vibration phenomenon to work.
But in some cases, the vibration phenomenon may also cause harm. For example: when an army crosses a bridge, their uniform steps can cause vibrations. If its frequency is close to the natural frequency of the bridge, it may cause the bridge to vibrate to the point of fracture. Therefore, troops must use walking steps to cross the bridge.
In northwest my country, the mountains are covered with snow all year round. Whenever spring comes, the ice and snow on the mountains melt, and the snow layer will slide away from its original place. Often an accidental roar will cause the thick snow layer to collapse due to vibration. Therefore, it is stipulated that the survey team members and mountaineers climbing the snow-capped mountains cannot speak loudly.
We must fully apply electromagnetic vibration to various scientific fields, and we must also prevent electromagnetic vibration from causing harm to life, work, and the environment. This requires us to continue to study and explore. TOP->>
The Mystery of Beer
Beer is a new type of beverage that can not only cool and quench thirst, benefit the body, but also invigorate people's spirits. But when you drink beer, have you ever paid attention to the following issues?
1. Why does a cold air come out when you open a beer bottle that has been at room temperature for a long time?
It turns out that there is a part of high-pressure CO2 gas sealed in the beer bottle, and its temperature is the same as room temperature. When the bottle is opened, the gas inside the bottle expands rapidly, and there is no time to exchange heat with the outside world. According to the first law of thermodynamics, it can be known , the gas in the bottle overcomes the external air pressure to do work, thereby reducing the internal energy, causing the pressure of the gas to decrease and the temperature to drop. Therefore, I felt a cold air coming out of the bottle.
If the beer bottle is shaken before opening. The temperature inside the bottle increases and the air pressure increases. When the bottle is opened, a large amount of CO2 will emerge rapidly and the beer will be sprayed and overflowed. It is common to see scenes of celebrating victory like this on TV: A bicycle race champion, wearing a yellow jersey, holds a large beer bottle and shakes it vigorously. The beer spurts out from the mouth of the bottle like a fountain, and hop foam splashes in the air. , also for this reason. Sometimes, when beer is being transported, due to uneven roads, the beer rolls in the bottle, the temperature rises, the air pressure increases, and sometimes the beer bottle can even explode.
2． Why do so many bubbles rise when beer is poured into a glass, causing a large amount of foam to form on the liquid surface?
It turns out that there is carbon dioxide dissolved in the beer, and there are air pockets containing air in places where there are defects or dust on the bottom of the cup or on the wall surface, providing a liquid surface for the precipitation of carbon dioxide. CO2 in the beer enters these air pockets to form bubbles. Since CO2 is lighter than beer, the bubbles leave the cup wall under the action of buoyancy. As CO2 continues to enter the bubbles, the volume of the bubbles gradually becomes larger, and the buoyancy becomes greater. The bubbles rise, and finally the bubbles Floats to the liquid surface and forms foam. Due to the interaction between gravity and the internal surface tension of the bubble, the thickness of the bubble wall becomes thinner and bursts as the foam rises.
3. When slowly pouring beer into a cup, you can often see that the beer is not poured directly into the cup vertically from the mouth of the bottle under the action of gravity, but instead flows around the mouth of the bottle and flows down the outer wall of the bottle for a while before being poured into the cup. , why does this happen?
It turns out that for liquids, the smaller the radius of the flowing trajectory, the greater the flow speed. The mouth of the beer bottle is small. When the beer flows through, the speed increases. According to Bernoulli's principle, the pressure at the bottle mouth is the smallest. When its value is less than the outside air pressure, the atmospheric pressure causes the beer to flow down the outer wall of the bottle and along the bottle wall. The distance traveled is related to the surface tension of the liquid and the viscosity of the fluid. When there is a small disturbance from the outside world, the beer will break away from the bottle wall and be poured into the cup. This phenomenon will be more obvious when pouring oil, so the outside of the oil bottle in the kitchen is often greasy.
"Clay doll peeing" and the gas law
"Clay doll" is a brown ceramic toy made of clay that looks like a child, because it is similar to the color of earth. , so it is called clay doll. In addition, if operated correctly, it will spray water from below the navel, just like a child peeing, so it is also known as the "peeing person".
Watching him "pee" is quite interesting and is loved by children. So why does the "mud baby" "pee"? The reason is that the three laws of gas properties are cleverly used. The analysis is as follows:
1. Isobaric water filling (Guy-Lussac's law) If you want to make the "mud baby" pee, you must let it drink water (water filling) first. How to make it drink water is the first problem to be solved. The picture is the structural principle diagram of "Clay Baby". As can be seen from the picture, it is a hollow cavity with a pinhole-sized opening under the umbilicus connected to the cavity through a channel on the front cavity wall. Since the diameter of the hole is less than half a millimeter, it is not possible to directly fill it with water. You must try to make the air in the cavity discharge outward, and then let the gas in the cavity shrink, and then rely on the outside atmospheric pressure to press the water into the cavity to achieve water filling. The specific method is: immerse the "mud baby" in hot water. The water temperature should be higher than the ambient temperature T0 at that time. When the "mud baby" is submerged in hot water, you can see bubbles coming out from the opening under its navel (hereinafter referred to as the water spout). When the temperature of the cavity and the hot water are the same (reaching thermal equilibrium), the exhaust stops. , prepare for watering. When the exhaust stops, use your fingers to block the water spout, quickly take the "mud baby" out of the hot water, and put it into cold water. Remove the finger blocking the water spout, and the cold water will naturally flow from the water spout. Pour into the cavity. When the gas temperature in the cavity is the same as the cold water temperature T0 (heat balance is reached again), the water filling process ends. This process is a gas isobaric cooling process. Complies with Guy-Lussac's law. Because at the moment when water filling starts, water has blocked the water nozzle, the quality of the gas in the cavity is fixed. At this time, the volume of the gas in the cavity is V1, the temperature is T, and the pressure is p0 (atmospheric pressure). After time t, the gas temperature is the same as the cold water temperature, and the volume is V2. According to Gay-Lussac's law, V2=T0V1/T can be obtained. Since T0 2. Isovolumetric pressure increase and energy storage process (Charlie's law). After the water filling is completed, block the water spout with your fingers and press the clay doll into the hot water again to heat it. At first, because the water spout was blocked by fingers, water could not flow out, so the volume of gas in the cavity was constant, and its pressure and temperature were still the same as the external pressure p0 and temperature T0. As the heating proceeds, when the gas and hot water in the cavity reach thermal equilibrium again, the temperature becomes T and the pressure is p. According to Charley's law, p=Tp0/T0. Because T>T0, then p>p0. It can be seen that after the temperature of the gas in the cavity increases, the pressure increases. 3. Isothermal expansion process, water spray (Boyle's law). After the isovolumetric pressure increase process is completed, the "mud baby" can be taken out of the hot water, put it in an upright position, and remove the finger pressing the water spout. , you can immediately see water spraying from the water spout. As the amount of water in the cavity decreases, the gas volume expands, the pressure naturally decreases, and the water spray range gradually decreases. When the pressure in the cavity becomes equal to the external pressure p0, the water spray ends. Since the water spraying process is extremely short, only a few seconds, and the temperature of the cavity has not yet dropped, the gas temperature is basically constant. It can be approximately considered to be an isothermal expansion process, following Boyle's law. From this, V2=pV1/p0 can be obtained. Since p>p0, then V2>V1. It is natural for the gas to expand and push the water out. Looking at the whole process of "Clay Baby Peeing", every part of the process makes excellent use of the relevant properties of gases, and it can be regarded as a model of the application of the three laws of gases. We can also use the p-V diagram to vividly represent each process, as shown in the figure. In the figure, state I to state II are isobaric contraction-water injection process; state II to state III are isovolumetric pressure increase-energy storage process; state III to state I are isothermal expansion-water injection process. There are also toys such as "Buddha rains" and "Turtle splashes in water", which are similar in principle to "Clay baby peeing", but the shape of the cavity is different. Application of Physics in the Kitchen In the kitchen, if we pay attention to the stoves, utensils and some phenomena that occur during cooking and stir-frying, we will definitely find many Physics knowledge is used everywhere. 1. When heating cold porridge or cold rice, the pot makes a "plop, plop" sound and bubbles keep coming out. However, when you taste it, the porridge or rice is not hot. This is why. ? Reheating cold porridge or rice is different from boiling water. Although water is a bad body for heat and conducts heat very slowly, water has very good fluidity. When the water at the bottom of the pot is heated, it expands, floats when its density decreases, and the surrounding cold water flows to fill it. Through this convection, the heat at the bottom of the pot is continuously transferred to all parts of the water, causing the water to change. hot. However, cold porridge or rice has poor fluidity and is not easy to conduct heat. Therefore, when the porridge or rice at the bottom of the pot absorbs heat, the temperature rises quickly, but it cannot flow upward or around quickly. A large amount of heat is concentrated at the bottom of the pot and burns the porridge at the bottom of the pot. Because heat is difficult to transfer to the top of the porridge, the porridge on top is still cold. When heating cold porridge or rice, add more water to the pot to thin the porridge and enhance its fluidity. In addition, stir frequently to force convection so that the porridge can be heated evenly. 2. When cooking meat or making soup in a casserole, when the soup is taken off the stove after boiling, the soup will continue to boil for a while, but this phenomenon does not occur in iron and aluminum pots. This is Why? Because the casserole is made of clay, the specific heat of non-metal is much greater than that of metal, and the heat transfer capacity is much worse than that of metal. When the casserole is heated on the stove, the temperature of the outer layer of the pot greatly exceeds 100°C, and the temperature of the inner layer is slightly higher than 100°C. At this time, the pot absorbs a lot of heat and stores a lot of thermal energy. After taking the casserole off the stove, the outer layer of the pot, which is well above 100°C, continues to transfer heat to the inner layer, so that the soup in the pot still reaches 100°C and can continue to boil for a period of time. Iron and aluminum pots will not This phenomenon occurs (students are asked to analyze the reasons themselves). 3. "Being familiar with each other" in stir-fried meat. During festivals, people always stir-fry some meat and vegetables, so how to stir-fry meat slices? If you put the meat slices directly into a hot oil pan for stir-frying, the water contained in the lean meat fiber will evaporate rapidly, causing the meat slices to become dry and hard, and even the meat will be charred and fried. Paste, the flavor is greatly lost. In order to stir-fry the meat slices deliciously, chefs often mix the meat slices with an appropriate amount of starch in advance. After the meat slices are placed in the hot oil pan, the water in the starch paste attached to the outside of the meat slices evaporates, but the water in the meat slices is difficult to evaporate. , the original tenderness of the meat is still maintained, and the loss of nutrients is reduced. The meat is cooked quickly and is "cooked". The meat slices fried in this way are both tender, delicious and nutritious. 4. What is the best way to thaw frozen meat? How to defrost frozen meat and chicken when you take them out of the refrigerator? It is best to use cold water close to 0℃. Because the temperature of frozen meat is below 0℃, if it is thawed in hot water, the frozen meat will absorb heat from the hot water, and its outer layer will thaw quickly and the temperature will quickly rise above 0℃. With gaps, the ability to transfer heat is reduced, making it difficult for the frozen meat inside to absorb heat and thaw to form a hard core. If frozen meat is placed in cold water, the temperature of the cold water will quickly drop to 0°C due to the heat absorbed by the frozen meat and frozen chicken, and some of the water will freeze. Because 1 gram of water can release 80 calories when it freezes (and 1 gram of water only releases 1 calorie when it is lowered by 1°C). After so much heat released is absorbed by the frozen meat, the temperature of the outer layer of the meat rises quickly. , and the inner layer easily absorbs heat, so the temperature of the whole piece of meat rises to 0°C relatively quickly. Repeat this several times until the frozen meat is defrosted. From a nutritional perspective, this method of uniform and slow heating is also scientific. Physical knowledge in life - electricity and life Life is inseparable from physical knowledge, and physical knowledge is closely related to life. Electricity is part of physical knowledge. Every day life is inseparable from electricity. Without electricity, we cannot live and study normally. So we have to save every kilowatt hour of electricity. My home has refrigerators, washing machines, televisions, electric fans, electric fans, rice cookers and other electrical equipment. These appliances have various uses. They are refrigerators and are tools used to store food and keep food fresh. Washing machines are used to wash clothes, which can reduce labor intensity and improve work efficiency. Television is used to tell people about the important events happening in the world every day. In winter, hair dryers are used to dry hair and beautify life. Electric fans can be used to escape the heat and cool down at night. Electric rice cookers can be used to cook rice, reducing people's burden. As can be seen from the above, our lives are closely related to electricity. If we do not have electricity in our lives, it will be very difficult. It will also be very difficult to do homework. Nowadays, most people have electricity in their homes. We all have electrical appliances. It can be seen that society is progressing, living standards are improving, and the scope of electricity consumption is becoming more and more extensive. In the future, when I am in the second or third year of junior high school, I will work hard to learn about electricity, study physics knowledge seriously for the purpose of building socialist modernization, and lay a solid foundation for my future career.