When the air in the sky lamp is heated, its volume will expand, but there is only a certain space (the volume of the sky lamp). So the extra air molecules will run out of the sky lamp (as shown below), and the quality will be reduced. Density = mass/volume, if the volume remains the same, the mass will become smaller, then the air density in the sky lamp will become smaller and the sky lamp will float! 4. Cupping find a water glass or glass jar bottle and an old cotton cloth. Wet the cotton cloth, fold it into several layers, lay it flat on the table, then put a ball of cotton in the bottle and burn it. Before the fire goes out, buckle the bottle on the wet cloth, and the bottle will absorb the wet cloth.
This is because some air in the bottle ran away after being heated and expanded. After the bottle is buckled on the wet cloth, the air inside the bottle quickly cools down, and the pressure of the air inside the bottle is lower than that outside. Under the pressure difference between inside and outside, the wet cloth seems to be held down by an invisible hand and cannot fall off. Cupping is the use of this truth. Anyone who has pulled out the cupping will feel that the cupping mouth has upward pulling force, in order to promote the metabolism of the body and achieve certain therapeutic purposes. Cupping has a long history in China, and it was used in the 4th century. This shows that more than 1500 years ago, our ancestors knew the phenomenon of gas expanding with heat and contracting with cold, and used it.
Peeled eggs and spiced tea eggs are very popular, especially when they are very hot. Careful people will find that when the eggs are just taken out of the boiling gravy, if they are eager to shell and eat them, it is inevitable to peel them off with the "meat". To solve this problem, there is a trick, that is, soak the eggs just out of the pot in cold water for a while, and then peel them, so that the eggshells can be peeled easily. General substances (with a few exceptions) have the characteristics of thermal expansion and cold contraction. However, when different substances are heated or cooled, the speed and amplitude of expansion and contraction are different. Generally speaking, substances with low density are easier to expand and contract than substances with high density, and substances with fast heat transfer are easier to expand and contract than substances with slow heat transfer. An egg consists of a hard shell, a soft protein and a yolk, and their expansion and contraction are different. When the temperature changes slightly or slowly and evenly, nothing can be displayed; Once the temperature changes dramatically, the expansion and contraction speeds of eggshells and protein are inconsistent. When the boiled egg is immediately immersed in cold water, the temperature of the eggshell will decrease and shrink rapidly, while protein is still at its original temperature and does not shrink. At this time, a small part of protein will be squeezed into the short position of the egg by the eggshell. Then, due to the decrease of temperature, protein gradually contracted. By this time, the contraction of the eggshell was already very slow, and protein was separated from the eggshell. Therefore, when peeling, the shell and the "meat" will not come off together. It is very useful for us to understand the truth that heat expands and contracts. If all things that need to undergo great temperature changes are made of two different materials together, their thermal expansion performance must be considered when selecting materials, and the closer they are, the better. Reinforced concrete is widely used when engineers design houses and bridges. Because the expansion degree of steel bars and concrete is similar. Although the temperature is different in spring, summer, autumn and winter, it will not produce harmful forces, so reinforced concrete buildings are very strong. 6. Use a beauty towel to apply ice to swollen eyelids for 3 minutes, and use the principle of heat expansion and cold contraction to reduce swelling. 7. The closure of Nanpu, Yangpu and Xupu cable-stayed bridges on the Huangpu River is based on the principle that steel expands when heated and contracts when cooled. Hundreds of bolt holes on both sides of the last steel box girder are respectively butted with the bolt holes on the east and west banks above the center of the river, and finally the closure is completed by anchoring. 8. Some automatic switches are realized by using the different degrees of thermal expansion and cold contraction of two metals in bimetallic strips. 9. Large rocks will be broken due to thermal expansion and cold contraction after alternating pouring with water and fire. From big to small, so that rocks can be easily removed. 2. Disadvantages of thermal expansion and cold contraction 1. The thin seam rail on the rail will expand when heated, and the temperature will rise 1℃, and the rail length of 1 km will extend 1 cm, which looks very small. However, if the temperature difference between winter and summer is 55℃, taking Tianjin to Beijing as an example, the length of railway track is 120km, which is 66m higher in summer than in winter. This is a big number. If the rails are tightly connected when laying, the expanded 66 meters will destroy the whole line, so when laying the rails, there should be a certain gap at the joint. When it is cold, the length of metal wire will shrink. If the wire is too tight in summer, it will break in winter when it is cold, which will cause fatal danger. Therefore, when people set up wires in summer, they often pull them longer. 3. When building a bridge, there is a certain gap on the deck. The crevice stones on the bridge deck will expand when heated. If there is no gap when building a stone arch bridge, the bridge will easily collapse, so people usually leave a certain gap when building a bridge. 4. Ceramic tiles have high water absorption rate, which will lead to cracking and peeling of the whole tile surface after thermal expansion and cold contraction. 5. Kitchen countertops have to endure fire roasting and water immersion during cooking, especially when they expand with heat and contract with cold. Over time, it will cause other problems and bring a lot of inconvenience. 6. If the toilet is installed with cement, the expansion and contraction of cement will cause the lower part of the toilet to crack, thus shortening the service life of the toilet. 7. Warm stimulation will accelerate the metabolism of skin epidermal cells. It stands to reason that this should be one of the ways to exfoliate. Unfortunately, after warm stimulation, blood vessels are the first reaction, but under the stimulation of long-term thermal expansion and cold contraction, blood vessels cannot contract after expansion. At this time, the whole face will blush easily, and with the use of exfoliating products, not only blushing, but repeated peeling is the real source of pain. Through investigation, I found that there are many phenomena and applications around us, but everything always has its two sides, and so does the expansion and contraction of an object, which has both advantages and disadvantages. We should prevent its shortcomings and make use of its advantages to serve mankind.