General repetition of program material in physics in high school in the form of solving illustrated qualitative problems. General repetition of program material in physics in high school in the form of solving illustrated qualitative problems.

What associations do you have when you hear the words “burning wood”? A melted fireplace or stove, a blazing fire; pleasant warmth and heat from an open fire; flying sparks and, of course, crackling sounds. Few people wonder why firewood crackles. Depending on the type of wood and the degree of its drying, logs can either make almost no acoustic noise, or continuously hiss, click, crackle... In silence, wood never burns.

Perhaps as a child you loved books and TV shows from the series “Fascinating Physics for Kids”. Simple phenomena that occur every day were popularly explained empirically. Try repeating a similar elementary experiment from childhood.

Open the burning stove or sit next to the fire, and then hold a metal ladle, ladle or spoon upside down to the flame. The surface of the metal will soon be covered with droplets. If you bring the dishes even closer to the fire, they will turn black with soot.

Dip a burning stick into a deep vessel at least twice. The flame will go out, and each time faster. The fact is that it accumulates in the container carbon dioxide. Water, coal and gas - this is what an armful of firewood in your stove will ultimately turn into.

It is believed that good firewood(producing a lot of heat) must be dried before use until the wood moisture content is approximately 15-16% (the permissible maximum is 25%). However, even the driest tree retains moisture: this is the so-called “capillary” water in the cavities of the cells, and “colloidal” water in the cell membranes, and a very small amount of chemically bound water.

When the fire flares up and the wood is covered with the first charred layer, all the water turns into steam. The continuous crackling of logs in the stove is nothing more than a series of real micro-explosions. The charcoal fibers break and water vapor escapes. Charred wood fragments bounce off each other, making loud clicks.

Maybe you heard loud bangs during fires. These were episodic emissions of hot gases from pyrolysis - a process during which the thermal decomposition of wood into various chemical elements. Experts have even learned how to heat wood in a special way without access to air (or with limited access) for various production needs.

So, the burning logs crackle and, interestingly, each in its own way. If you put dried beech wood in the stove, you may hardly hear the familiar crackling sound. The aspen will crackle peacefully, while the pine in the flame will simply “shoot” - the accumulated resin will be released from its cavities and fill the room with a pleasant ether.

Burning wood not only “talks” in its own way, but also has different distinctive features. For example, the same fragrant pine should not be used in an open fireplace - sparks will fly. It is better to burn cherry or apple tree branches there and enjoy the warmth and pleasant aroma. If you use a home fireplace or bathhouse with wood stove, be sure to consult with a specialist before preparing firewood.

Good day, dear readers. As a child, each of us asked the question “Why?” We are growing up, and our children also often ask us this question. Some “whys” are quite difficult to answer. Because of this, I thought it would be quite interesting and educational to answer some of the “whys.” And since there are a lot of questions, a whole series of articles will be devoted to these answers.

My article today will be devoted to answering the question why firewood crackles when burning.

Everything is quite simple and can be explained by simple physical phenomena.

Wood, like everything else organic matter, consists of tiny cells. All cells contain fluid. Like any other liquid, when exposed to high temperatures evaporates and turns into gas. When heated, the gas expands, the pressure on the cell increases and, in the end, it bursts like a balloon.

Naturally, we won’t even notice such an “explosion” of one cell. But there are millions of cells, and it is impossible not to hear the “explosion” of so many cells.

As a result, we hear crackling and hissing noises when wood burns.

It’s interesting that no matter whether the firewood is dry or wet, we still hear a crackling sound. Why? No matter how well the firewood is dried, some moisture will still remain in it.

By the way, the drier the wood, the louder the crackling sound. And the strength of the crack also depends on the type of wood. Scientists still haven’t really figured out why this happens. Most likely, it depends on the structure of the wood itself and the presence of resins in it.

What associations do you have when you hear the words “burning wood”? A melted fireplace or stove, a blazing fire; pleasant warmth and heat from an open fire; flying sparks and, of course, crackling sounds. Few people think about why firewood crackles. Depending on the type of wood and the degree of its drying, logs can either emit almost no acoustic noise, or continuously hiss, click, crackle... In silence, wood never burns.

Perhaps as a child you loved books and TV shows from the series "Fascinating Physics for Kids." Simple phenomena that occur every day were popularly explained experimentally. Try repeating a similar elementary experiment from childhood.

Dip a burning stick into a deep vessel at least twice. The flame will go out, and each time faster. The fact is that carbon dioxide accumulates in the container. Water, coal and gas - this is what the armful of firewood in your stove will ultimately turn into.

It is believed that good firewood (which produces a lot of heat) should be dried before use until the wood moisture content is approximately 15-16% (the permissible maximum is 25%). However, even the driest tree retains moisture: this is the so-called “capillary” water in the cavities of the cells, and “colloidal” water in the cell membranes, and a very small amount of chemically bound water.

Maybe you heard loud bangs during fires. These were occasional emissions of hot gases pyrolysis. To put it simply, the combustion process consists of two stages: pyrolysis of wood and combustion of its products. Pyrolysis- decomposition of complex organic matter at temperatures up to 450 °C. In this case, about a quarter of the products are immediately released in the form of gases: carbon dioxide, hydrogen, methane and carbon monoxide. Another approximately half of the mass of wood when burned forms a liquid consisting of water and various organics, including acetic acid, alcohols and other compounds.

But in a fire or stove these liquids are invisible because they instantly evaporate. The rest of the mass forms charcoal, consisting of 80–90% carbon. It is very porous and smolders due to interaction with oxygen penetrating into its pores. If a log burns from the surface, its inner layers heat up and pyrolysis starts in them. The resulting gases accumulate between the layers of wood and rupture them with a loud crack as the thickness of the outer layer contracts due to burning and the pressure underneath increases due to heat. And firewood constantly crackles due to the fact that the wood dries out under the influence of heat and becomes deformed, causing the destruction of the brittle coal formed on the logs during pyrolysis.

Experts have even learned how to heat wood in a special way without access to air (or with limited access) for various production needs.

Burning wood not only “talks” in its own way, but also has other distinctive features. For example, the same fragrant pine should not be used in an open fireplace - sparks will fly. It is better to burn cherry or apple tree branches there and enjoy the warmth and pleasant aroma. If you use a home fireplace or sauna with a wood-burning stove, be sure to consult with a specialist before preparing firewood.