When completing task 22 with a detailed answer, first write down the task number and then the answer to it. A complete answer should include not only the answer to the question, but also its detailed, logically connected rationale.
A glass of hot tea was left in a large, cool room. Over time, the temperature of the tea became equal to the temperature of the surrounding air. How did the intensities of thermal radiation and thermal absorption of tea change? Explain your answer.
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Sample possible answer
The intensity of thermal radiation decreased, the intensity of thermal absorption remained virtually unchanged.
Tea, on the one hand, emits heat rays, on the other hand, absorbs heat radiation from the surrounding air. Initially, the radiation process predominates and the tea cools. As the temperature decreases, the intensity of thermal radiation from tea decreases until it equals the intensity of absorption of thermal radiation from the air in the room. Further, the temperature of the tea does not change.
When completing tasks 23–26, write down the task number first and then the answer to it.
Assemble an experimental setup to study the dependence of the electric current in a resistor on the voltage at its ends. Use a 4.5 V current source, voltmeter, ammeter, key, rheostat, connecting wires, resistor labeled R 1.
In the answer form
1) draw an electrical diagram of the experiment;
2) using a rheostat to set the current strength in turn. circuits 0.4 A, 0.5 A and 0.6 A and measuring in each case the value of the electrical voltage at the ends of the resistor, indicate the results of measuring the current and voltage for three cases in the form of a table (or graph);
3) formulate a conclusion about the dependence of the electric current in the resistor on the voltage at its ends.
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1) Scheme of the experimental setup
2) 
3) Conclusion: as the current in the conductor increases, the voltage arising at the ends of the conductor also increases.
Task 24 is a question that requires a written answer. A complete answer should include not only the answer to the question, but also its detailed, logically connected rationale.
A model boat floats in a jar of water. Will the submersion depth (sediment) of the boat change (and if it changes, how) if it is moved from the Earth to the Moon? Explain your answer.
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Sample possible answer
Will not change.
The boat is immersed in the water until the buoyant force acting on the boat from the water balances the force of gravity. The depth of immersion (draft) of the boat is determined by fulfilling the condition: F heavy = F out (1). The acceleration of gravity on the Moon is less than on Earth. But since both forces are directly proportional to the acceleration of free fall, then both forces F heavy and F ext will decrease by the same number of times, and equality (1) will not be violated.
For tasks 25–26, it is necessary to write a complete solution, which includes writing a brief condition of the problem (Given), writing formulas, the use of which is necessary and sufficient to solve the problem, as well as mathematical transformations and calculations leading to a numerical answer.
We know that sound travels through the air. That's why we can hear. No sounds can exist in a vacuum. But if sound is transmitted through the air, due to the interaction of its particles, will it not also be transmitted by other substances? Will.
Propagation and speed of sound in different media
Sound is not transmitted only by air. Probably everyone knows that if you put your ear to the wall, you can hear conversations in the next room. In this case, the sound is transmitted by the wall. Sounds travel in water and other media. Moreover, sound propagation occurs differently in different environments. The speed of sound varies depending on the substance.
It is curious that the speed of sound in water is almost four times higher than in air. That is, fish hear “faster” than we do. In metals and glass, sound travels even faster. This is because sound is a vibration of a medium, and sound waves travel faster in better conductive media.
The density and conductivity of water is greater than that of air, but less than that of metal. Accordingly, sound is transmitted differently. When moving from one medium to another, the speed of sound changes.
The length of the sound wave also changes as it passes from one medium to another. Only its frequency remains the same. But this is precisely why we can discern who exactly is speaking even through walls.
Since sound is vibrations, all laws and formulas for vibrations and waves are well applicable to sound vibrations. When calculating the speed of sound in air, it should also be taken into account that this speed depends on the air temperature. As temperature increases, the speed of sound propagation increases. Under normal conditions, the speed of sound in air is 340,344 m/s.
Sound waves
Sound waves, as is known from physics, propagate in elastic media. This is why sounds are well transmitted by the earth. By placing your ear to the ground, you can hear the sound of footsteps, clattering hooves, and so on from afar.
As a child, everyone probably had fun putting their ear to the rails. The sound of train wheels is transmitted along the rails for several kilometers. To create the reverse sound absorption effect, soft and porous materials are used.
For example, in order to protect a room from extraneous sounds, or, conversely, to prevent sounds from escaping from the room to the outside, the room is treated and soundproofed. The walls, floor and ceiling are covered with special materials based on foamed polymers. In such upholstery all sounds fade away very quickly.
For sound to propagate, an elastic medium is required. Sound waves cannot propagate in a vacuum, since there is nothing to vibrate there. This can be verified by simple experiment. If we place an electric bell under a glass bell, then as the air is pumped out from under the bell, we will find that the sound from the bell will become weaker and weaker until it stops completely.
Sound in gases. It is known that during a thunderstorm we first see a flash of lightning and only after some time we hear the rumble of thunder (Fig. 52). This delay occurs because the speed of sound in air is much less than the speed of light coming from lightning.
The speed of sound in air was first measured in 1636 by the French scientist M. Mersenne. At a temperature of 20 °C it is equal to 343 m/s, i.e. 1235 km/h. Note that it is to this value that the speed of a bullet fired from a Kalashnikov machine gun (PK) decreases at a distance of 800 m. The initial speed of the bullet is 825 m/s, which significantly exceeds the speed of sound in air. Therefore, a person who hears the sound of a shot or the whistle of a bullet need not worry: this bullet has already passed him. The bullet outruns the sound of the shot and reaches its victim before the sound arrives.
The speed of sound depends on the temperature of the medium: with increasing air temperature it increases, and with decreasing air temperature it decreases. At 0 °C, the speed of sound in air is 331 m/s.
Sound travels at different speeds in different gases. The greater the mass of gas molecules, the lower the speed of sound in it. Thus, at a temperature of 0 °C, the speed of sound in hydrogen is 1284 m/s, in helium - 965 m/s, and in oxygen - 316 m/s.
Sound in liquids. The speed of sound in liquids is usually greater than the speed of sound in gases. The speed of sound in water was first measured in 1826 by J. Colladon and J. Sturm. They carried out their experiments on Lake Geneva in Switzerland (Fig. 53). On one boat they set fire to gunpowder and at the same time struck a bell lowered into the water. The sound of this bell, using a special horn, also lowered into the water, was captured on another boat, which was located at a distance of 14 km from the first. Based on the time interval between the flash of light and the arrival of the sound signal, the speed of sound in water was determined. At a temperature of 8 °C it turned out to be approximately 1440 m/s. 
At the boundary between two different media, part of the sound wave is reflected, and part travels further. When sound passes from air into water, 99.9% of the sound energy is reflected back, but the pressure in the sound wave transmitted into the water is almost 2 times greater. The hearing system of fish reacts precisely to this. Therefore, for example, screams and noises above the surface of the water are a sure way to scare away marine life. A person who finds himself under water will not be deafened by these screams: when immersed in water, air “plugs” will remain in his ears, which will save him from sound overload.
When sound passes from water to air, 99.9% of the energy is reflected again. But if during the transition from air to water the sound pressure increased, now, on the contrary, it sharply decreases. It is for this reason, for example, that the sound that occurs under water when one stone hits another does not reach a person in the air.
This behavior of sound at the boundary between water and air gave our ancestors the basis to consider the underwater world a “world of silence.” Hence the expression: “Mute as a fish.” However, Leonardo da Vinci also suggested listening to underwater sounds by putting your ear to an oar lowered into the water. Using this method, you can make sure that the fish are actually quite talkative.
Sound in solids. The speed of sound in solids is greater than in liquids and gases. If you put your ear to the rail, you will hear two sounds after hitting the other end of the rail. One of them will reach your ear by rail, the other by air.
The earth has good sound conductivity. Therefore, in the old days, during a siege, “listeners” were placed in the fortress walls, who, by the sound transmitted by the ground, could determine whether the enemy was digging into the walls or not. By putting their ear to the ground, they also monitored the approach of the enemy cavalry.
Solids conduct sound well. Thanks to this, people who have lost their hearing are sometimes able to dance to music that reaches their auditory nerves not through the air and the outer ear, but through the floor and bones.
1. Why during a thunderstorm do we first see lightning and only then hear thunder? 2. What does the speed of sound in gases depend on? 3. Why doesn’t a person standing on the river bank hear sounds arising under water? 4. Why were the “hearers” who in ancient times monitored the enemy’s excavation work often blind people?
Experimental task. Place your wristwatch on one end of a board (or long wooden ruler) and place your ear on the other end. What do you hear? Explain the phenomenon.