Calculate the amount of heat required for heating. Presentation on the topic "Calculation of the amount of heat required to heat a body and released by it when it cools"
§ 9. Calculation of the amount of heat required to heat the body or released by it during cooling - Physics Grade 8 (Peryshkin)
Short description:
In a paragraph with such a long title, a formula for calculating the amount of heat is finally obtained. All the reasoning carried out in the two previous paragraphs, briefly, in the form of letters denoting physical quantities are combined into one formula. Quantities: body weight, change in body temperature, specific heat capacity. This is the first formula in the eighth grade course. Certainly. after paragraph nine, tasks will follow in which it will be necessary to calculate the amount of heat that is required or released. An example of solving such a problem is in the textbook. Even two tasks. Specific heat capacity, if it is not indicated in the condition of the problem, take from the table in paragraph 8.
The amount of heat is related to the internal energy of the body. if the body gives off heat, then the internal energy decreases, and if it receives, then vice versa. Therefore, in tasks, sometimes they are asked to calculate not heat, but the change in internal energy. This is how the question of the problem is formulated: “How much has the internal energy changed?” This must be done according to the same formula for heat, which you will get acquainted with in this paragraph.
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The purpose of the lesson:
determine the formula for calculating the amount of heat required to change body temperature; analyze the formula; development of practical skills in solving problems; continue to learn to analyze the conditions of the task; analyze and evaluate the response of classmates;
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Without warmth, there is no life. But too much cold and heat destroys all life. All bodies, even blocks of ice, radiate energy, but slightly heated bodies radiate little energy, and this radiation is not perceived by the human eye. In the eighteenth century, many scientists believed that heat is a special substance called "caloric", a weightless "liquid" contained in bodies. Now we know. It is not so. Today we will talk about heat and thermal phenomena, and also learn how to calculate the amount of heat needed to heat a body and released when it cools.
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Comprehensive knowledge test
1. The energy of motion and interaction of the particles that make up the body is called internal energy. 2. The internal energy of a body cannot be increased by doing work on it. 3. The transfer of energy from a colder body to a hotter one is called heat conduction. 4. With heat conduction, the substance does not move from one end of the body to the other. 5. Convection occurs in solids. 6. The energy that the body gives or receives during heat transfer is called the amount of heat. 7. Radiation is a type of heat transfer. 8. The transfer of energy from one body to another or from one part of it to another is carried out by molecules or other particles. 9. Internal energy is measured in Newtons. 10. The amount of heat required to heat the body depends on the type of substance
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Answers to the task:
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Which diagram shows the three modes of heat transfer: conduction, radiation, and convection? a/ c/ b/
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Through heat conduction through the bottom and walls of the pot, the internal energy of the flame passes into the internal energy of the tourist stew. By radiation - into the internal energy of the palms of the tourist and his clothes. And by convection - into the internal energy of the air above the fire.
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Qualitative tasks
From the Russian fairy tale "The little sister and the gray wolf". The wolf went to the river, lowered his tail into the hole and began to say: “Catch, fish, both small and large! Catch, small and big fish! Following him, the fox appeared; walks around the wolf and laments: “Clear, clear the stars in the sky! Freeze, freeze wolf tail! Tail and freeze. How did the wolf's tail leave the heat? (Radiation).
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From the Altai fairy tale "The Ermine and the Hare". The wise bear thought silently. A large fire crackled hotly in front of him, above the fire on an iron tripod stood a golden cauldron with seven bronze ears. The bear never cleaned this favorite cauldron: he was afraid that happiness would go away with the dirt, and the golden cauldron was always covered with a hundred layers of soot like velvet. Did the fact that the boiler was covered with “a hundred layers of soot” affect the heating of the water?
Yes, since the soot is porous, the heating of water will be slower.
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Before taking off, the night butterfly flutters its wings for a long time. Why?
Butterfly "warms up", like an athlete doing a warm-up before the start. Part of the mechanical work done by it goes to increase internal energy.
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Focus "Fireproof paper". The nail is tightly wrapped in paper and heated in the flame of an alcohol lamp. The paper does not burn. Why? Focus "Fireproof paper". The nail is tightly wrapped in paper and heated in the flame of an alcohol lamp. The paper does not burn. Why?
Iron has great thermal conductivity, so almost all the heat is transferred to the nail, and the paper does not burn. Experimental task.
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Experimental task. Experience with a striped glass I paste over a glass of thin glass from the inside with strips of white and black paper of the same width. Outside, I glue buttons to the glass with plasticine at the same height, one against each white and black strip. I put the glass on a saucer and put a candle in it strictly in the center. I light a candle. After a while, the buttons begin to fall off. Explain the results of the experiment. Answer: First, those buttons that are glued against black strips of paper will disappear, since here the glass heats up more, black surfaces absorb the energy of the radiation incident on them more than white ones.
In practice, thermal calculations are often used. For example, when constructing buildings, it is necessary to take into account how much heat the entire heating system should give to the building. You should also know how much heat will go into the surrounding space through windows, walls, doors.
We will show by examples how to carry out the simplest calculations.
So, you need to find out how much heat the copper part received when heated. Its mass is 2 kg, and the temperature increased from 20 to 280 °C. First, according to Table 1, we determine the specific heat capacity of copper with m = 400 J / kg ° C). This means that it takes 400 J to heat a copper part weighing 1 kg by 1 °C. large quantity heat - 800 J. The temperature of the copper part must be increased not by 1 ° C, but by 260 ° C, which means that 260 times more heat will be required, i.e. 800 J 260 \u003d 208,000 J.
If we denote the mass m, the difference between the final (t 2) and initial (t 1) temperatures - t 2 - t 1 we get a formula for calculating the amount of heat:
Q \u003d cm (t 2 - t 1).
Example 1. An iron cauldron of mass 5 kg is filled with water of mass 10 kg. How much heat must be transferred to the boiler with water to change their temperature from 10 to 100 °C?
When solving the problem, it must be taken into account that both bodies - both the boiler and the water - will be heated together. Heat exchange takes place between them. Their temperatures can be considered the same, i.e. the temperature of the boiler and water changes by 100 °C - 10 °C = 90 °C. But the amounts of heat received by the boiler and water will not be the same. After all, their masses and specific heat capacities different.
Heating water in a kettle
Example 2. Mixed water weighing 0.8 kg, having a temperature of 25 ° C, and water at a temperature of 100 ° C, weighing 0.2 kg. The temperature of the resulting mixture was measured and found to be 40°C. Calculate how much heat the hot water gave off when it cooled and the cold water received when it was heated. Compare these amounts of heat.
Let's write down the condition of the problem and solve it.
We see that the amount of heat given off hot water, and the amount of heat received by cold water are equal to each other. This is not a random result. Experience shows that if heat exchange occurs between bodies, then the internal energy of all heating bodies increases by as much as the internal energy of cooling bodies decreases.
When conducting experiments, it usually turns out that the energy given off by hot water is greater than the energy received by cold water. This is explained by the fact that part of the energy is transferred to the surrounding air, and part of the energy is transferred to the vessel in which water was mixed. The equality of the given and received energies will be the more accurate, the less energy loss is allowed in the experiment. If you calculate and take into account these losses, then the equality will be accurate.
Questions
- What you need to know to calculate the amount of heat received by the body when heated?
- Explain with an example how the amount of heat imparted to a body when it is heated or released when it is cooled is calculated.
- Write a formula to calculate the amount of heat.
- What conclusion can be drawn from the experience of mixing cold and hot water? Why are these energies not equal in practice?
Exercise 8
- How much heat is required to raise the temperature of 0.1 kg of water by 1°C?
- Calculate the amount of heat required to heat: a) a cast-iron iron weighing 1.5 kg to change its temperature by 200 °C; b) an aluminum spoon weighing 50 g from 20 to 90 °C; c) a brick fireplace weighing 2 tons from 10 to 40 °C.
- What is the amount of heat released during the cooling of water, the volume of which is 20 liters, if the temperature changes from 100 to 50 °C?