Calculate the amount of heat to heat the cast iron. Presentation on the topic "Calculation of the amount of heat required to heat a body and emitted by it when it cools"
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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; practicing practical skills in solving problems; continue to learn to analyze the conditions of the assignment; analyze and evaluate the response of classmates;
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There is no life without warmth. But too much cold and heat destroys all living things. All bodies, even blocks of ice, emit energy, but weakly heated bodies emit 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. We now know. It is not so. Today we will talk about heat and thermal phenomena, and also learn how to calculate the amount of heat required 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 the body cannot be increased by doing work on it. 3. The transfer of energy from a colder body to a hotter one is called thermal conductivity. 4. With thermal conductivity, matter 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 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 a body depends on the type of substance
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Answers to the task:
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Which figure shows three methods of heat transfer: heat conduction, radiation and convection? a / b / b /
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By means of heat conduction through the bottom and walls of the pot, the internal energy of the flame is converted into the internal energy of the tourist soup. By radiation - into the internal energy of the tourist's palms 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 "Fox - a sister and a gray wolf". The wolf went to the river, lowered its tail into the hole and began to say: “Catch, fish, both small and large! Catch, fish, both small and large! ”. The fox followed him; walks around the wolf and wails: “Clear, clear the stars in the sky! Freeze, freeze wolf's tail! ”. The tail is frozen. How did the wolf's tail leave the warmth? (By radiation).
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From the Altai fairy tale “The Ermine and the Hare”. Silently, the wise bear thought his thoughts. In front of him a large fire was crackling hotly; over the fire, on an iron tripod, stood a golden cauldron with seven bronze ears. The bear never cleaned this beloved 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. Was the heating of the water affected by the fact that the boiler was covered with “one hundred layers of soot”?
Yes, since the soot is porous, the water heating will be slower
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Before taking off, the moth flutters its wings for a long time. Why?
The butterfly “warms up” like an athlete warming up before starting. Part of the mechanical work she does goes to increase the 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 is off. Why? Focus "Fireproof paper". The nail is tightly wrapped in paper and heated in the flame of an alcohol lamp. The paper is off. Why?
Iron has a high thermal conductivity, so almost all the heat is transferred to the nail, and the paper does not burn. Experimental assignment.
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Experimental assignment. Experiment 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 a glass on a saucer and 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 the 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 must give to the building. You should also know how much heat will go into the surrounding space through windows, walls, doors.
Let's show with examples how to carry out the simplest calculations.
So, you need to find out how much heat the copper part received when heating. Its mass is 2 kg, and the temperature increased from 20 to 280 ° C. First, according to table 1, we determine the specific heat of copper with m = 400 J / kg ° C). This means that heating a copper part weighing 1 kg per 1 ° C will require 400 J. ° С, and at 260 ° С, which means that 260 times more heat will be required, i.e. 800 J 260 = 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 obtain the formula for calculating the amount of heat:
Q = cm (t 2 - t 1).
Example 1... Water weighing 10 kg is poured into an iron cauldron weighing 5 kg. How much heat needs to be transferred to the boiler with water to change their temperature from 10 to 100 ° C?
When solving the problem, it is necessary to take into account that both bodies - both the boiler and the water - will heat up 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 are different.
Heating water in a pot
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 during cooling and received cold water when 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 by hot water and the amount of heat received by cold water are equal. This is not an accidental 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 due to 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 the water was mixed. The equality of the given and received energies will be the more accurate, the less energy losses are allowed in the experiment. If you calculate and take into account these losses, then the equality will be exact.
Questions
- What do 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 heats up or released when it cools is calculated.
- Write a formula for calculating 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
- What amount of heat is required to heat water with a mass of 0.1 kg per 1 ° C?
- Calculate the amount of heat required for heating: 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) brick fireplace weighing 2 tons from 10 to 40 ° C.
- What amount of heat was released during the cooling of water, the volume of which is 20 liters, if the temperature changed from 100 to 50 ° С?
§ 9. Calculation of the amount of heat required for heating a 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 previous two paragraphs, briefly, in the form of letters denoting physical quantities, are collected in one formula. Quantities: body weight, change in body temperature, specific heat. This is the first formula in an eighth grade course. Sure. 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 if it is not indicated in the problem statement, take it 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 problems it is sometimes asked to calculate not the heat, but the change in the internal energy. This is how the question of the problem is formulated: "How much has the internal energy changed?" This should be done according to the same formula for warmth, which you will get acquainted with in this paragraph.
