The amount of heat that is released. "Quantity of heat. Specific heat

Learning Objective: Introduce the concepts of heat quantity and specific heat capacity.

Developmental goal: To cultivate attentiveness; teach to think, draw conclusions.

1. Updating the topic

2. Explanation of new material. 50 min.

You already know that internal energy body can change both by doing work and by heat transfer (without doing work).

The energy that a body gains or loses during heat transfer is called the amount of heat. (write in notebook)

This means that the units for measuring the amount of heat are also Joules ( J).

We conduct an experiment: two glasses in one with 300 g of water, and in the other 150 g, and an iron cylinder weighing 150 g. Both glasses are placed on the same tile. After some time, thermometers will show that the water in the vessel in which the body is located heats up faster.

This means that less iron is required to heat 150 g. quantity of heat than for heating 150 g of water.

The amount of heat transferred to a body depends on the type of substance from which the body is made. (write in notebook)

We pose the question: is the same amount of heat required to heat bodies of equal mass, but consisting of different substances, to the same temperature?

We conduct an experiment with Tyndall's device to determine specific heat capacity.

We conclude: bodies made of different substances, but of the same mass, give up when cooled and require different amounts of heat when heated by the same number of degrees.

We draw conclusions:

1. To heat bodies of equal mass, consisting of different substances, to the same temperature, it is required different quantity warmth.

2. Bodies of equal mass, consisting of different substances and heated to the same temperature. When cooled by the same number of degrees, different amounts of heat are released.

We conclude that the amount of heat required to heat a unit mass of different substances by one degree will vary.

We give the definition of specific heat capacity.

A physical quantity numerically equal to the amount of heat that must be transferred to a body weighing 1 kg in order for its temperature to change by 1 degree is called the specific heat capacity of a substance.

Enter the unit of measurement for specific heat capacity: 1J/kg*degree.

Physical meaning of the term : Specific heat capacity shows by what amount the internal energy of 1g (kg) of a substance changes when it is heated or cooled by 1 degree.

Let's look at the table of specific heat capacities of some substances.

We solve the problem analytically

How much heat is required to heat a glass of water (200 g) from 20 0 to 70 0 C.

To heat 1 g per 1 g, 4.2 J is required.

And to heat 200 g by 1 g, it will take 200 more - 200 * 4.2 J.

And to heat 200 g by (70 0 -20 0) it will take another (70-20) more - 200 * (70-20) * 4.2 J

Substituting the data, we get Q = 200 * 50 * 4.2 J = 42000 J.

Let us write the resulting formula in terms of the corresponding quantities

4. What determines the amount of heat received by a body when heated?

Please note that the amount of heat required to heat any body is proportional to the mass of the body and the change in its temperature.,

There are two cylinders of equal mass: iron and brass. Is the same amount of heat required to heat them the same number of degrees? Why?

What amount of heat is needed to heat 250 g of water from 20 o to 60 0 C.

What is the relationship between calorie and joule?

A calorie is the amount of heat required to heat 1 g of water by 1 degree.

1 cal = 4.19 = 4.2 J

1kcal=1000cal

1kcal=4190J=4200J

3. Problem solving. 28 min.

If cylinders of lead, tin and steel weighing 1 kg heated in boiling water are placed on ice, they will cool and part of the ice under them will melt. How will the internal energy of the cylinders change? Under which cylinder will it melt? more ice, under which – less?

A heated stone weighing 5 kg. Cooling in water by 1 degree, it transfers 2.1 kJ of energy to it. What is the specific heat capacity of the stone?

When hardening a chisel, it was first heated to 650 0, then lowered into oil, where it cooled to 50 0 C. What amount of heat was released if its mass was 500 grams.

How much heat was used to heat a steel blank for the compressor crankshaft weighing 35 kg from 20 0 to 1220 0 C.

Independent work

What type of heat transfer?

Students fill out the table.

1. The air in the room is heated through the walls.
2. Through an open window into which warm air enters.
3. Through glass that lets in the sun's rays.
4. The earth is heated by the sun's rays.
5. The liquid is heated on the stove.
6. The steel spoon is heated by the tea.
7. The air is heated by the candle.
8. The gas moves near the fuel-generating parts of the machine.
9. Heating a machine gun barrel.
10. Boiling milk.

5. Homework: Peryshkin A.V. “Physics 8” § §7, 8; collection of problems 7-8 Lukashik V.I. No. 778-780, 792,793 2 min.

The internal energy of a body depends on its temperature and external conditions - volume, etc. If external conditions remain unchanged, i.e. the volume and other parameters are constant, then the internal energy of the body depends only on its temperature.

You can change the internal energy of a body not only by heating it in a flame or performing mechanical work on it (without changing the position of the body, for example, the work of friction), but also by bringing it into contact with another body that has a temperature different from the temperature of this body, i.e. through heat transfer.

The amount of internal energy that a body gains or loses during heat transfer is called the “amount of heat.” The amount of heat is usually denoted by the letter `Q`. If the internal energy of a body increases during the process of heat transfer, then the heat is assigned a plus sign, and the body is said to have been given heat `Q`. When the internal energy decreases during the process of heat transfer, the heat is considered negative, and it is said that the amount of heat `Q` has been removed (or removed) from the body.

The amount of heat can be measured in the same units in which mechanical energy is measured. In SI it is `1` joule. There is another unit of heat measurement - the calorie. Calorie is the amount of heat required to heat `1` g of water by `1^@ bb"C"`. The relationship between these units was established by Joule: `1` cal `= 4.18` J. This means that due to the work of `4.18` kJ, the temperature of `1` kilogram of water will increase by `1` degree.

The amount of heat required to heat a body by `1^@ bb"C"` is called the heat capacity of the body. The heat capacity of a body is designated by the letter `C`. If the body is given a small amount of heat `Delta Q`, and the body temperature changes to `Delta t` degrees, then

If a body is surrounded by a shell that does not conduct heat well, then the temperature of the body, if left to its own devices, will remain practically constant for a long time. Such ideal shells, of course, do not exist in nature, but it is possible to create shells that are close to such in their properties.

Examples include cladding spaceships, Dewar vessels used in physics and technology. A Dewar flask is a glass or metal cylinder with double mirror walls, between which a high vacuum is created. The glass flask of a home thermos is also a Dewar flask.

The shell is insulating calorimeter- a device that allows you to measure the amount of heat. The calorimeter is a large thin-walled glass, placed on pieces of cork inside another large glass so that a layer of air remains between the walls, and closed on top with a heat-insulating lid.

If two or more bodies having different temperatures, and wait, then after some time it will be established inside the calorimeter thermal equilibrium. In the process of transition to thermal equilibrium, some bodies will give off heat (total amount of heat `Q_(sf"floor")`), others will receive heat (total amount of heat `Q_(sf"floor")`). And since the calorimeter and the bodies contained in it do not exchange heat with the surrounding space, but only with each other, we can write down a relationship, also called heat balance equation:

In a number of thermal processes, heat can be absorbed or released by a body without changing its temperature. Such thermal processes occur when the aggregate state of a substance changes - melting, crystallization, evaporation, condensation and boiling. Let us briefly discuss the main characteristics of these processes.

Melting- the process of turning a crystalline solid into a liquid. The melting process occurs when constant temperature, heat is absorbed.

The specific heat of fusion `lambda` is equal to the amount of heat required to melt `1` kg of a crystalline substance taken at its melting point. The amount of heat `Q_(sf"pl")` that is required to convert a solid body of mass `m` at the melting point into a liquid state is equal to

Since the melting point remains constant, the amount of heat imparted to the body goes to increase the potential energy of interaction between molecules, and the crystal lattice is destroyed.

Process crystallization- This is a process reverse to the melting process. During crystallization, the liquid turns into a solid and an amount of heat is released, also determined by formula (1.5).

Evaporation is the process of converting liquid into vapor. Evaporation occurs from the open surface of the liquid. During the process of evaporation, the fastest molecules leave the liquid, i.e., molecules that can overcome the attractive forces exerted by the liquid molecules. As a result, if the liquid is thermally insulated, it cools during the evaporation process.

The specific heat of vaporization `L` is equal to the amount of heat required to turn `1` kg of liquid into steam. The amount of heat `Q_(sf"use")` that is required to convert a liquid of mass `m` into a vapor state is equal to

Condensation- a process reverse to the evaporation process. When condensation occurs, steam turns into liquid. This generates heat. The amount of heat released during steam condensation is determined by formula (1.6).

Boiling- a process in which pressure saturated vapors liquid equals atmospheric pressure, therefore, evaporation occurs not only from the surface, but throughout the entire volume (there are always air bubbles in the liquid; when boiling, the vapor pressure in them reaches atmospheric pressure, and the bubbles rise upward).

The focus of our article is the amount of heat. We will consider the concept of internal energy, which is transformed when this quantity changes. We will also show some examples of the application of calculations in human activity.

Heat

Every person has their own associations with any word in their native language. They are determined personal experience and irrational feelings. What do you usually think of when you hear the word “warmth”? Soft blanket, working central heating radiator in winter, first sunlight spring, cat Or a mother’s look, a friend’s comforting word, timely attention.

Physicists mean a very specific term by this. And very important, especially in some sections of this complex but fascinating science.

Thermodynamics

It is not worth considering the amount of heat in isolation from the simplest processes on which the law of conservation of energy is based - nothing will be clear. Therefore, first let us remind our readers of them.

Thermodynamics considers any thing or object as a very large quantity elementary parts - atoms, ions, molecules. Its equations describe any change in the collective state of the system as a whole and as a part of the whole when macroparameters change. The latter refers to temperature (denoted as T), pressure (P), concentration of components (usually C).

Internal energy

Internal energy is a rather complex term, the meaning of which is worth understanding before talking about the amount of heat. It denotes the energy that changes when the value of the macroparameters of an object increases or decreases and does not depend on the reference system. It is part of the total energy. It coincides with it in conditions when the center of mass of the thing under study is at rest (that is, there is no kinetic component).

When a person feels that an object (say a bicycle) has become hot or cold, it shows that all the molecules and atoms that make up this system, experienced a change in internal energy. However, the constant temperature does not mean the preservation of this indicator.

Work and heat

The internal energy of any thermodynamic system can be transformed in two ways:

• by doing work on it;
• during heat exchange with the environment.

The formula for this process looks like this:

dU=Q-A, where U is internal energy, Q is heat, A is work.

Let the reader not be deceived by the simplicity of the expression. The rearrangement shows that Q=dU+A, however, the introduction of entropy (S) brings the formula to the form dQ=dSxT.

Since in this case the equation takes the form of a differential one, the first expression requires the same. Next, depending on the forces acting in the object under study and the parameter that is being calculated, the required ratio is derived.

Let's take a metal ball as an example of a thermodynamic system. If you press on it, throw it up, drop it into a deep well, then this means doing work on it. Outwardly, all these harmless actions will not cause any harm to the ball, but its internal energy will change, albeit very slightly.

The second method is heat exchange. Now we come to the main goal of this article: a description of what the amount of heat is. This is a change in the internal energy of a thermodynamic system that occurs during heat exchange (see formula above). It is measured in joules or calories. Obviously, if you hold the ball over a lighter, in the sun, or simply in a warm hand, it will heat up. And then you can use the change in temperature to find the amount of heat that was communicated to him.

Why gas is the best example of a change in internal energy, and why schoolchildren don’t like physics because of this

Above we described changes in the thermodynamic parameters of a metal ball. They are not very noticeable without special devices, and the reader can only take the word about the processes occurring with the object. It's another matter if the system is gas. Press on it - it will be visible, heat it - the pressure will rise, lower it underground - and it can be easily recorded. Therefore, in textbooks, gas is most often used as a visual thermodynamic system.

But, alas, in modern education real experiences not much attention is paid. Scientist who writes Toolkit, understands perfectly what he's talking about we're talking about. It seems to him that, using the example of gas molecules, all thermodynamic parameters will be in the right way demonstrated. But a student who is just discovering this world is bored hearing about an ideal flask with a theoretical piston. If the school had real research laboratories and allocated hours to work in them, things would be different. So far, unfortunately, the experiments are only on paper. And, most likely, this is precisely the reason that people consider this branch of physics to be something purely theoretical, far from life and unnecessary.

Therefore, we decided to use the bicycle already mentioned above as an example. A person presses on the pedals and does work on them. In addition to imparting torque to the entire mechanism (thanks to which the bicycle moves in space), the internal energy of the materials from which the levers are made changes. The cyclist presses the handles to turn, and again does the work.

The internal energy of the outer coating (plastic or metal) increases. A man drives out into a clearing under bright sun- the bicycle heats up, its amount of heat changes. Stops to rest in the shade of an old oak tree and the system cools, losing calories or joules. Increases speed - increases energy exchange. However, calculating the amount of heat in all these cases will show a very small, imperceptible value. Therefore, it seems that the manifestations of thermodynamic physics in real life No.

Application of calculations for changes in the amount of heat

The reader will probably say that all this is very educational, but why are we so tormented at school with these formulas? And now we will give examples in which areas of human activity they are directly needed and how this concerns anyone in their everyday life.

First, look around you and count: how many metal objects surround you? Probably more than ten. But before becoming a paper clip, a carriage, a ring or a flash drive, any metal undergoes smelting. Every plant that processes, say, iron ore, must understand how much fuel is required in order to optimize costs. And when calculating this, it is necessary to know the heat capacity of the metal-containing raw material and the amount of heat that needs to be imparted to it in order for everything to happen. technological processes. Since the energy released by a unit of fuel is calculated in joules or calories, the formulas are needed directly.

Or another example: most supermarkets have a department with frozen goods - fish, meat, fruit. Where raw materials from animal meat or seafood are transformed into semi-finished products, they must know how much electricity refrigeration and freezing units will consume per ton or unit of finished product. To do this, you need to calculate how much heat a kilogram of strawberries or squid loses when cooled by one degree Celsius. And in the end, this will show how much electricity a freezer of a certain power will consume.

Planes, ships, trains

Above we showed examples of relatively motionless, static objects to which a certain amount of heat is imparted or from which, on the contrary, a certain amount of heat is taken away. For objects that move in conditions of constantly changing temperature during operation, calculations of the amount of heat are important for another reason.

There is such a thing as “metal fatigue”. It also includes maximum permissible loads at a certain rate of temperature change. Imagine a plane taking off from humid tropics into the frozen upper atmosphere. Engineers have to work hard to ensure that it does not fall apart due to cracks in the metal that appear when the temperature changes. They are looking for an alloy composition that can withstand real loads and have a large margin of safety. And in order not to search blindly, hoping to accidentally stumble upon the desired composition, you have to do a lot of calculations, including those that include changes in the amount of heat.

In this lesson we will learn how to calculate the amount of heat required to heat a body or released by it when cooling. To do this, we will summarize the knowledge that was acquired in previous lessons.

In addition, we will learn, using the formula for the amount of heat, to express the remaining quantities from this formula and calculate them, knowing other quantities. An example of a problem with a solution for calculating the amount of heat will also be considered.

This lesson is devoted to calculating the amount of heat when a body is heated or released when cooled.

The ability to calculate the required amount of heat is very important. This may be needed, for example, when calculating the amount of heat that needs to be imparted to water to heat a room.

Rice. 1. The amount of heat that must be imparted to the water to heat the room

Or to calculate the amount of heat that is released when fuel is burned in various engines:

Rice. 2. The amount of heat that is released when fuel is burned in the engine

This knowledge is also needed, for example, to determine the amount of heat that is released by the Sun and falls on the Earth:

Rice. 3. The amount of heat released by the Sun and falling on the Earth

To calculate the amount of heat, you need to know three things (Fig. 4):

• body weight (which can usually be measured using a scale);
• the temperature difference by which a body must be heated or cooled (usually measured using a thermometer);
• specific heat capacity of the body (which can be determined from the table).

Rice. 4. What you need to know to determine

The formula by which the amount of heat is calculated looks like this:

The following quantities appear in this formula:

The amount of heat measured in joules (J);

The specific heat capacity of a substance is measured in ;

- temperature difference, measured in degrees Celsius ().

Let's consider the problem of calculating the amount of heat.

Task

A copper glass with a mass of grams contains water with a volume of liter at a temperature. How much heat must be transferred to a glass of water so that its temperature becomes equal to ?

Rice. 5. Illustration of the problem conditions

First let's write down short condition (Given) and convert all quantities to the international system (SI).

Solution:

First, determine what other quantities we need to solve this problem. Using the table of specific heat capacity (Table 1) we find (specific heat capacity of copper, since by condition the glass is copper), (specific heat capacity of water, since by condition there is water in the glass). In addition, we know that to calculate the amount of heat we need a mass of water. According to the condition, we are given only the volume. Therefore, from the table we take the density of water: (Table 2).

Table 1. Specific heat capacity of some substances,

Table 2. Densities of some liquids

Now we have everything we need to solve this problem.

Note that the final amount of heat will consist of the sum of the amount of heat required to heat the copper glass and the amount of heat required to heat the water in it:

Let's first calculate the amount of heat required to heat a copper glass:

Before calculating the amount of heat required to heat water, let’s calculate the mass of water using a formula that is familiar to us from grade 7:

Now we can calculate:

Then we can calculate:

Let's remember what kilojoules mean. The prefix "kilo" means .

Answer:.

For the convenience of solving problems of finding the amount of heat (the so-called direct problems) and quantities associated with this concept, you can use the following table.

What will heat up faster on the stove - a kettle or a bucket of water? The answer is obvious - a teapot. Then the second question is why?

The answer is no less obvious - because the mass of water in the kettle is less. Great. And now you can do a real physical experience yourself at home. To do this you will need two identical small saucepans, an equal amount of water and vegetable oil, for example, half a liter and a stove. Place saucepans with oil and water on the same heat. Now just watch what will heat up faster. If you have a thermometer for liquids, you can use it; if not, you can simply test the temperature with your finger from time to time, just be careful not to get burned. In any case, you will soon see that the oil heats up much faster than water. And one more question, which can also be implemented in the form of experience. Which will boil faster - warm water or cold? Everything is obvious again - the warm one will be first at the finish line. Why all these strange questions and experiments? To determine physical quantity, called the “amount of heat”.

Quantity of heat

The amount of heat is the energy that a body loses or gains during heat transfer. This is clear from the name. When cooling, the body will lose a certain amount of heat, and when heating, it will absorb. And the answers to our questions showed us What does the amount of heat depend on? Firstly, than more mass body, the more heat must be expended to change its temperature by one degree. Secondly, the amount of heat required to heat a body depends on the substance of which it consists, that is, on the type of substance. And thirdly, the difference in body temperature before and after heat transfer is also important for our calculations. Based on the above, we can determine the amount of heat using the formula:

Q=cm(t_2-t_1) ,

where Q is the amount of heat,
m- body mass,
(t_2-t_1) - difference between initial and final body temperatures,
c is the specific heat capacity of the substance, found from the corresponding tables.

Using this formula, you can calculate the amount of heat that is necessary to heat any body or that this body will release when cooling.

The amount of heat is measured in joules (1 J), like any type of energy. However, this value was introduced not so long ago, and people began measuring the amount of heat much earlier. And they used a unit that is widely used in our time - calorie (1 cal). 1 calorie is the amount of heat required to heat 1 gram of water by 1 degree Celsius. Guided by these data, those who like to count calories in the food they eat can, just for fun, calculate how many liters of water can be boiled with the energy they consume with food during the day.