How is the amount of heat found? Quantity of heat

(or heat transfer).

Specific heat capacity of a substance.

Heat capacity- this is the amount of heat absorbed by a body when heated by 1 degree.

The heat capacity of a body is indicated by a capital Latin letter WITH.

What does the heat capacity of a body depend on? First of all, from its mass. It is clear that heating, for example, 1 kilogram of water will require more heat than heating 200 grams.

What about the type of substance? Let's do an experiment. Let's take two identical vessels and, having poured water weighing 400 g into one of them, and vegetable oil weighing 400 g into the other, we will begin to heat them using identical burners. By observing the thermometer readings, we will see that the oil heats up quickly. To heat water and oil to the same temperature, the water must be heated longer. But the longer we heat the water, the more heat it receives from the burner.

Thus, heating the same mass of different substances to the same temperature requires different amounts of heat. The amount of heat required to heat a body and, therefore, its heat capacity depend on the type of substance of which the body is composed.

So, for example, to increase the temperature of water weighing 1 kg by 1°C, an amount of heat equal to 4200 J is required, and to heat the same mass of sunflower oil by 1°C, an amount of heat equal to 1700 J is required.

A physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat capacity of this substance.

Each substance has its own specific heat capacity, which is denoted by the Latin letter c and measured in joules per kilogram degree (J/(kg °C)).

The specific heat capacity of the same substance in different states of aggregation (solid, liquid and gaseous) is different. For example, the specific heat capacity of water is 4200 J/(kg °C), and the specific heat capacity of ice is 2100 J/(kg °C); aluminum in the solid state has a specific heat capacity of 920 J/(kg - °C), and in the liquid state - 1080 J/(kg - °C).

Note that water has a very high specific heat capacity. Therefore, water in the seas and oceans, heating up in summer, absorbs a large amount of heat from the air. Thanks to this, in those places that are located near large bodies of water, summer is not as hot as in places far from the water.

Calculation of the amount of heat required to heat a body or released by it during cooling.

From the above it is clear that the amount of heat required to heat a body depends on the type of substance of which the body consists (i.e., its specific heat capacity) and on the mass of the body. It is also clear that the amount of heat depends on how many degrees we are going to increase the body temperature.

So, to determine the amount of heat required to heat a body or released by it during cooling, you need to multiply the specific heat capacity of the body by its mass and by the difference between its final and initial temperatures:

Q = cm (t 2 - t 1 ) ,

Where Q- quantity of heat, c— specific heat capacity, m- body mass , t 1 — initial temperature, t 2 — final temperature.

When the body heats up t 2 > t 1 and therefore Q > 0 . When the body cools down t 2i< t 1 and therefore Q< 0 .

If the heat capacity of the entire body is known WITH, Q determined by the formula:

Q = C (t 2 - t 1 ) .

Heat capacity- this is the amount of heat absorbed by the body when heated by 1 degree.

The heat capacity of a body is indicated by a capital Latin letter WITH.

What does the heat capacity of a body depend on? First of all, from its mass. It is clear that heating, for example, 1 kilogram of water will require more heat than heating 200 grams.

Thus, different amounts of heat are required to heat the same mass of different substances to the same temperature. The amount of heat required to heat a body and, therefore, its heat capacity depend on the type of substance of which the body is composed.

A physical quantity showing how much heat is required to heat 1 kg of a substance by 1 ºС is called specific heat capacity of this substance.

Each substance has its own specific heat capacity, which is denoted by the Latin letter c and measured in joules per kilogram degree (J/(kg °C)).

Calculation of the amount of heat required to heat a body or released by it during cooling.

Q= cm (t 2 -t 1),

Where Q- quantity of heat, c- specific heat capacity, m- body mass, t 1- initial temperature, t 2- final temperature.

When the body heats up t 2> t 1 and therefore Q >0 . When the body cools down t 2i< t 1 and therefore Q< 0 .

If the heat capacity of the entire body is known WITH, Q determined by the formula: Q = C (t 2 - t 1).

22) Melting: definition, calculation of the amount of heat for melting or solidification, specific heat of fusion, graph of t 0 (Q).

Thermodynamics

A branch of molecular physics that studies the transfer of energy, the patterns of transformation of one type of energy into another. Unlike molecular kinetic theory, thermodynamics does not take into account the internal structure of substances and microparameters.

Thermodynamic system

It is a collection of bodies that exchange energy (in the form of work or heat) with each other or with the environment. For example, the water in the kettle cools down, and heat is exchanged between the water and the kettle and the heat of the kettle with the environment. A cylinder with gas under the piston: the piston performs work, as a result of which the gas receives energy and its macroparameters change.

Quantity of heat

This energy, which the system receives or releases during the heat exchange process. Denoted by the symbol Q, it is measured, like any energy, in Joules.

As a result of various heat exchange processes, the energy that is transferred is determined in its own way.

Heating and cooling

This process is characterized by a change in the temperature of the system. The amount of heat is determined by the formula

Specific heat capacity of a substance with measured by the amount of heat required to warm up units of mass of this substance by 1K. Heating 1kg of glass or 1kg of water requires different amounts of energy. Specific heat capacity is a known quantity, already calculated for all substances; see the value in physical tables.

Heat capacity of substance C- this is the amount of heat that is necessary to heat a body without taking into account its mass by 1K.

Melting and crystallization

Melting is the transition of a substance from a solid to a liquid state. The reverse transition is called crystallization.

The energy that is spent on the destruction of the crystal lattice of a substance is determined by the formula

The specific heat of fusion is a known value for each substance; see the value in physical tables.

Vaporization (evaporation or boiling) and condensation

Vaporization is the transition of a substance from a liquid (solid) state to a gaseous state. The reverse process is called condensation.

The specific heat of vaporization is a known value for each substance; see the value in physical tables.

Combustion

The amount of heat released when a substance burns

The specific heat of combustion is a known value for each substance; see the value in physical tables.

For a closed and adiabatically isolated system of bodies, the heat balance equation is satisfied. The algebraic sum of the amounts of heat given and received by all bodies participating in heat exchange is equal to zero:

Q 1 +Q 2 +...+Q n =0

23) The structure of liquids. Surface layer. Surface tension force: examples of manifestation, calculation, surface tension coefficient.

From time to time, any molecule may move to a nearby vacant location. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely located molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called close order(Fig. 3.5.1).

The coefficient β is called temperature coefficient of volumetric expansion . This coefficient for liquids is tens of times greater than for solids. For water, for example, at a temperature of 20 °C β in ≈ 2 10 – 4 K – 1, for steel β st ≈ 3.6 10 – 5 K – 1, for quartz glass β kv ≈ 9 10 – 6 K - 1 .

The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands as the temperature decreases (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.

When water freezes, it expands, so ice remains floating on the surface of a freezing body of water. The temperature of freezing water under the ice is 0 °C. In denser layers of water at the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.

The most interesting feature of liquids is the presence free surface . Liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between the liquid and gas (or vapor), which is in special conditions compared to the rest of the liquid. It should be borne in mind that due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid . If a molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depths of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must perform positive work Δ A external, proportional to the change Δ S surface area:

It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces .

The presence of surface tension forces makes the surface of a liquid look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area of the liquid.

Some liquids, such as soapy water, have the ability to form thin films. Well-known soap bubbles have a regular spherical shape - this also shows the effect of surface tension forces. If a wire frame, one of whose sides is movable, is lowered into a soap solution, then the entire frame will be covered with a film of liquid (Fig. 3.5.3).

Surface tension forces tend to reduce the surface of the film. To balance the movable side of the frame, an external force must be applied to it. If, under the influence of force, the crossbar moves by Δ x, then work Δ will be performed A vn = F vn Δ x = Δ E p = σΔ S, where Δ S = 2LΔ x– increment in the surface area of both sides of the soap film. Since the moduli of forces and are the same, we can write:

Thus, the surface tension coefficient σ can be defined as modulus of the surface tension force acting per unit length of the line bounding the surface.

Due to the action of surface tension forces in drops of liquid and inside soap bubbles, excess pressure Δ arises p. If you mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the cut boundary of length 2π R and excess pressure forces acting on the area π R 2 sections (Fig. 3.5.4). The equilibrium condition is written as

If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets surface of a solid. In this case, the liquid approaches the surface of the solid at a certain acute angle θ, characteristic of a given liquid-solid pair. The angle θ is called contact angle . If the forces of interaction between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 3.5.5). In this case they say that the liquid does not wet surface of a solid. At complete wettingθ = 0, at complete non-wettingθ = 180°.

Capillary phenomena called the rise or fall of liquid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.

In Fig. 3.5.6 shows a capillary tube of a certain radius r, lowered at the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of liquid in the capillary continues until the force of gravity acting on the column of liquid in the capillary becomes equal in magnitude to the resultant F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.

This implies:

With complete non-wetting θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.

Water almost completely wets the clean glass surface. On the contrary, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary drops below the level in the vessel.

24) Vaporization: definition, types (evaporation, boiling), calculation of the amount of heat for vaporization and condensation, specific heat of vaporization.

Evaporation and condensation. Explanation of the phenomenon of evaporation based on ideas about the molecular structure of matter. Specific heat of vaporization. Its units.

The phenomenon of turning a liquid into vapor is called vaporization.

Evaporation - the process of vaporization occurring from an open surface.

Liquid molecules move at different speeds. If any molecule ends up at the surface of a liquid, it can overcome the attraction of neighboring molecules and fly out of the liquid. The ejected molecules form steam. The remaining molecules of the liquid change speed upon collision. At the same time, some molecules acquire a speed sufficient to fly out of the liquid. This process continues so the liquids evaporate slowly.

*The rate of evaporation depends on the type of liquid. Those liquids whose molecules are attracted with less force evaporate faster.

*Evaporation can occur at any temperature. But at high temperatures evaporation occurs faster .

*The rate of evaporation depends on its surface area.

*With wind (air flow), evaporation occurs faster.

During evaporation, the internal energy decreases, because During evaporation, the liquid leaves fast molecules, therefore, the average speed of the remaining molecules decreases. This means that if there is no influx of energy from outside, then the temperature of the liquid decreases.

The phenomenon of vapor turning into liquid is called condensation. It is accompanied by the release of energy.

Steam condensation explains the formation of clouds. Water vapor rising above the ground forms clouds in the upper cold layers of air, which consist of tiny drops of water.

Specific heat of vaporization – physical a value showing how much heat is needed to convert a liquid weighing 1 kg into steam without changing temperature.

Ud. heat of vaporization denoted by the letter L and measured in J/kg

Ud. heat of vaporization of water: L=2.3×10 6 J/kg, alcohol L=0.9×10 6

Amount of heat required to convert liquid into vapor: Q = Lm

>>Physics: Calculation of the amount of heat required to heat a body and released by it during cooling

To learn how to calculate the amount of heat that is necessary to heat a body, let us first establish on what quantities it depends.
From the previous paragraph we already know that this amount of heat depends on the type of substance of which the body consists (i.e., its specific heat capacity):
Q depends on c
But that is not all.

If we want to heat the water in the kettle so that it becomes only warm, then we will not heat it for long. And in order for the water to become hot, we will heat it longer. But the longer the kettle is in contact with the heater, the more heat it will receive from it.

Consequently, the more the body temperature changes when heated, the greater the amount of heat that needs to be transferred to it.

Let the initial temperature of the body be tbegin, and the final temperature be tend. Then the change in body temperature will be expressed by the difference:

Finally, everyone knows that for heating For example, 2 kg of water requires more time (and therefore more heat) than to heat 1 kg of water. This means that the amount of heat required to heat a body depends on the mass of that body:

So, to calculate the amount of heat, you need to know the specific heat capacity of the substance from which the body is made, the mass of this body and the difference between its final and initial temperatures.

Let, for example, you need to determine how much heat is needed to heat an iron part weighing 5 kg, provided that its initial temperature is 20 °C, and the final temperature should be equal to 620 °C.

From Table 8 we find that the specific heat capacity of iron is c = 460 J/(kg°C). This means that heating 1 kg of iron by 1 °C requires 460 J.
To heat 5 kg of iron by 1 °C, 5 times more heat will be required, i.e. 460 J * 5 = 2300 J.

To heat iron not by 1 °C, but by A t = 600°C, another 600 times more amount of heat will be required, i.e. 2300 J X 600 = 1,380,000 J. Exactly the same (modulo) amount of heat will be released when this iron cools from 620 to 20 °C.

So, to find the amount of heat required to heat a body or released by it during cooling, you need to multiply the specific heat capacity of the body by its mass and by the difference between its final and initial temperatures:

??? 1. Give examples showing that the amount of heat received by a body when heated depends on its mass and temperature changes. 2. What formula is used to calculate the amount of heat required to heat a body or released by it when cooling?

S.V. Gromov, N.A. Rodina, Physics 8th grade

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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 each 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. What 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 the physical quantity called “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, the greater the mass of a body, the greater the amount of heat that 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:

where Q is the amount of heat,
m - body weight,
(t_2-t_1) - the difference between the 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.

The concept of the amount of heat was formed in the early stages of the development of modern physics, when there were no clear ideas about the internal structure of matter, what energy is, what forms of energy exist in nature and about energy as a form of movement and transformation of matter.

The amount of heat is understood as a physical quantity equivalent to the energy transferred to a material body in the process of heat exchange.

The outdated unit of heat is the calorie, equal to 4.2 J; today this unit is practically not used, and the joule has taken its place.

Initially, it was assumed that the carrier of thermal energy was some completely weightless medium with the properties of a liquid. Numerous physical problems of heat transfer have been and are still being solved based on this premise. The existence of hypothetical caloric was the basis for many essentially correct constructions. It was believed that caloric is released and absorbed in the phenomena of heating and cooling, melting and crystallization. The correct equations for heat transfer processes were obtained based on incorrect physical concepts. There is a known law according to which the amount of heat is directly proportional to the mass of the body participating in heat exchange and the temperature gradient:

Where Q is the amount of heat, m is the body mass, and the coefficient With– a quantity called specific heat capacity. Specific heat capacity is a characteristic of a substance involved in a process.

Work in thermodynamics

As a result of thermal processes, purely mechanical work can be performed. For example, when a gas heats up, it increases its volume. Let's take a situation like the picture below:

In this case, the mechanical work will be equal to the force of gas pressure on the piston multiplied by the path traveled by the piston under pressure. Of course, this is the simplest case. But even in it one can notice one difficulty: the pressure force will depend on the volume of the gas, which means that we are not dealing with constants, but with variable quantities. Since all three variables: pressure, temperature and volume are related to each other, calculating work becomes significantly more complicated. There are some ideal, infinitely slow processes: isobaric, isothermal, adiabatic and isochoric - for which such calculations can be performed relatively simply. A graph of pressure versus volume is plotted and the work is calculated as an integral of the form.