Presentation for the lesson "The first law of thermodynamics. Application of the first law of thermodynamics to thermal processes." Didactic goal: to formulate the first law of thermodynamics and consider its application in various processes Lesson on the topic first law of those

08.11.2019 Thrush

Educational goal: to achieve students' assimilation of the law of conservation and transformation of energy for thermal processes - the first law of thermodynamics; show the practical significance of the law

Basic knowledge and skills: know the formulation of the law, the definition of an adiabatic process and be able to interpret natural phenomena based on the laws of thermodynamics

Organizing moment (report the lesson plan) SLIDE 1
Repetition of the studied material: give names to various processes on the graph, choose formulas for each section, answer questions SLIDES 2 - 4

1. Why does the temperature not change in two areas?

2. What happens to the molecules at each site?

3. In what cases Q>0 and Q<0?

4. In what state is the substance in these areas?

5. Define isoprocesses.

6. What is called internal energy and what does it depend on?

7. In what case does the gas do work? What does the sign of work depend on?

8. What is called the amount of heat?

9. What formulas do we use when calculating the amount of heat?

3. Problem solving. While the oral survey is being conducted, the rest of the students solve problems on

calculation of the amount of heat according to the options SLIDE 5

Problem solution check
Repetition: ways to change internal energy
Repetition: the law of conservation of energy and examples of its manifestation in nature
First law of thermodynamics: definition and formula (write down)
The first law of thermodynamics for an isochoric process (write down)
The first law of thermodynamics for an isothermal process (write down)
The first law of thermodynamics for an isobaric process (write down)
Adiabatic process (write down). Consider examples
Heat balance equation (write down)
Sample solution of the problem for the heat balance equation (write down)
Lesson summary:

1. The wording of the first law

2. How does the equation change for different processes?

3. What process is called adiabatic?

4. Examples of adiabatic processes?

5. Why does the atmosphere cool when moving away from the Earth's surface?

15. Homework:

Know the wording of the law

First law of thermodynamics

On fig. 3.9.1 conditionally depicts the energy flows between the selected thermodynamic system and the surrounding bodies. The value of Q > 0 if the heat flow is directed towards the thermodynamic system. The value A > 0 if the system performs positive work on the surrounding bodies.

Figure 3.9.1.

The exchange of energy between a thermodynamic system and surrounding bodies as a result of heat transfer and the work done.

If the system exchanges heat with surrounding bodies and does work (positive or negative), then the state of the system changes, i.e., its macroscopic parameters (temperature, pressure, volume) change. Since the internal energy U is uniquely determined by the macroscopic parameters that characterize the state of the system, it follows that the processes of heat transfer and work are accompanied by a change in the internal energy of the system ΔU.

The first law of thermodynamics is a generalization of the law of conservation and transformation of energy for a thermodynamic system. It is formulated as follows:

The change ΔU of the internal energy of a non-isolated thermodynamic system is equal to the difference between the amount of heat Q transferred to the system and the work A performed by the system on external bodies.

The relation expressing the first law of thermodynamics is often written in a different form:

The amount of heat received by the system is used to change its internal energy and perform work on external bodies.

The first law of thermodynamics is a generalization of experimental facts. According to this law, energy cannot be created or destroyed; it is transferred from one system to another and is transformed from one form into another. An important consequence of the first law of thermodynamics is the assertion that it is impossible to create a machine capable of doing useful work without consuming energy from outside and without any changes inside the machine itself. Such a hypothetical machine was called a perpetual motion machine (perpetuum mobile) of the first kind. Numerous attempts to create such a machine invariably ended in failure. Any machine can perform positive work A on external bodies only by obtaining some amount of heat Q from the surrounding bodies or by reducing ΔU of its internal energy.

Let us apply the first law of thermodynamics to isoprocesses in gases.

In an isochoric process (V = const), the gas does no work, A = 0. Therefore,

Q = ∆U = U(T2) - U(T1).

Here U(T1) and U(T2) are the internal energies of the gas in the initial and final states. The internal energy of an ideal gas depends only on temperature (Joule's law). During isochoric heating, heat is absorbed by the gas (Q > 0), and its internal energy increases. During cooling, heat is transferred to external bodies (Q< 0).
In an isobaric process (p = const), the work done by the gas is expressed by the relation

A = p(V2 - V1) = pΔV.

The first law of thermodynamics for an isobaric process gives:

Q = U(T2) - U(T1) + p(V2 - V1) = ΔU + pΔV.

With isobaric expansion Q > 0, heat is absorbed by the gas, and the gas does positive work. Under isobaric compression Q< 0 - тепло отдается внешним телам. В этом случае A < 0. Температура газа при изобарном сжатии уменьшается, T2 < T1; внутренняя энергия убывает, ΔU < 0.
In an isothermal process, the gas temperature does not change, therefore, the internal energy of the gas does not change either, ΔU = 0.

The first law of thermodynamics for an isothermal process is expressed by the relation

The amount of heat Q received by the gas in the process of isothermal expansion is converted into work on external bodies. Under isothermal compression, the work of external forces produced on the gas is converted into heat, which is transferred to the surrounding bodies.

Along with isochoric, isobaric, and isothermal processes, thermodynamics often considers processes that occur in the absence of heat exchange with surrounding bodies. Vessels with heat-impermeable walls are called adiabatic shells, and the processes of gas expansion or compression in such vessels are called adiabatic.

Model. adiabatic process.

In an adiabatic process Q = 0; so the first law of thermodynamics takes the form

i.e., the gas does work due to the loss of its internal energy.

On the plane (p, V), the process of adiabatic expansion (or compression) of a gas is represented by a curve called an adiabat. During adiabatic expansion, the gas does positive work (A > 0); therefore its internal energy decreases (ΔU< 0). Это приводит к понижению температуры газа. Вследствие этого давление газа при адиабатическом расширении убывает быстрее, чем при изотермическом расширении (рис. 3.9.2).

Figure 3.9.2.

Families of isotherms (red curves) and adiabats (blue curves) of an ideal gas.

In thermodynamics, an equation for an adiabatic process is derived for an ideal gas. In coordinates (p, V), this equation has the form

This relation is called the Poisson equation. Here γ = Cp / CV is the adiabatic index, Cp and CV are the heat capacities of the gas in constant pressure and constant volume processes (see §3.10). For a monatomic gas for a diatomic gas for a polyatomic gas

The work of a gas in an adiabatic process is simply expressed in terms of the temperatures T1 and T2 of the initial and final states:

A = CV(T2 - T1).

An adiabatic process can also be classified as an isoprocess. In thermodynamics important role is played by a physical quantity called entropy (see §3.12). The change in entropy in any quasi-static process is equal to the reduced heat ΔQ / T received by the system. Since in any part of the adiabatic process ΔQ = 0, the entropy in this process remains unchanged.

An adiabatic process (as well as other isoprocesses) is a quasi-static process. All intermediate states of the gas in this process are close to the states of thermodynamic equilibrium (see §3.3). Any point on the adiabat describes an equilibrium state.

Not every process carried out in an adiabatic shell, i.e., without heat exchange with surrounding bodies, satisfies this condition. An example of a non-quasistatic process in which the intermediate states are nonequilibrium is the expansion of a gas into a vacuum. On fig. 3.9.3 shows a rigid adiabatic shell consisting of two communicating vessels separated by a valve K. In the initial state, the gas fills one of the vessels, and vacuum in the other vessel. After opening the valve, the gas expands, fills both vessels, and a new equilibrium state is established. In this process, Q = 0, because there is no heat exchange with the surrounding bodies, and A = 0, because shell is not deformable. It follows from the first law of thermodynamics: ΔU = 0, i.e., the internal energy of the gas remained unchanged. Since the internal energy of an ideal gas depends only on temperature, the temperatures of the gas in the initial and final states are the same - the points on the plane (p, V, depicting these states, lie on the same isotherm. All intermediate states of the gas are nonequilibrium and cannot be shown on the diagram.

Lesson Objectives:

deepen knowledge about isoprocesses, develop problem solving skills on this topic, develop communication skills, skills, teach self-esteem.

During the classes

Preparation for work in groups.

Work with the class (orally).

What is called internal energy?

How can the internal energy of a gas be changed?

How to determine the amount of heat required to heat the body?

Write the heat balance equation for three bodies.

When is the amount of heat negative?

How to determine the work of a gas during expansion?

What is the difference between the work of a gas and the work of external forces?

Formulate the first law of thermodynamics for the work of external forces.

Formulate the first law of thermodynamics for the work of a gas.

Application of the first law of thermodynamics to the isochoric process.

Application of the first law of thermodynamics to an isobaric process.

Application of the first law of thermodynamics to an isothermal process.

What process is called adiabatic?

Application of the first law of thermodynamics to an adiabatic process.

Group work.

Each group receives a sheet on which theoretical tasks and tasks are indicated. The theoretical part contains five questions. The group takes to prepare for the answer the question corresponding to its number. The practical part contains ten tasks, two for each of the indicated topics in theory. Tasks are arranged randomly. This means that students must first find problems that match their theoretical question, then solve them. Additional data for solving problems are taken from directories.

After the end of the work of the groups, two students are called in turn from each group: one answers the theory, the other writes a brief condition of one problem on the board. (Another problem in this group can be checked selectively in the same lesson or in the next.) All members of the group should be able to answer the theory and explain the problems; the use of additional material in the theoretical part.

Tasks in notebooks are written by all students.

A clear organization of work leads to the vigorous activity of all the guys. Group coordinators at the end of the lesson hand over sheets on which they note the contribution of group members to its work.

The activity of groups and individual students is finally evaluated by the teacher.

Sheet sample.

Theoretical part

1. Isochoric process.

2. Isothermal process.

3. Isobaric process.

4. Adiabatic process.

5. Heat transfer in a closed system.

Practical part

1. There is 1.25 kg of air in the cylinder under the piston. To heat it by 40C at constant pressure, 5 kJ of heat was expended. Determine the change in the internal energy of the gas.

2. 0.02 kg of carbon dioxide is heated at a constant volume. Determine the change in the internal energy of the gas during heating from 200C to 1080C (c = 655 J/(kg K)).

3. In a heat-insulated cylinder with a piston, there is nitrogen weighing 0.3 kg at a temperature of 200C. Nitrogen, expanding, does the work of 6705 J. Determine the change in the internal energy of nitrogen and its temperature after expansion (c \u003d 745 J / (kg K)).

4. The amount of heat is imparted to the gas, as a result of which it expands isothermally from a volume of 2 liters to a volume of 12 liters. The initial pressure is 1.2 106 Pa. Determine the work done by the gas.

5. A certain amount of mercury was poured into a 50 g glass flask, containing 185 g of water at 200C, at 1000C, and the temperature of the water in the flask rose to 220C. Determine the mass of mercury.

6. 1.43 kg of air occupy a volume of 0.5 m3 at 00C. A certain amount of heat was imparted to the air and it expanded isobarically to a volume of 0.55 m3. Find the perfect work, the amount of absorbed heat, the change in temperature and the internal energy of the air.

7. There is 1.5 kg of oxygen in the cylinder under the piston. The piston is stationary. How much heat must be imparted to the gas to raise its temperature by 80°C? What is the change in internal energy? (cv= 675 J/(kg K))

8. In the cylinder under the piston is 1.6 kg of oxygen at a temperature of 170C and a pressure of 4 105 Pa. The gas did work at an isothermal expansion of 20J. How much heat is imparted to the gas? What is the change in the internal energy of the gas? What was the initial volume of gas?

9. How much heat will be released during the condensation of 0.2 kg of water vapor, which has a temperature of 1000C, and when the water obtained from it is cooled to 200C?

10. The gas cylinder is placed in a heat-resistant shell. How will the temperature of the gas change if the volume of the cylinder is gradually increased? What is the change in the internal energy of the gas if 6000 J of work is done on the gas?

Review questions:

What is internal energy?
Name the ways of changing the internal energy.
How to determine the work of a gas?
How to determine the amount of heat?
Explain the physical meaning of specific quantities.

The change in the internal energy of the system during its transition from one state to another is equal to the sum of the work of external forces and the amount of heat transferred to the system.

The amount of heat transferred to the system is used to perform work by the system and change its internal energy

Isothermal process

(T = const) :  U =0

Because ΔT=0, Δ U=0 and then Q= A.

If Q

Application of the first law of thermodynamics to isoprocesses

Isobaric process:

(p = const, ∆p=0 )

A=p  V = vR  T

0 "width="640"

0, then ΔU 0 is gas heating, if Q "width="640"

isochoric process.

1. What is an isochoric process?

2. Because ΔV=0, → A=0 →ΔU=Q

If Q 0, then ΔU 0 is gas heating, if Q

Application of the first law of thermodynamics to isoprocesses

Isochoric process:

( V = const): A = 0

0, then Δ U0 is gas heating, if Q" width="640"

Because ΔV=0, then A=0 and ΔU=Q

If Q0, then Δ U0 is gas heating, if Q

Application of the first law of thermodynamics to isoprocesses

Adiabatic process: A process that does not exchange heat with the environment.

Q=0

Temperature changes only when work is done

adiabatic process

All fast processes and processes occurring in a thermally insulated medium can be considered adiabatic.

The adiabat is steeper than any isotherm that intersects it

Thermodynamics of a cyclic process.

For an arbitrary cyclic process 1–2–3–4–1 the work done by the gas in a cycle is numerically equal to the area of the figure bounded by the cycle diagram in coordinates p – V

Irreversibility of processes in nature .

Irreversible - processes that can spontaneously proceed in only one direction. In the opposite direction, they can proceed only as one of the links in a more complex process.

What will happen to pendulum oscillations over time?

All processes in nature IRREVERSIBLE!

II law of thermodynamics.

Clausius' formulation(1850): a process is impossible in which heat would spontaneously transfer from less heated bodies to more heated bodies.
Thomson's formulation(1851): a circular process is impossible, the only result of which would be the production of work at the expense of a decrease in internal energy.
Clausius' formulation(1865): all spontaneous processes in a closed non-equilibrium system occur in such a direction in which the entropy of the system increases; in a state of thermal equilibrium, it is maximum and constant.
Boltzmann's formulation(1877): a closed system of many particles spontaneously passes from a more ordered state to a less ordered one. The spontaneous exit of the system from the equilibrium position is impossible. Boltzmann introduced a quantitative measure of disorder in a system consisting of many bodies - entropy .

deepen knowledge of isoprocesses,

develop skills in problem solving on a given topic,

develop communication skills,

teach self-esteem.

During the classes.

Preparation for work in groups.

Work with the class (orally).

What is called internal energy?

How can the internal energy of a gas be changed?

How to determine the amount of heat required to heat the body?

Write the heat balance equation for three bodies.

When is the amount of heat negative?

How to determine the work of a gas during expansion?

What is the difference between the work of a gas and the work of external forces?

Formulate the first law of thermodynamics for the work of external forces.

Formulate the first law of thermodynamics for the work of a gas.

Application of the first law of thermodynamics to the isochoric process.

Application of the first law of thermodynamics to an isobaric process.

Application of the first law of thermodynamics to an isothermal process.

What process is called adiabatic?

Application of the first law of thermodynamics to an adiabatic process.