Thermodynamics is an important branch of physics. We can safely say that its achievements led to the emergence of the technological era and largely determined the course of human history over the past 300 years. The article discusses the first, second and third laws of thermodynamics and their application in practice.
What is thermodynamics?
Before giving the formulations of the laws of thermodynamics, we will understand what this section of physics does.
The word "thermodynamics" is of Greek origin and means "movement due to heat." That is, this branch of physics is engaged in the study of any processes as a result of which thermal energy is converted into mechanical motion and vice versa.
The basic laws of thermodynamics were formulated in the middle of the XIX century. The science of motion and heat considers the behavior of the entire system as a whole, studying the change in its macroscopic parameters - temperature, pressure and volume, and not paying attention to its microscopic structure. Moreover, the first of them plays a fundamental role in the formulation of the laws of thermodynamics in physics. It is interesting to note that they are derived exclusively from experimental observations.
The concept of a thermodynamic system
By it is meant any group of atoms, molecules or other elements that are considered as a whole. All three laws are formulated for the so-called thermodynamic system. Examples are: Earth's atmosphere, any living organism, gas mixture in an internal combustion engine, etc.
All systems in thermodynamics belong to one of three types:
- Open. In them there is an exchange of both heat and matter with the environment. For example, if food is being cooked in an open fire in a pot, then this is a striking example of an open system, since the pot receives energy from the external environment (bonfire), while it emits energy in the form of heat, and also water evaporates from it (exchange of matter).
- Closed. In such systems, there is no exchange of matter with the medium, although an exchange of energy occurs. Returning to the previous case: if you cover the pot, you can get a closed system.
- Isolated. This is a type of thermodynamic systems that exchange neither matter nor energy with the space surrounding them. An example is a thermos containing hot tea.
Thermodynamic temperature
By this concept is meant the kinetic energy of the particles forming the surrounding bodies, which reflects the speed of the chaotic movement of particles. The larger it is, the higher the temperature. Accordingly, reducing the kinetic energy of the system, we cool it.
By this concept is meant the kinetic energy of the particles forming the surrounding bodies, which reflects the speed of the chaotic movement of particles. The larger it is, the higher the temperature. Accordingly, reducing the kinetic energy of the system, we cool it.
The thermodynamic temperature is expressed in SI (International System of Units of Measurement) in kelvin (in honor of the British scientist William Kelvin, who proposed this scale for the first time). Understanding the first, second and third laws of thermodynamics is impossible without determining the temperature.
The division of one degree on the Kelvin scale also corresponds to one degree Celsius. The translation between these units is carried out according to the formula: T K = T C + 273.15, where T K and T C are the temperatures in kelvins and degrees Celsius, respectively.
A feature of the Kelvin scale is that it does not have negative values. Zero in it (T C = -273.15 o C) corresponds to the state when the thermal motion of the particles of the system is completely absent, they appear to be "frozen".
Energy conservation and 1 law of thermodynamics
In 1824, Nicola Leonard Sadi Carnot, a French engineer and physicist, put forward a bold assumption, which not only led to the development of physics, but also became an important step for improving technology. It can be formulated as follows: "Energy cannot be created or destroyed, it can only be transferred from one state to another."
In essence, the phrase Sadi Carnot postulates the law of conservation of energy, which formed the basis of 1 law of thermodynamics: "Whenever a system receives energy from outside, it translates it into other forms, the main of which are thermal and mechanical."
The mathematical formula for the 1st law is written as follows:
Q = ฮU + A,
here Q is the amount of heat transferred by the environment to the system, ฮU is the change in the internal energy of this system, A is the perfect mechanical work.
Adiabatic processes
A good example of them is the movement of air masses along mountain slopes. Such masses are huge (kilometers or more), and air is an excellent heat insulator. The noted properties make it possible to consider any processes with air masses that occur within a short time as adiabatic. When the air rises along the hillside, its pressure drops, it expands, that is, performs mechanical work, and, as a result, cools. On the contrary, the movement of the air mass down is accompanied by an increase in pressure in it, it is compressed and due to this it is very hot.
The application of the law of thermodynamics, which was considered in the previous subheading, is easiest to demonstrate using the adiabatic process as an example.
According to the definition, as a result of it, there is no energy exchange with the environment, that is, in the equation above Q = 0. This leads to the following expression: ฮU = -A. The minus sign here means that the system performs mechanical work by reducing its own internal energy. It should be recalled that internal energy is directly dependent on the temperature of the system.
The direction of thermal processes
The 2nd law of thermodynamics deals with this issue. Surely everyone noticed that if you bring two objects into contact with different temperatures, the cold will always heat up, and the hot will cool. Note that the reverse process can occur within the framework of the first law of thermodynamics, however, it is never implemented in practice.
The reason for the irreversibility of this process (and all known processes in the Universe) is the transition of the system to a more probable state. In the considered example with the contact of two bodies of different temperature, the most probable state will be that in which all particles of the system will have the same kinetic energy.
The second law of thermodynamics can be formulated as follows: "Heat can never be transferred spontaneously from a cold body to a hot one." If we introduce the concept of entropy as a measure of disorder, then it can be represented in the following form: "Any thermodynamic process proceeds with an increase in entropy."
Thermal machine
This term refers to a system that, thanks to the supply of external energy to it, can perform mechanical work. The first heat engines were steam engines and were invented at the end of the 17th century.
The second law of thermodynamics plays a decisive role in determining their effectiveness. Sadi Carnot found that the maximum efficiency of this device is: Efficiency = (T 2 - T 1 ) / T 2 , here T 2 and T 1 are the temperatures of the heater and the refrigerator. Mechanical work can be accomplished only when there is a stream of heat from a hot body to a cold one, and this stream cannot be 100% converted into useful energy.
The figure below shows the principle of operation of a heat engine (Q abs - heat transferred to the machine, Q ced - heat loss, W - useful work, P and V - pressure and gas volume in the piston).
Absolute Zero and Nernst's Postulate
Finally, we turn to the consideration of the third law of thermodynamics. It is also called the Nernst postulate (the name of the German physicist who first formulated it at the beginning of the 20th century). The law says: โWith a finite number of processes, absolute zero cannot be achieved.โ That is, it is impossible in any way to completely "freeze" the molecules and atoms of a substance. The reason for this is the constant existing heat exchange with the environment.
One of the useful conclusions made from the third law of thermodynamics is to reduce the entropy when moving to absolute zero. This means that the system seeks to organize itself. This fact can be used, for example, to transfer paramagnets to the ferromagnetic state during cooling.
It is interesting to note that the lowest temperature that has been reached at present is 5 ยท 10 โ10 K (2003, laboratory of the Massachusetts Institute of Technology, USA).